Atoms and Nuclei

Let’s carefully match them:

• (a) Davisson and Germer Experiment → Demonstrated wave nature of electrons via electron diffraction. → (i)

• (b) Millikan’s Oil Drop Experiment → Measured charge of an electron. → (ii)

• (c) Rutherford Experiment → Showed the existence of nucleus in the atom through alpha-particle scattering. → (iv)

• (d) Franck–Hertz Experiment → Demonstrated quantisation of energy levels in atoms (mercury). → (iii)

So correct matching is:

(a)-(i), (b)-(ii), (c)-(iv), (d)-(iii)

✅ Answer: Option B

• Fresh wood: 20 decays/min (current activity if alive / just cut).

• Old museum sample: 2 decays/min.

• Half-life of C‑14 = 5730 years.

• Find the age of wooden piece in museum.

Step 1: Ratio of activities

$ \frac{A}{A_0} = \frac{2}{20} = 0.1 $

So, the museum sample’s activity is 10% of that in fresh wood.

Step 2: Radioactive decay law

$ A = A_0 e^{-\lambda t} $

$ \frac{A}{A_0} = e^{-\lambda t} $

So,

$ 0.1 = e^{-\lambda t} $

$ t = \frac{\ln(0.1)}{-\lambda} $

Step 3: Decay constant

$ \lambda = \frac{\ln 2}{T_{1/2}} = \frac{0.693}{5730 \,\text{yr}} \approx 1.2097 \times 10^{-4} \,\text{yr}^{-1} $

Step 4: Solve for $t$

$ t = \frac{\ln(0.1)}{-1.2097 \times 10^{-4}} $

$ \ln(0.1) = -2.3026 $

$ t = \frac{2.3026}{1.2097 \times 10^{-4}} $

$ t \approx 19039 \,\text{years} $

✅ Final Answer:

Option C: 19039 years

The nucleus is assumed as a sphere where the volume is proportional to the total number of nucleons (mass number, A).

Because the density of nuclear matter is approximately constant, the radius R of a nucleus is related to its mass number A by the following formula :

$ \mathrm{R}=\mathrm{R}_0 \mathrm{~A}^{1 / 3} $

Where R is the nuclear radius.

$R_0$ is a constant (approximately $1.2 \times 10^{-15} \mathrm{~m}$ or 1.2 fm ).

A is the mass number.

For the first nucleus : $\mathrm{R}_\alpha=\mathrm{R}_0(\alpha)^{1 / 3}$

For the second nucleus : $R_\beta=R_0(\beta)^{1 / 3}$

Taking the ratio $\frac{\mathrm{R}_\alpha}{\mathrm{R}_\beta}$ :

$ \frac{\mathrm{R}_\alpha}{\mathrm{R}_\beta}=\frac{\mathrm{R}_0 \alpha^{1 / 3}}{\mathrm{R}_0 \beta^{1 / 3}}=\left(\frac{\alpha}{\beta}\right)^{1 / 3} $

It is given that $\beta=8 \alpha$.

$ \frac{\mathrm{R}_\alpha}{\mathrm{R}_\beta}=\left(\frac{\alpha}{8 \alpha}\right)^{1 / 3}=\frac{1}{2}=0.5 $

Therefore, the ratio of the radius of the first nucleus to the second is 0.5 . Hence, the correct option is (D).

The mass number of initial nucleus is $\mathrm{A}_1=8$

The radius of a nucleus is proportional to $A^{1 / 3}\left(R=R_0 A^{1 / 3}\right)$.

So, the nuclear radius of initial nucleus is $R_1=R_0(8)^{1 / 3}=2 R_0$

Hence, initial surface area

$ \mathrm{S}_1=4 \pi \mathrm{R}_1^2 $

$\Rightarrow $ $ S_1=4 \pi\left(2 R_0\right)^2=16 \pi R_0^2 $

The atom absorbs 10 protons and 9 neutrons.

Total nucleons in final state of nucleus is 10 protons +9 neutrons $=19$ nucleons.

So, new mass number is $\mathrm{A}_2=\mathrm{A}_1+19=8+19=27$

The final radius,

$ \mathrm{R}_2=\mathrm{R}_0(27)^{1 / 3}=3 \mathrm{R}_0 $

So, final surface area is

$ S_2=4 \pi\left(3 R_0\right)^2=36 \pi R_0^2 $

The question has asked about the percentage growth recorded, that means final surface area is what percentage

So, the percentage growth is,

So, the percentage growth is,

$ \frac{\Delta S}{\mathrm{~S}_1} \times 100 \%=\frac{36 \pi \mathrm{R}_0^2-16 \pi \mathrm{R}_0^2}{16 \pi \mathrm{R}_0^2} \times 100 \%=125 \% $

A nucleus is more stable if it has a higher binding energy i.e., is in a lower energy state.

In exothermic reaction energy is released, $\Delta Q>0$. The products are more stable than the reactants.

In endothermic reaction energy is absorbed, $\Delta Q<0$. The products are less stable than the reactants.

The given two nuclear reactions are,

$ { }_2^3 \mathrm{He}+{ }_0^1 \mathrm{n} \rightarrow{ }_2^4 \mathrm{He}+20 \mathrm{MeV} \ldots . (1)$

$ { }_2^4 \mathrm{He}+{ }_0^1 \mathrm{n} \rightarrow{ }_2^5 \mathrm{He}-0.9 \mathrm{MeV} $......(2)

In ${ }_2^3 \mathrm{He}+{ }_0^1 \mathrm{n} \rightarrow{ }_2^4 \mathrm{He}+20 \mathrm{MeV}$ the released energy is $20 \mathrm{MeV}(\mathrm{Q}=+20 \mathrm{MeV})$.

Since energy is released, the product ${ }_2^4 \mathrm{He}$ is significantly more stable than the reactant ${ }_2^3 \mathrm{He}$.

Therefore, $\mathrm{X}_4>\mathrm{X}_3$.

In ${ }_2^4 \mathrm{He}+{ }_0^1 \mathrm{n} \rightarrow{ }_2^5 \mathrm{He}-0.9 \mathrm{MeV}$ the absorbed energy is $0.9 \mathrm{MeV}(\mathrm{Q}=-0.9 \mathrm{MeV})$.

Since energy must be supplied to this reaction, the product ${ }_2^5 \mathrm{He}$ is less stable than the reactant ${ }_2^4 \mathrm{He}$.

Therefore, $\mathrm{X}_4>\mathrm{X}_5$.

From reaction 1 , the jump in stability from $\mathrm{X}_3$ to $\mathrm{X}_4$ is very large ( 20 MeV ).

From reaction 2, the drop in stability from $\mathrm{X}_4$ to $\mathrm{X}_5$ is very small ( 0.9 MeV ).

Because $\mathrm{X}_5$ is only slighty less stable than $\mathrm{X}_4$, but $\mathrm{X}_3$ is significantly less stable than $\mathrm{X}_4$, we can conclude that $\mathrm{X}_5$ is still more stable than $\mathrm{X}_3$.

Therefore, the order of stability is $X_4>X_5>X_3$.

Hence, the correct option is (A).

The velocity of an electron in the $\mathrm{n}^{\text {th }}$ orbit is proportional to the atomic number Z and inversely proportional to the principal quantum number n .

$ \mathrm{v} \propto \frac{\mathrm{Z}}{\mathrm{n}} $

$\Rightarrow $ $ \mathrm{v}=\mathrm{k}_{\mathrm{v}} \cdot \frac{\mathrm{Z}}{\mathrm{n}} \ldots (i)$

Where $\mathrm{k}_{\mathrm{v}}$ is constant for the velocity.

The radius of the $\mathrm{n}^{\text {th }}$ orbit is proportional to the square of the principal quantum number n and inversely proportional to the atomic number Z.

$ r \propto \frac{n^2}{Z} $

$\Rightarrow $ $ \mathrm{r}=\mathrm{k}_{\mathrm{r}} \cdot \frac{\mathrm{n}^2}{\mathrm{Z}} \ldots(\mathrm{ii}) $

Let $\mathrm{r}_1$ be the radius for the first atom (with state $\mathrm{n}_1, \mathrm{Z}_1$ ) and $\mathrm{r}_2$ for the second (with state $\mathrm{n}_2, \mathrm{Z}_2$ ).

$ \mathrm{r}_1=\mathrm{r}_2 $

$\Rightarrow $ $\mathrm{k}_{\mathrm{r}} \cdot \frac{\mathrm{n}_1^2}{\mathrm{Z}_1}=\mathrm{k}_{\mathrm{r}} \cdot \frac{\mathrm{n}_2^2}{\mathrm{Z}_2}$

$\Rightarrow $ $\frac{\mathrm{n}_1^2}{\mathrm{Z}_1}=\frac{\mathrm{n}_2^2}{\mathrm{Z}_2}$

$ \frac{\mathrm{Z}_1}{\mathrm{Z}_2}=\frac{\mathrm{n}_1^2}{\mathrm{n}_2^2}=\left(\frac{\mathrm{n}_1}{\mathrm{n}_2}\right)^2 \ldots (iii)$

Let $\mathrm{v}_1$ and $\mathrm{v}_2$ be the speeds.

$ \mathrm{v}_1=\mathrm{k}_{\mathrm{v}} \frac{\mathrm{Z}_1}{\mathrm{n}_1} \ldots (iv)$

$\mathrm{v}_2=\mathrm{k}_{\mathrm{v}} \frac{\mathrm{Z}_2}{\mathrm{n}_2} \ldots(\mathrm{v})$

$\therefore $ $\frac{\mathrm{v}_1}{\mathrm{v}_2}=\frac{\mathrm{k}_{\mathrm{v}}\left(\frac{\mathrm{Z}_1}{\mathrm{n}_1}\right)}{\mathrm{k}_{\mathrm{v}}\left(\frac{\mathrm{Z}_2}{\mathrm{n}_2}\right)}$

$\Rightarrow $ $\frac{\mathrm{v}_1}{\mathrm{v}_2}=\frac{\mathrm{Z}_1}{\mathrm{n}_1} \times \frac{\mathrm{n}_2}{\mathrm{Z}_2}$ .....(vi)

$\Rightarrow $ $ \frac{\mathrm{v}_1}{\mathrm{v}_2}=\left[\left(\frac{\mathrm{n}_1}{\mathrm{n}_2}\right)^2\right] \cdot\left(\frac{\mathrm{n}_2}{\mathrm{n}_1}\right)=\frac{\mathrm{n}_1}{\mathrm{n}_2} $

It is given that the speed of first electron is $\mathrm{v}_1=3 \times 10^5 \mathrm{~m} / \mathrm{s}$ and that of electron in second hydrogen like atom is $\mathrm{v}_2=2.5 \times 10^5 \mathrm{~m} / \mathrm{s}$

$ \frac{n_1}{n_2}=\frac{3 \times 10^5}{2.5 \times 10^5} $

$\Rightarrow $ $\frac{\mathrm{n}_1}{\mathrm{n}_2}=\frac{3}{2.5}$

$\Rightarrow $ $\frac{\mathrm{n}_1}{\mathrm{n}_2}=\frac{6}{5}$

So, the smallest possible energy states in two atoms are 6 and 5.

Hence, the correct option is (D).

For light nuclei, the binding energy per nucleon $\left(\frac{B E}{A}\right)$ increases rapidly as mass increases. The curve reaches a maximum around Mass Number $\mathrm{A} \approx 56$ (Iron, Fe ). This is the most stable region. For heavy nuclei the $\frac{\mathrm{BE}}{\mathrm{A}}$ actually decreases as mass increases.

So, the binding energy per nucleon does not constantly increase with mass. It increases initially, peaks, and then decreases.

Hence, statement I is false.

Nature always tends towards the state of lowest potential energy (highest stability). Higher Binding Energy per Nucleon means the nucleus is more tightly bound and therefore more stable.

So, in nuclear fission a heavy nucleus (low $\frac{B E}{A}$ ) splits into two lighter nuclei (higher $\frac{B E}{A}$ ). And in nuclear fusion the light nuclei (low BE/A) combine to form a heavier nucleus (higher BE/A).

In both fundamental nuclear processes (fission and fusion), the system evolves from a state of lower binding energy per nucleon to a state of higher binding energy per nucleon. This is the driving force behind nuclear energy release.

Hence, statement II is true.

Therefore, statement I is false, statement II is true. Hence, the correct option is (A).

Atoms are represented by the notation $\mathrm{X}_{\mathrm{Z}}^{\mathrm{A}}$, where :

• 1. A is the Mass Number (total number of protons and neutrons).

• 2. $ \mathrm{Z}$ is the Atomic Number (number of protons).

Isobars are atoms of different chemical elements that have the same mass number (A) but different atomic numbers (Z).

Option A : $\mathrm{H}_1^3$ and $\mathrm{He}_2^3$

• Mass numbers: 3 and 3 (Same)

• Atomic numbers: 1 and 2 (Different)

Since they have the same mass number ( $A=3$ ) but represent different elements ( $Z=1$ vs $Z=2$ ), they are Isobars. Hence, option (A) is correct.

Option B : $\mathrm{Hg}_{80}^{198}$ and $\mathrm{Au}_{79}^{197}$

• Mass numbers: 198 and 197 (Different)

• Atomic numbers: 80 and 79 (Different)

So, $\mathrm{Hg}_{80}^{198}$ and $\mathrm{Au}_{79}^{197}$ are neither isobars nor isotopes. Hence, option (B) is incorrect.

Option C : $\mathrm{H}_1^2$ and $\mathrm{H}_1^3$

• Mass numbers: 2 and 3 (Different)

• Atomic numbers: 1 and 1 (Same)

So, $\mathrm{H}_1^2$ and $\mathrm{H}_1^3$ are Isotopes of Hydrogen (Deuterium and Tritium). Hence, option (C) is incorrect.

Option D : $\mathrm{U}_{92}^{236}$ and $\mathrm{U}_{92}^{238}$

• Mass numbers: 236 and 238 (Different)

• Atomic numbers: 92 and 92 (Same)

So, $\mathrm{U}_{92}^{236}$ and $\mathrm{U}_{92}^{238}$ are Isotopes (same element, different mass) not isobars. Hence, option (D) is incorrect.

Using the Rydberg formula :

$ \frac{1}{\lambda}=\mathrm{R}\left(\frac{1}{\mathrm{n}_1^2}-\frac{1}{\mathrm{n}_2^2}\right) $

Where, $\lambda$ is the wavelength of radiated photon when electron makes a transition from initial energy state $n_2$ to final energy state $\mathrm{n}_1$, and R is the Rydberg's constant.

For statement A

The given series is Lyman ( $\mathrm{n}_1=1$ ).

Maximum wavelength implies minimum energy. The transition corresponds to the smallest energy gap, which is between the first excited state ( $\mathrm{n}_2=2$ ) and the ground state ( $\mathrm{n}_1=1$ ).

$ \frac{1}{\lambda_{\max }}=\mathrm{R}\left(\frac{1}{1^2}-\frac{1}{2^2}\right) $

$\Rightarrow $ $ \frac{1}{\lambda_{\max }}=\mathrm{R}\left(1-\frac{1}{4}\right)=\frac{3 \mathrm{R}}{4} \Rightarrow \lambda_{\max }=\frac{4}{3 \mathrm{R}} $

Hence, the statement A is true.

For statement B

The given series is Balmer ( $\mathrm{n}_1=2$ ).

The Balmer series corresponds to electron transitions from higher shells ( $n=3,4,5 \ldots$ ) to the second shell $(\mathrm{n}=2)$. The wavelengths produced (e.g., $656 \mathrm{~nm}, 486 \mathrm{~nm})$ fall within the range of approximately 400 nm to 700 nm , which is the visible spectrum

So, Balmer series lines are indeed visible. Hence, the statement B is true.

For Statement C

The given series is Paschen ( $\mathrm{n}_1=3$ ).

Minimum wavelength implies maximum energy. The transition corresponds to the largest energy gap, which is from infinity $\left(\mathrm{n}_2=\infty\right)$ to the level $\mathrm{n}_1=3$.

$ \frac{1}{\lambda_{\min }}=R\left(\frac{1}{3^2}-\frac{1}{\infty^2}\right) $

$\Rightarrow $ $ \frac{1}{\lambda_{\min }}=\mathrm{R}\left(\frac{1}{9}\right)=\frac{\mathrm{R}}{9} \Rightarrow \lambda_{\min }=\frac{9}{\mathrm{R}} $

Hence, the statement C is also true.

For statement D

The given series is Lyman ( $\mathrm{n}_1=1$ ).

Minimum wavelength means ( $\mathrm{n}_2=\infty \rightarrow \mathrm{n}_1=1$ ).

$ \frac{1}{\lambda_{\min }}=\mathrm{R}\left(\frac{1}{1^2}-\frac{1}{\infty^2}\right) $

$\Rightarrow $ $\frac{1}{\lambda_{\min }}=\mathrm{R}(1-0)=\mathrm{R}$

$\Rightarrow $ $ \lambda_{\min }=1 \mathrm{R} $

Hence, the statement D is false.

• Statement A is Correct.

• Statement B is Correct.

• Statement C is Correct.

• Statement D is Incorrect.

Therefore, the correct option is (D).

The energy of $n^{\text {th }}$ energy state of hydrogen atom is $E_n=-\frac{13.6}{n^2}$

The wavelengths of the spectral lines of hydrogen-like atoms are governed by the Rydberg formula

$ \frac{1}{\lambda}=\mathrm{R}\left(\frac{1}{\mathrm{n}_1^2}-\frac{1}{\mathrm{n}_2^2}\right) $

Where :

• $\lambda$ is the wavelength of the emitted photon.

• R is the Rydberg constant.

• $\mathrm{n}_1$ is the principal quantum number of the lower energy level.

• $\mathrm{n}_2$ is the principal quantum number of the higher energy level ( $\mathrm{n}_2>\mathrm{n}_1$ ).

The smallest wavelength corresponds to the highest energy transition (from $\mathrm{n}_2=\infty$ to $\mathrm{n}_1$ ).

The largest wavelength corresponds to the lowest energy transition (from the immediate next energy level, $\mathrm{n}_2= \mathrm{n}_1+1$, down to $\mathrm{n}_1$ ).

For the Lyman series, the electron transitions fall to the ground state, so $\mathrm{n}_1=1$. The smallest wavelength $\left(\lambda_{\mathrm{L}(\min )}\right)$ occurs when $\mathrm{n}_2=\infty$.

It is given that $\lambda_{\mathrm{L}, \min }=91 \mathrm{~nm}$.

Using the Rydberg formula :

$ \frac{1}{\lambda_{\mathrm{L}, \min }}=\mathrm{R}\left(\frac{1}{1^2}-\frac{1}{\infty^2}\right) $

$\Rightarrow $ $ \frac{1}{91}=\mathrm{R}(1-0) \Rightarrow \mathrm{R}=\frac{1}{91} \mathrm{~nm}^{-1} $

For the Balmer series, the lower energy level is $\mathrm{n}_1=2$.

The largest wavelength ( $\lambda_{B, \max }$ corresponds to the smallest energy jump, which is from $\mathrm{n}_2=3$ to $\mathrm{n}_1=2$.

Applying the formula :

$ \frac{1}{\lambda_{\mathrm{B}, \max }}=\mathrm{R}\left(\frac{1}{2^2}-\frac{1}{3^2}\right)=\mathrm{R}\left(\frac{5}{36}\right) $

$\Rightarrow $ $\frac{1}{\lambda_{\mathrm{B}, \max }}=\frac{1}{91} \times \frac{5}{36} \mathrm{~nm}^{-1}$

$\Rightarrow $ $ \lambda_{\mathrm{B}, \max }=\frac{91 \times 36}{5} \mathrm{~nm}=655.2 \mathrm{~nm} $

For the Paschen series, the lower energy level is $\mathrm{n}_1=3$.

The largest wavelength ( $\lambda_{P, \max }$ ) corresponds to the smallest energy jump, which is from $\mathrm{n}_2=4$ to $\mathrm{n}_1=3$.

Applying the formula :

$ \frac{1}{\lambda_{\mathrm{P}, \max }}=\mathrm{R}\left(\frac{1}{3^2}-\frac{1}{4^2}\right)=\mathrm{R}\left(\frac{1}{9}-\frac{1}{16}\right) $

$\Rightarrow $ $\frac{1}{\lambda_{\mathrm{P}, \max }}=\frac{1}{91} \times \frac{7}{144} \mathrm{~nm}^{-1}$

$ \Rightarrow \lambda_{P, \max }=\frac{91 \times 144}{7} \mathrm{~nm}=1871 \mathrm{~nm} $

So, the difference between these two largest wavelengths :

$ \Delta \lambda=\lambda_{\mathrm{P}, \max }-\lambda_{\mathrm{B}, \max } $

$\Rightarrow $ $\Delta \lambda=1872 \mathrm{~nm}-655.2 \mathrm{~nm}$

$\Rightarrow $ $ \Delta \lambda=1216.8 \mathrm{~nm} \approx 1217 \mathrm{~nm} $

Therefore, the difference between the largest wavelengths of Paschen and Balmer series is nearly 1217 nm.

Hence, the correct option is (C).

Einstein's equation tells us that mass and energy are interchangeable.

$ \mathrm{E}=\mathrm{mc}^2 $

When a nuclear reaction occurs, the total mass of the products is often slightly different from the initial mass of the reactants.

• If the products are lighter, the missing mass is released as energy.

• If the products are heavier, energy must be supplied to create that extra mass.

It is given that a particle breaks into $4 \alpha$ particles.

Initial mass of the single particle. $\mathrm{M}_{\text {initial }}=15.348 \mathrm{amu}$

Final mass of $4 \alpha$ particles $=\mathrm{M}_{\text {final }}=4 \times$ mass of He nucleus

$ \mathrm{M}_{\mathrm{final}}=4 \times 4.002 \mathrm{amu}=16.008 \mathrm{amu} $

The mass defect ( $\Delta \mathrm{m}$ ) during this nuclear reaction is,

$ \Delta \mathrm{m}=\mathrm{M}_{\text {final }}-\mathrm{M}_{\text {initial }} $

$\Rightarrow $ $\Delta \mathrm{m}=16.008 \mathrm{amu}-15.348 \mathrm{amu}$

$\Rightarrow $ $ \Delta \mathrm{m}=0.66 \mathrm{amu} $

It is given that $1 \mathrm{amu}=1.66 \times 10^{-27} \mathrm{~kg}$.

$ \Delta \mathrm{m}=0.66 \times\left(1.66 \times 10^{-27}\right) \mathrm{kg} $

$\Rightarrow $ $ \Delta \mathrm{m}=1.0956 \times 10^{-27} \mathrm{~kg} $

Now using Einstein's equation,

$\mathrm{E}=\Delta \mathrm{m} \cdot \mathrm{c}^2$

$\Rightarrow $ $E=\left(1.0956 \times 10^{-27}\right) \times\left(3 \times 10^8\right)^2$

$\Rightarrow $ $E=\left(1.0956 \times 10^{-27}\right) \times\left(9 \times 10^{16}\right)$

$\Rightarrow $ $ E=9.8604 \times 10^{-11} \text { Joules } $

This energy is provided by a single photon. The energy of a photon is directly proportional to its frequency ( $v$ ), given by Planck's equation:

$ \mathrm{E}=\mathrm{h} \nu $

Where h is Planck's constant $\left(6.6 \times 10^{-34} \mathrm{~J} \cdot \mathrm{~s}\right)$.

$ v=\frac{E}{h} $

$\Rightarrow $ $v=\frac{9.8604 \times 10^{-11}}{6.6 \times 10^{-34}}$

$\Rightarrow $$ v=1.494 \times 10^{23} \mathrm{~Hz} $

$\Rightarrow $$ v_{\mathrm{kHz}}=1.494 \times 10^{23} \times 10^{-3} \mathrm{kHz} $

$\Rightarrow $ $ v_{\mathrm{kHz}}=14.94 \times 10^{19} \mathrm{kHz} $

Therefore, the minimum frequency of photon required is $14.94 \times 10^{19} \mathrm{kHz}$.

Hence, the correct option is (D).

Let an $\alpha$-particle fired directly at the center of a target nucleus.

The $\alpha$-particle has maximum kinetic energy $(\mathrm{K})$ and practically zero electrostatic potential energy at very far away from target material.

As it gets closer, the positive charge of the $\alpha$-particle and the positive charge of the target nucleus repel each other. The $\alpha$-particle slows down, converting its kinetic energy into electrostatic potential energy.

At the closest approach ( $\mathrm{r}_0$ ) it momentarily stops before reversing direction. At this exact point, all its initial kinetic energy has been converted into potential energy.

By the Law of Conservation of Energy:

Initial Kinetic Energy $\left(\mathrm{K}_0\right)=$ Final Potential Energy $(\mathrm{U})$

The electrostatic potential energy between two point charges ( $\mathrm{q}_1$ and $\mathrm{q}_2$ ) separated by a distance r is :

$ U=\frac{1}{4 \pi \epsilon_0} \frac{q_1 q_2}{r} $

An $\alpha$-particle is a Helium nucleus, so it has 2 protons. So, the charge of $\alpha$-particle $\left(\mathrm{q}_1\right)$ is $\mathrm{q}_1=+2 \mathrm{e}$

The atomic number of the target is $\mathrm{Z}=79$, so it has 79 protons, So, the charge of target nucleus $\left(\mathrm{q}_2\right)$ is $\mathrm{q}_2= +79 \mathrm{e}$

Substituting into our energy conservation equation :

$ \mathrm{K}_0=\frac{1}{4 \pi \epsilon_0} \frac{(2 \mathrm{e})(79 \mathrm{e})}{\mathrm{r}_0} $

$\Rightarrow $ $ \mathrm{r}_0=\frac{1}{4 \pi \epsilon_0} \frac{158 \mathrm{e}^2}{\mathrm{~K}_0} $

The kinetic energy of alpha particle is,

$ \mathrm{K}_0=7.9 \mathrm{MeV}=7.9 \times 10^6 \mathrm{eV} $

Since $1 \mathrm{eV}=1.6 \times 10^{-19} \mathrm{~J}$, we get:

$ K_0=\left(7.9 \times 10^6\right) \times\left(1.6 \times 10^{-19}\right) \mathrm{J} $

$\Rightarrow $ $ \mathrm{K}_0=12.64 \times 10^{-13} \mathrm{~J} $

Putting in the closest approach formula,

$ r_0=\frac{\left(9 \times 10^9\right) \times 158 \times\left(1.6 \times 10^{-19}\right)^2}{12.64 \times 10^{-13}} $

$\Rightarrow $ $r_0=\frac{(9 \times 158 \times 2.56) \times 10^{-29}}{12.64 \times 10^{-13}}$

$\Rightarrow $ $\mathrm{r}_0=288 \times 10^{-16} \mathrm{~m}$

$\Rightarrow $ $ \mathrm{r}_0=2.88 \times 10^{-14} \mathrm{~m} $

Therefore, the distance of closest approach is $\mathrm{r}_0=2.88 \times 10^{-14} \mathrm{~m}$.

The distance from the centre of the nucleus to this stopping point is the distance of closest approach. Because the $\alpha$-particle cannot physically penetrate the nucleus using just the electrostatic force in this classical model, $r_0$ gives us an upper estimate for the radius of the target nucleus.

Diameter $=2 \times$ Radius

Diameter $=2 \times \mathrm{r}_0$

Diameter $=2 \times\left(2.88 \times 10^{-14} \mathrm{~m}\right)$

Diameter $=5.76 \times 10^{-14} \mathrm{~m}$

Therefore, the diameter of nuclei of the target material is approximately $5.76 \times 10^{-14} \mathrm{~m}$.

Hence, the correct option is (B).

In the Bohr model of the atom, the energy of an electron in the $n^{\text {th }}$ orbit $\left(E_n\right)$ is related to the ground state energy ( $\mathrm{E}_0$ ) by the formula :

$ \mathrm{E}_{\mathrm{n}}=-\frac{\mathrm{E}_0}{\mathrm{n}^2} $

It is given that the energy in the current orbit is $-0.04 \mathrm{E}_0 \mathrm{eV}$.

$ -0.04 \mathrm{E}_0=-\frac{\mathrm{E}_0}{\mathrm{n}^2} $

$\Rightarrow $ $0.04=\frac{1}{\mathrm{n}^2}$

$\Rightarrow $ $\mathrm{n}^2=\frac{100}{4}=25$

$\Rightarrow $ $ \mathrm{n}=\sqrt{25}=5 $

So, the electron is in the $5^{\text {th }}$ orbit of the Bohr's atom.

Bohr's second postulate states that the angular momentum of an electron is quantized and is an integral multiple of $\mathrm{h} / 2 \pi$ :

$ \mathrm{L}=\frac{\mathrm{nh}}{2 \pi} $

Where n is the principal quantum number (orbit number).

$ \frac{2 \pi L}{h}=n=5 $

Therefore, the value of $\frac{2 \pi L}{h}$ is 5 .

Hence, the correct option is (C).

Let an $\alpha$-particle fired directly at the centre of a target nucleus, gold foil.

The $\alpha$-particle has maximum kinetic energy $(\mathrm{K})$ and practically zero electrostatic potential energy at very far away from target material.

The initial Kinetic Energy ( $\mathrm{K}_0$ ) of the alpha particle is :

$ \mathrm{K}_0=7.7 \mathrm{MeV} $

As it gets closer, the positive charge of the $\alpha$-particle and the positive charge of the target nucleus repel each other. The $\alpha$-particle slows down, converting its kinetic energy into electrostatic potential energy.

At the closest approach ( $\mathrm{r}_0$ ) it momentarily stops before reversing direction. At this exact point, all its initial kinetic energy has been converted into potential energy.

The electrostatic potential energy $(U)$ between two point charges $q_1$ and $q_2$ separated by a distance $r_0$ is :

$ \mathrm{U}=\frac{1}{4 \pi \epsilon_0} \frac{\mathrm{q}_1 \mathrm{q}_2}{\mathrm{r}_0} $

By the Law of Conservation of Energy :

Initial Kinetic Energy $\left(\mathrm{K}_0\right)=$ Final Potential Energy $(\mathrm{U})$

The electrostatic potential energy between two point charges ( $\mathrm{q}_1$ and $\mathrm{q}_2$ ) separated by a distance r is :

$ U=\frac{1}{4 \pi \epsilon_0} \frac{q_1 q_2}{r} $

An $\alpha$-particle is a Helium nucleus, so it has 2 protons. So, the charge of $\alpha$-particle $\left(\mathrm{q}_1\right)$ is $\mathrm{q}_1=+2 \mathrm{e}$ The atomic number of the target is $\mathrm{Z}=79$, so it has 79 protons, So, the charge of target nucleus $\left(\mathrm{q}_2\right)$ is $\mathrm{q}_2= +79 \mathrm{e}$

Substituting into our energy conservation equation :

$ \mathrm{K}_0=\frac{1}{4 \pi \epsilon_0} \frac{(2 \mathrm{e})(79 \mathrm{e})}{\mathrm{r}_0} $

$\Rightarrow $ $ \mathrm{r}_0=\frac{1}{4 \pi \epsilon_0} \frac{158 \mathrm{e}^2}{\mathrm{~K}_0} $

The kinetic energy of alpha particle is,

$ \mathrm{K}_0=7.7 \mathrm{MeV}=7.7 \times 10^6 \mathrm{eV} $

Since $1 \mathrm{eV}=1.6 \times 10^{-19} \mathrm{~J}$, we get :

$ K_0=\left(7.7 \times 10^6\right) \times\left(1.6 \times 10^{-19}\right) \mathrm{J} $

$\Rightarrow $ $ \mathrm{K}_0=12.32 \times 10^{-13} \mathrm{~J} $

Putting in the closest approach formula,

$ r_0=\frac{\left(9 \times 10^9\right) \times 158 \times\left(1.6 \times 10^{-19}\right)^2}{12.32 \times 10^{-13}} $

$\Rightarrow $ $r_0=\frac{(9 \times 158 \times 2.56) \times 10^{-29}}{12.32 \times 10^{-13}}$

$\Rightarrow $ $\mathrm{r}_0=295 \times 10^{-16} \mathrm{~m}$

$\Rightarrow $ $ \mathrm{r}_0=2.95 \times 10^{-14} \mathrm{~m} $

Therefore, the distance of closest approach is $\mathrm{r}_0=2.95 \times 10^{-14} \mathrm{~m}$.

Hence, the correct option is (C).

The radius $ R $ of a nucleus is given by the empirical relation:

$ R = r_0 A^{\frac{1}{3}}, $

where $ r_0 $ is a constant and $ A $ is the mass number.

The volume $ V $ of the nucleus is approximately that of a sphere:

$ V = \frac{4}{3}\pi R^3. $

Substituting the expression for $ R $ into the volume formula:

$ V = \frac{4}{3}\pi \left(r_0 A^{\frac{1}{3}}\right)^3 = \frac{4}{3}\pi r_0^3 A. $

The mass density $ \rho $ is defined as the mass (which is proportional to $ A $) divided by the volume:

$ \rho = \frac{A}{V} = \frac{A}{\frac{4}{3}\pi r_0^3 A} = \frac{1}{\frac{4}{3}\pi r_0^3}. $

Notice that $ A $ cancels out in the numerator and denominator, so the density is:

$ \rho = \text{constant}, $

meaning it is independent of the mass number $ A $.

Thus, the correct option is:

Option B: Independent of A.

To evaluate the assertion and reason provided, we need to consider the formula for the density of a nucleus. The density $ \rho $ can be expressed as follows:

$ \rho = \frac{M}{V} = \frac{m_n \times A}{\frac{4}{3} \pi R^3} $

Here, $ m_n $ represents the nucleon mass, $ A $ is the mass number, and $ R $ is the radius of the nucleus. According to the formula for the radius:

$ R = R_0 A^{1/3} $

This indicates that the radius $ R $ is proportional to $ A^{1/3} $. By substituting the expression for $ R $ in terms of $ A $ back into the formula for density, we get:

$ \rho = \frac{m_n \times A}{\frac{4}{3} \pi (A^{1/3} R_0)^3} $

Simplifying the equation shows that the $ A $ terms cancel out:

$ \rho = \frac{m_n}{\frac{4}{3} \pi R_0^3} $

This reveals that the nuclear density $ \rho $ is approximately constant and independent of the mass number $ A $. Therefore, the densities of copper $(^{64}_{29} \text{Cu})$ and carbon $(^{12}_{6} \text{C})$ nuclei are effectively the same because the density formula results in a similar value regardless of the specific mass number.

To determine the atomic number (Z) of a hydrogen-like ion, given that the energy difference between its second excitation state and the ground state is 108.8 eV, we use the following formula for the energy difference between two energy levels in a hydrogen-like atom:

$ \Delta E = 13.6 \, \text{eV} \times Z^2 \left[\frac{1}{n_1^2} - \frac{1}{n_2^2}\right] $

Here, for the second excitation, $ n_2 = 3 $ (since excitation means moving to the second higher energy level beyond the ground state, which means $ n_1 = 1 $ for the ground state to $ n_2 = 3 $). Plug these into the energy difference formula:

$ \Delta E = 13.6 \, Z^2 \left[\frac{1}{1^2} - \frac{1}{3^2}\right] $

Simplifying the term in brackets:

$ \Delta E = 13.6 \, Z^2 \left[1 - \frac{1}{9}\right] = 13.6 \, Z^2 \times \frac{8}{9} $

Given that $\Delta E = 108.8 \, \text{eV}$, set the equations equal:

$ 13.6 \, \frac{8Z^2}{9} = 108.8 $

Solving for $ Z $, first simplify:

$ 13.6 \times \frac{8}{9} \times Z^2 = 108.8 $

$ \frac{108.8 \times 9}{13.6 \times 8} = Z^2 $

$ Z^2 = 9 $

Thus, the atomic number $ Z $ is:

$ Z = 3 $

The atomic number of the ion is 3.

Lyman

$\begin{aligned} & \frac{1}{\lambda_1}=\mathrm{R}\left[\frac{1}{1}-\frac{1}{4}\right]=\frac{3 \mathrm{R}}{4} \\ & \lambda_1=\frac{4}{3 \mathrm{R}}\quad\text{..... (1)} \end{aligned}$

and Balmer

$\begin{aligned} &\begin{aligned} & \frac{1}{\lambda_2}=\mathrm{R}\left[\frac{1}{4}-\frac{1}{9}\right]=\frac{5 \mathrm{R}}{36} \\ & \lambda_2=\frac{36}{5 \mathrm{R}} \end{aligned}\\ &\text { Then, } \frac{\lambda_1}{\lambda_2}=\frac{5}{27} \end{aligned}$

$\begin{aligned} &\begin{aligned} & \mathrm{E}=\mathrm{hf} \\ & \mathrm{ML}^2 \mathrm{~T}^{-2}=[\mathrm{h}] \times\left[\mathrm{T}^{-1}\right] \\ & {[\mathrm{h}]=\left[\mathrm{ML}^2 \mathrm{~T}^{-1}\right]} \\ & \mathrm{L}=[\mathrm{MVR}]=\left[\mathrm{ML}^2 \mathrm{~T}^{-1}\right] \\ & \mathrm{L}=\frac{\mathrm{nh}}{2 \pi} \end{aligned}\\ &\mathrm{L} \text { is integral multiple of } \frac{\mathrm{h}}{2 \pi} \end{aligned}$

In the Bohr model for a hydrogen-like atom, the radius of the $ n^{\text{th}} $ orbit is given by

$ r_n = \frac{n^2 a_0}{Z} $

where:

$ n $ is the principal quantum number,

$ a_0 $ is the Bohr radius, and

$ Z $ is the atomic number (the effective nuclear charge).

For the $ 5^{\text{th}} $ orbit ($ n = 5 $):

For $ \mathrm{Li}^{2+} $ (where $ Z = 3 $):

$ r_5(\mathrm{Li}^{2+}) = \frac{5^2 a_0}{3} = \frac{25a_0}{3} $

For $ \mathrm{He}^{+} $ (where $ Z = 2 $):

$ r_5(\mathrm{He}^{+}) = \frac{5^2 a_0}{2} = \frac{25a_0}{2} $

Now, the ratio of the radius of the 5th orbit of $ \mathrm{Li}^{2+} $ to that of $ \mathrm{He}^{+} $ is:

$ \frac{r_5(\mathrm{Li}^{2+})}{r_5(\mathrm{He}^{+})} = \frac{\frac{25a_0}{3}}{\frac{25a_0}{2}} = \frac{25a_0}{3} \times \frac{2}{25a_0} = \frac{2}{3} $

Thus, the ratio is $ \frac{2}{3} $.

The correct answer is Option B.

Option B

Both statements are true, and the absence of electron–electron repulsion in Bohr’s formulation is exactly why it works only for one‐electron systems (hydrogen and hydrogen‐like ions).

• Assertion A is true: Bohr’s model applies only to atoms with a single electron.

• Reason R is true: Bohr treated only the Coulomb attraction between nucleus and electron, omitting any repulsive force between multiple electrons.

• Since ignoring e–e repulsion prevents extension to multi–electron atoms, R correctly explains A.

Here’s the correct matching:

A.

$_0^1\text{n}+_{92}^{235}\text{U}\to_{54}^{140}\text{Xe}+_{38}^{94}\text{Sr}+2\,_0^1\text{n}$

–– This is nuclear fission ⇒ III.

B.

$2\text{H}_2+\text{O}_2\to2\text{H}_2\text{O}$

–– This is a chemical reaction ⇒ I.

C.

$_1^2\text{H}+_1^2\text{H}\to_2^3\text{He}+_0^1\text{n}$

–– This D–D fusion releases about 3.3 MeV ⇒ fusion with +ve Q ⇒ II.

D.

$_1^1\text{H}+_1^3\text{H}\to_1^2\text{H}+_1^2\text{H}$

–– The mass of products exceeds reactants ⇒ requires energy ⇒ fusion with –ve Q ⇒ IV.

So the answer is

Option B: A–III, B–I, C–II, D–IV.

To calculate the energy released when two deuterons (${}_1 \mathrm{H}^2$) fuse to form a helium nucleus (${}_2 \mathrm{He}^4$), consider the following:

The reaction is represented as:

$ {}_1{H^2} + {}_1{H^2} \to {}_2{He^4} $

Given the binding energy per nucleon:

For ${}_1 \mathrm{H}^2$: 1.1 MeV

For ${}_2 \mathrm{He}^4$: 7.0 MeV

Calculating Binding Energy

Binding Energy for Reactants:

Each deuteron (${}_1 \mathrm{H}^2$) has a binding energy of $1.1 \, \text{MeV}$ per nucleon.

Since there are two nucleons in a deuteron, the total binding energy for one deuteron is $1.1 \times 2 = 2.2 \, \text{MeV}$.

Therefore, for two deuterons: $2.2 \times 2 = 4.4 \, \text{MeV}$.

Binding Energy for Product:

For ${}_2 \mathrm{He}^4$: Each of the four nucleons has a binding energy of $7.0 \, \text{MeV}$.

Total binding energy for the helium nucleus: $7.0 \times 4 = 28.0 \, \text{MeV}$.

Energy Released (Q)

The energy released, $ \mathrm{Q} $, is the difference between the binding energy of the products and the reactants:

$ \mathrm{Q} = \mathrm{BE}_{\text{product}} - \mathrm{BE}_{\text{reactants}} = 28.0 \, \text{MeV} - 4.4 \, \text{MeV} = 23.6 \, \text{MeV} $

Thus, the energy released when two deuterons fuse to form a helium nucleus is 23.6 MeV.

The radius of a hydrogen-like ion according to Bohr's atomic model is given by the formula:

$ r = r_0 \frac{n^2}{Z} $

Where:

$ r_0 $ is the first Bohr radius ($ a_0 $)

$ n $ represents the principal quantum number

$ Z $ is the atomic number of the ion

For the ion $ \text{Li}^{++} $, the atomic number $ Z = 3 $ and we are considering the ground state, so $ n = 1 $.

Plugging these values into the formula, we get:

$ r = r_0 \frac{1^2}{3} = \frac{r_0}{3} $

This shows that the radius of $\text{Li}^{++}$ in its ground state is $\frac{1}{3} a_0$, meaning $ X = 3 $.

In Bohr's atomic model, the energy of an electron in an atom is given by the formula:

$ E \propto \frac{Z^2}{n^2} $

where $ Z $ is the atomic number and $ n $ is the principal quantum number of the electron's orbit. Let’s consider the implications:

For hydrogen ($ \mathrm{H} $), $ Z = 1 $.

For helium ion ($ \mathrm{He}^{+} $), $ Z = 2 $.

For lithium ion ($ \mathrm{Li}^{++} $), $ Z = 3 $.

Now, identifying the relevant states:

Ground State: This corresponds to $ n = 1 $.

First Excited State: This corresponds to $ n = 2 $.

Second Excited State: This corresponds to $ n = 3 $.

Let's evaluate the energy comparisons:

Hydrogen Atom in Ground State: $ E \propto \frac{1^2}{1^2} = 1 $.

Helium Ion ($ \mathrm{He}^{+} $) in the First Excited State:

$ E \propto \frac{2^2}{2^2} = 1 $

This matches the energy of hydrogen in its ground state.

Lithium Ion ($ \mathrm{Li}^{++} $) in the Second Excited State:

$ E \propto \frac{3^2}{3^2} = 1 $

This also matches the energy of hydrogen in its ground state.

From this analysis, we can conclude:

The energy of a hydrogen atom in its ground state is equal to the energy of a $ \mathrm{He}^{+} $ ion in its first excited state.

The energy of a hydrogen atom in its ground state also equals the energy of a $ \mathrm{Li}^{++} $ ion in its second excited state.

Hence, statements (A) and (B) are correct.

When an e transition from n$^{th}$ to ground state then $\sum {(n - 1)} $ spectral lines are produced.

Given, $n=4$

So, $\sum {(4 - 1) = \sum {3 = 3 + 2 + 1 = 6} } $

Hence, 6 spectral lines are emitted by atomic Hydrogen that is in the $4^{th}$ energy level

Frequency of revolution $\propto \frac{1}{\mathrm{n}^3}$

We know, theoretical equation for $\beta^-$ decay,

Hence, option 2 is correct.

$\begin{aligned} & {[E]=M^1 L^2 T^{-2}} \\ & {[P c]=M^1 L^1 T^{-1} \cdot L^1 T^{-1}=M^1 L^2 T^{-2}} \\ & {\left[\mathrm{mc}^2\right]=\mathrm{M}^1 L^2 T^{-2}} \\ & E^2=\mathrm{M}^1 \mathrm{~L}^2 \mathrm{~T}^{-2} \\ & E^2=\mathrm{P}^2 \mathrm{c}^2+\mathrm{m}^2 \mathrm{c}^4 \end{aligned}$

$ \frac{1}{\lambda_{AC}} = \frac{E_A - E_C}{hc}, \quad \frac{1}{\lambda_{BC}} = \frac{E_B - E_C}{hc} $

Since the electron transitions from state A to state C in two steps (A to B and then B to C), the energy difference for the transition from A to B is given by

$ E_A - E_B = (E_A - E_C) - (E_B - E_C) $

Expressing these energy differences in terms of wavelengths,

$ \frac{hc}{\lambda_{AB}} = \frac{hc}{\lambda_{AC}} - \frac{hc}{\lambda_{BC}} $

Cancelling out $hc$, we obtain

$ \frac{1}{\lambda_{AB}} = \frac{1}{\lambda_{AC}} - \frac{1}{\lambda_{BC}} $

Substitute the given values:

$ \frac{1}{\lambda_{AB}} = \frac{1}{2000} - \frac{1}{6000} $

Finding a common denominator:

$ \frac{1}{\lambda_{AB}} = \frac{3 - 1}{6000} = \frac{2}{6000} = \frac{1}{3000} $

Thus, the wavelength for the A to B transition is

$ \lambda_{AB} = 3000 \, \mathring{A}. $

This matches Option D.

Let's analyze both statements:

Assertion (A): “The binding energy per nucleon is found to be practically independent of the atomic number A, for nuclei with mass numbers between 30 and 170.”

In fact, for nuclei in this mass range, the binding energy per nucleon is nearly constant (around 7–8 MeV per nucleon). This is because the attractive nuclear force saturates, leading to a relatively uniform binding energy per nucleon in medium to heavy nuclei.

Therefore, Assertion (A) is true.

Reason (R): “Nuclear force is long range.”

The nuclear force (or strong force) that holds the nucleons (protons and neutrons) together is actually very short-ranged. Its effective range is approximately 1 to 3 femtometers (fm), after which the force rapidly decreases.

Hence, Reason (R) is false.

Since the assertion is correct and the reason is incorrect, the correct answer is:

Option A: (A) is true but (R) is false.

Let's work through the problem step by step.

The decay of a radioactive nucleus is given by the formula:

$N(t) = N_0 \, e^{-\lambda t},$

where:

$N(t)$ is the number of nuclei remaining at time $t$,

$N_0$ is the initial number of nuclei,

$\lambda$ is the decay constant.

For nucleus $n_1$ with decay constant $\lambda_1$, its half-life $t_{1/2}$ is:

$t_{1/2} = \frac{\ln 2}{\lambda_1}.$

After one half-life of $n_1$, the number of $n_1$ nuclei remaining is:

$N_1 = N_0 \, e^{-\lambda_1 \, t_{1/2}} = N_0 \, e^{-\lambda_1 \, \frac{\ln2}{\lambda_1}} = N_0 \, e^{-\ln2} = \frac{N_0}{2}.$

Nucleus $n_2$ has a decay constant that is 3 times that of $n_1$:

$\lambda_2 = 3\lambda_1.$

For nucleus $n_2$, the number of nuclei remaining after one half-life of $n_1$ (which is $t = \frac{\ln2}{\lambda_1}$) is:

$ N_2 = N_0 \, e^{-\lambda_2 \, t} = N_0\, e^{-3\lambda_1 \, \frac{\ln2}{\lambda_1}} = N_0 \, e^{-3\ln2} = N_0 \cdot \frac{1}{2^3} = \frac{N_0}{8}. $

Now, we find the ratio of $n_2$ nuclei to $n_1$ nuclei after this time:

$ \frac{N_2}{N_1} = \frac{\frac{N_0}{8}}{\frac{N_0}{2}} = \frac{1/8}{1/2} = \frac{1}{4}. $

Therefore, the correct ratio is $\frac{1}{4}$, which corresponds to Option A.

In nuclear disintegration, the conservation of momentum plays a crucial role in determining the motion of the resulting fragments. Since the original nucleus is at rest, its total initial momentum is zero. After the disintegration, the total momentum of the system must still be zero to conserve momentum.

Given the mass ratio of the resulting two smaller nuclei is $2:1$, let's denote the masses of the two nuclei as $2m$ and $m$, respectively.

Applying Conservation of Momentum

For the nucleus with mass $2m$ and velocity $v_1$ and for the nucleus with mass $m$ and velocity $v_2$, the conservation of momentum equation is:

$ 2m \cdot v_1 + m \cdot v_2 = 0 $

Given that momentum is a vector quantity, and the total initial momentum was zero, the nuclei must move in opposite directions for their momenta to cancel each other out. Therefore, we rearrange the equation:

$ 2m \cdot v_1 = -m \cdot v_2 $

$ 2 \cdot v_1 = -v_2 $

$ v_2 = -2 \cdot v_1 $

The negative sign indicates that $v_1$ and $v_2$ are in opposite directions. Taking magnitudes and considering the ratio:

$ \left| v_2 \right| = 2 \cdot \left| v_1 \right| $

This equation shows that the velocity of the lighter fragment (mass $m$) is twice the velocity of the heavier fragment (mass $2m$). Since they must move in opposite directions for momentum conservation, this confirms:

Correct Answer:

Option A - in opposite directions with speed in the ratio of $1:2$ respectively.

To determine the ratio $\frac{E_H}{E_U}$, let's first consider the energy released in the fusion of hydrogen and the energy released in the fission of $^{235}U$.

For the fusion reaction:

The given reaction is:

$4_1^1H + 2 \mathrm{e}^{-} \rightarrow {}_2^4 \mathrm{He} + 2 \mathrm{\nu} + 6 \gamma + 26.7 \, \mathrm{MeV}$

This reaction shows that 4 hydrogen atoms and 2 electrons produce 1 helium atom, 2 neutrinos, and 6 gamma photons while releasing 26.7 MeV of energy.

The mass of 1 mole of $H_1^1$ is approximately 1 gram, so 2 kg of hydrogen is equal to:

$2 \times 10^3 \, \mathrm{g}$

The number of moles of hydrogen in 2 kg is:

$\mathrm{N}_\text{moles} = \frac{2 \times 10^3}{1} = 2 \times 10^3 \, \mathrm{moles}$

The number of hydrogen atoms in 2 kg is:

$\mathrm{N}_\text{atoms} = N_A \times 2 \times 10^3 = 6.023 \times 10^{23} \times 2 \times 10^3 = 1.2046 \times 10^{27} \, \mathrm{atoms}$

Since 4 hydrogen atoms release 26.7 MeV, the total energy released ($E_H$) is:

$E_H = \frac{1.2046 \times 10^{27}}{4} \times 26.7 \, \mathrm{MeV}$

$E_H = 3.0115 \times 10^{26} \times 26.7 \, \mathrm{MeV}$

$E_H \approx 8.04 \times 10^{27} \, \mathrm{MeV}$

For the fission reaction:

The energy released per fission of one $^{235}U$ nucleus is 200 MeV.

The mass of 1 mole of $^{235}U$ is approximately 235 grams, so 2 kg of ${}^{235}U$ is equal to:

$2 \times 10^3 \, \mathrm{g}$

The number of moles of $^{235}U$ in 2 kg is:

$\mathrm{N}_\text{moles} = \frac{2 \times 10^3}{235} \approx 8.51 \, \mathrm{moles}$

The number of $^{235}U$ nuclei is:

$\mathrm{N}_\text{nuclei} = N_A \times 8.51 \approx 6.023 \times 10^{23} \times 8.51 \approx 5.12 \times 10^{24} \, \mathrm{nuclei}$

The total energy released ($E_U$) is:

$E_U = 200 \, \mathrm{MeV} \times 5.12 \times 10^{24} \approx 1.024 \times 10^{27} \, \mathrm{MeV}$

Finally, the ratio $\frac{E_H}{E_U}$ is:

$\frac{E_H}{E_U} \approx \frac{8.04 \times 10^{27}}{1.024 \times 10^{27}} \approx 7.85$

Therefore, the ratio is closest to option A: 7.62.

To determine how many spectral lines will be emitted due to transitions of electrons in a hydrogen atom when it is given an energy of $10.2 \, \text{eV}$, we first need to ascertain which energy level the electron will reach with this energy and then count the possible transitions (spectral lines) as it returns to the ground state.

Energy Levels of Hydrogen Atom :

The energy levels $ E_n $ of a hydrogen atom can be calculated using the formula:

$ E_n = -\frac{13.6 \, \text{eV}}{n^2} $

where $ n $ is the principal quantum number.

Ground State Energy :

The ground state (n=1) energy is $ E_1 = -13.6 \, \text{eV} $.

Determine the Excited State :

If the ground state electron is given $10.2 \, \text{eV}$, its total energy becomes:

$ E_{\text{total}} = E_1 + 10.2 \, \text{eV} = -13.6 \, \text{eV} + 10.2 \, \text{eV} = -3.4 \, \text{eV} $

Now, we find the principal quantum number $ n $ for which the energy is closest to $-3.4 \, \text{eV}$:

• For $ n = 2 $: $ E_2 = -\frac{13.6}{2^2} = -3.4 \, \text{eV} $


• For $ n = 3 $: $ E_3 = -\frac{13.6}{3^2} = -1.51 \, \text{eV} $

Since $-3.4 \, \text{eV}$ matches exactly with $ E_2 $, the electron reaches the second energy level ($ n = 2 $).

Counting Spectral Lines :

When the electron falls back to the ground state from $ n = 2 $, it can do so in a single transition:

• $ n = 2 $ to $ n = 1 $

Thus, only one spectral line will be emitted during this transition.

Conclusion :

The number of spectral lines emitted when a hydrogen atom in the ground state is given $10.2 \, \text{eV}$ and the electron transitions back to the ground state from $ n = 2 $ is just one.

Therefore, the correct answer is:

To determine the energy equivalent of a mass, we use Einstein's mass-energy equivalence principle given by the equation:

$E = mc^2$

where:

- $E$ is the energy

- $m$ is the mass

- $c$ is the speed of light in a vacuum, which is approximately $3 \times 10^8 \mathrm{~m/s}$

Given the mass $m = 1 \mathrm{~g} = 1 \times 10^{-3} \mathrm{~kg}$, we can substitute these values into the equation:

$E = (1 \times 10^{-3} \mathrm{~kg}) \times (3 \times 10^8 \mathrm{~m/s})^2$

Calculating this, we get:

$E = 1 \times 10^{-3} \times 9 \times 10^{16}$

$E = 9 \times 10^{13} \mathrm{~J}$

Next, to convert this energy into electron volts ($\mathrm{eV}$), we use the conversion factor: $1 \mathrm{~J} = 6.242 \times 10^{12} \mathrm{~MeV}$.

Therefore:

$E = 9 \times 10^{13} \mathrm{~J} \times 6.242 \times 10^{12} \mathrm{~MeV/J}$

Calculating this, we get:

$E = 5.6178 \times 10^{26} \mathrm{~MeV}$

Therefore, the energy equivalent of $1 \mathrm{~g}$ of a substance is:

Option A: $5.6 \times 10^{26} \mathrm{~MeV}$

To determine the nuclear binding energy of the isotope $_5^{12}B$, we need to consider the mass defect concept. The mass defect is the difference between the sum of the individual masses of nucleons (protons and neutrons) and the actual mass of the nucleus.

Let's calculate the mass defect first. The isotope $_5^{12}B$ has 5 protons and 7 neutrons (since the total number of nucleons is 12). Therefore, the mass defect can be written as:

$\Delta M = (5 M_p + 7 M_n) - M_0$

Once we have the mass defect, the binding energy can be found using Einstein's mass-energy equivalence principle, which is given by:

$E = \Delta M \cdot c^2$

Substituting the mass defect into this equation, we get:

$E = ((5 M_p + 7 M_n) - M_0) \cdot c^2$

Therefore, the correct answer is Option A:

$ (5 M_p + 7 M_n - M_0) \cdot c^2 $

In the given hypothetical fission reaction:

$^{236}_{92} X \rightarrow \, ^{141}_{56} Y + \, ^{92}_{36} Z + 3 R$

We need to determine the identity of particles denoted by $ R $. Let's use the conservation of charge and mass number (nucleon number) to identify $ R $.

First, for the conservation of nucleon number (mass number), we have:

$236 = 141 + 92 + 3 \times A_R$

Where $ A_R $ is the mass number of $ R $. This simplifies to:

$236 = 233 + 3A_R$

$3A_R = 236 - 233$

$3A_R = 3$

$A_R = 1$

Next, we use the conservation of charge (atomic number), we have:

$92 = 56 + 36 + 3Z_R$

Where $ Z_R $ is the atomic number of $ R $. This simplifies to:

$92 = 92 + 3Z_R$

$3Z_R = 92 - 92$

$3Z_R = 0$

$Z_R = 0$

Since the particle $ R $ has a mass number of 1 and atomic number of 0, it must be a neutron.

So, the identity of the emitted particles $ R $ is:

Option B: Neutron

To determine the difference between the total mass of all the nucleons and the nuclear mass of the given nucleus, we need to use the concept of mass-energy equivalence provided by Einstein's famous equation:

$ E= \Delta m \cdot c^2 $

where:

• E is the binding energy.
• $ \Delta m $ is the mass defect (difference in mass).
• $ c $ is the speed of light in vacuum, which is approximately $3 \times 10^8 \, \text{m/s}$.

Given:

• Binding energy, $ E = 18 \times 10^8 \, \text{J} $.

We need to find $ \Delta m $, so rearranging the equation:

$ \Delta m = \frac{E}{c^2} $

Substitute the given values:

$ \Delta m = \frac{18 \times 10^8}{(3 \times 10^8)^2} $

Calculate the value:

$ \Delta m = \frac{18 \times 10^8}{9 \times 10^{16}} $

$ \Delta m = \frac{18}{9} \times 10^{-8} $

$ \Delta m = 2 \times 10^{-8} \, \text{kg} $

To convert the mass from kilograms to micrograms ($ \mu g $), we use the conversion factor $ 1 \, \text{kg} = 10^9 \, \mu g $:

$ \Delta m = 2 \times 10^{-8} \, \text{kg} \times 10^9 \, \frac{\mu g}{\text{kg}} $

$ \Delta m = 2 \times 10^{1} \, \mu g $

$ \Delta m = 20 \, \mu g $

Therefore, the difference between the total mass of all the nucleons and the nuclear mass of the given nucleus is 20 $ \mu g $.

Option A (20 $ \mu g $) is the correct answer.

To determine the longest wavelength associated with the Paschen series, we need to understand what the Paschen series is and how its wavelength can be calculated. The Paschen series pertains to the spectral line emissions of the hydrogen atom as an electron transitions from higher energy levels (n > 3) down to n = 3. The formula used to calculate the wavelength ($\lambda$) of the emitted photon during such a transition in the hydrogen atom is given by the Rydberg formula:

$\frac{1}{\lambda} = R_{H} \left( \frac{1}{3^2} - \frac{1}{n^2} \right)$

where,

• $R_{H}$ is the Rydberg constant for hydrogen, $R_{H} = 1.097 \times 10^7 \, \text{m}^{-1}$.
• $n$ is the principal quantum number of the higher energy level (For the longest wavelength, we use the smallest possible value of $n$ that is larger than 3, which is $n=4$, since for longer wavelength transitions, the difference in energy levels should be the smallest).

To find the longest wavelength, we plug in $n=4$ into the Rydberg equation, as this corresponds to the smallest energy transition within the Paschen series ($n=4$ to $n=3$):

$\frac{1}{\lambda} = 1.097 \times 10^7 \left( \frac{1}{3^2} - \frac{1}{4^2} \right)$

$\frac{1}{\lambda} = 1.097 \times 10^7 \left( \frac{1}{9} - \frac{1}{16} \right)$

$\frac{1}{\lambda} = 1.097 \times 10^7 \left( \frac{16 - 9}{144} \right)$

$\frac{1}{\lambda} = 1.097 \times 10^7 \times \frac{7}{144}$

$\frac{1}{\lambda} = 1.097 \times 10^7 \times \frac{7}{144}$

$\frac{1}{\lambda} = 1.097 \times 10^7 \times \frac{7}{144}$

$\frac{1}{\lambda} = 1.097 \times 10^7 \times \frac{7}{144} = 76403.47\, \text{m}^{-1}$

Finally, we calculate $\lambda$ by taking the reciprocal of this value:

$\lambda = \frac{1}{76403.47} \approx 1.308 \times 10^{-5} \, \text{m} = 1.308 \times 10^{-5} \times 10^{2} \, \text{cm} = 1.308 \times 10^{-3} \, \text{cm}$

It seems there was a mistake in my calculation. Revising the calculation properly:

$\lambda = \frac{1}{1.097 \times 10^7 \times \frac{7}{144}}$

Correctly evaluating this, we should directly compute:

$\lambda \approx \frac{144}{7 \times 1.097 \times 10^7} = \frac{144}{7.679 \times 10^7} \approx 1.876 \times 10^{-6} \, \text{m}$

Therefore, the correct option that matches our calculation for the longest wavelength in the Paschen series is:

Option B: $1.876 \times 10^{-6} \, \text{m}$.

The wavelength of light emitted when an electron transitions between energy levels in a hydrogen atom is given by the Rydberg formula:

$\frac{1}{\lambda} = R \left( \frac{1}{n_1^2} - \frac{1}{n_2^2} \right)$

where:

• $\lambda$ is the wavelength of the emitted light,
• $R$ is the Rydberg constant,
• $n_1$ is the lower energy level, and
• $n_2$ is the higher energy level.

The Balmer series corresponds to electron transitions where the lower energy level, $n_1$, is 2, and the Lyman series corresponds to transitions where $n_1$ is 1. The shortest wavelength in each series occurs for transitions from the highest possible energy level ($n_2 = \infty$) to the specified lower level ($n_1$).

For the Balmer series (shortest wavelength):

$n_1 = 2, n_2 = \infty$,

Putting these values into the Rydberg formula gives:

$\frac{1}{\lambda_{B}} = R \left( \frac{1}{2^2} - \frac{1}{\infty^2} \right)$

$\frac{1}{\lambda_{B}} = R \left( \frac{1}{4} \right)$

$\lambda_{B} = \frac{1}{R \cdot \frac{1}{4}} = \frac{4}{R}$

For the Lyman series (shortest wavelength):

$n_1 = 1, n_2 = \infty$,

Putting these values into the Rydberg formula gives:

$\frac{1}{\lambda_{L}} = R \left( \frac{1}{1^2} - \frac{1}{\infty^2} \right)$

$\frac{1}{\lambda_{L}} = R \cdot 1$

$\lambda_{L} = \frac{1}{R}$

The ratio of the shortest wavelength of Balmer series ($\lambda_{B}$) to the shortest wavelength of Lyman series ($\lambda_{L}$) is:

$\frac{\lambda_{B}}{\lambda_{L}} = \frac{\frac{4}{R}}{\frac{1}{R}} = \frac{4}{1} = 4:1$

Therefore, the correct answer is Option D: $4: 1$.

$\begin{aligned} & L=m v r \propto n \\ & \therefore \quad m v r \propto \sqrt{r} \end{aligned}$

In the context of an electron orbiting around a nucleus with a positive charge of $\mathrm{Ze}$, we are dealing with classical physics approximations and the electrostatic force between the electron and the nucleus. In such a setup, the electron's potential energy (U) is due to electrostatic interaction, and it is given by Coulomb's law:

$U = -\frac{kZe^2}{r}$

Where:

• $U$ is the potential energy of the electron,
• $k$ is Coulomb's constant,
• $Z$ is the atomic number (number of protons in the nucleus),
• $e$ is the charge of an electron, and
• $r$ is the radius of the orbit of the electron around the nucleus.

The negative sign indicates that the potential energy is negative because the electron and nucleus attract each other.

The total energy (E) of the electron in orbit is the sum of its kinetic energy (K) and its potential energy (U). Since the electron is in a stable orbit, its kinetic energy can be shown to be exactly half the magnitude of its potential energy but positive:

$K = -\frac{1}{2}U$

Therefore,

$E = K + U = -\frac{1}{2}U + U = \frac{1}{2}U$

To find a relation between total energy (E) and potential energy (U), we rearrange the equation as follows:

$2E = U$

This is to say, the total energy (E) is half the magnitude of potential energy (U) but negative, and the correct relationship between them, when looking for a positive proportionality, yields to $2E = U$. Hence, the correct option is:

Option C: $2 \mathrm{E} = \mathrm{U}$

According to Bohr's theory, one of the postulates specifies that the angular momentum of an electron in orbit around a nucleus is quantized. This quantization can be expressed by the formula:

$ L = n\frac{h}{2\pi} $

Where:

• $L$ is the angular momentum of the electron,
• $n$ is the principal quantum number (or the orbit number in simpler terms), which can take positive integer values (1, 2, 3, ...),
• $h$ is Planck's constant ($6.62607015 \times 10^{-34} m^2 kg / s$), and
• $\frac{h}{2\pi}$ is often denoted as $\hbar$ (h-bar), known as the reduced Planck's constant.

For an electron in the 4th orbit ($n = 4$) of a hydrogen atom, we substitute $n = 4$ into the equation:

$ L = 4\frac{h}{2\pi} $

Therefore, the moment of momentum (or angular momentum) of an electron in the $4^{\text{th}}$ orbit of a hydrogen atom is:

$ L = 4\frac{h}{2\pi} = 2\frac{2h}{2\pi} = 2\frac{h}{\pi} $

Hence, the correct option is:

Option A: $2 \frac{h}{\pi}$

In order to identify the correct nuclear fragments resulting from the fission of uranium-235 by a neutron, $\left({ }_0^1 \mathrm{n}\right) + \left({ }_{92}^{235} \mathrm{U}\right)$, we need to apply the conservation of mass number and atomic number. These conservation laws tell us that the sum of mass numbers (top numbers, representing the total count of protons and neutrons) and the sum of atomic numbers (bottom numbers, representing the total count of protons) before and after the fission must be equal. Let's apply these rules to each option:

For the uranium-235 fission reaction, before the reaction, the total mass number is $235 + 1 = 236$ and the total atomic number is $92$ (since a neutron doesn't contribute to the atomic number).

Option A: ${ }_{56}^{140} \mathrm{Xe}+{ }_{38}^{94} \mathrm{Sr}+3{ }_0^1 \mathrm{n}$

• Total mass number after = $140 + 94 + 3(1) = 234 + 3 = 237$
• Total atomic number after = $56 + 38 = 94$

Option B: ${ }_{51}^{153} \mathrm{Sb}+{ }_{41}^{99} \mathrm{Nb}+3{ }_0^1 \mathrm{n}$

• Total mass number after = $153 + 99 + 3(1) = 252 + 3 = 255$
• Total atomic number after = $51 + 41 = 92$

Option C: ${ }_{56}^{144} \mathrm{Ba}+{ }_{36}^{89} \mathrm{Kr}+4{ }_0^1 \mathrm{n}$

• Total mass number after = $144 + 89 + 4(1) = 233 + 4 = 237$
• Total atomic number after = $56 + 36 = 92$

Option D: ${ }_{56}^{144} \mathrm{Ba}+{ }_{36}^{89} \mathrm{Kr}+3{ }_0^1 \mathrm{n}$

• Total mass number after = $144 + 89 + 3(1) = 233 + 3 = 236$
• Total atomic number after = $56 + 36 = 92$

By examining the conservation of mass numbers and atomic numbers, Option D is identified as the correct option because both the total mass number and atomic number after the reaction match exactly with the total mass number and atomic number before the reaction. Therefore, the nuclear fission fragments and the neutron count for the reaction are accurately represented by ${ }_{56}^{144} \mathrm{Ba}+{ }_{36}^{89} \mathrm{Kr}+3{ }_0^1 \mathrm{n}$.

Let's analyze each of the given statements to determine which ones are correct:

(A) The angular momentum of an electron in $n^{\text{th}}$ orbit is an integral multiple of $\hbar$.

This statement reflects Bohr's quantization rule for angular momentum in the Bohr model of the hydrogen atom. According to this rule, the angular momentum of an electron in a stationary orbit is quantized and given by:

where $n$ is a principal quantum number (which can be any positive integer), and $\hbar$ is the reduced Planck's constant. Therefore, this statement (A) is correct.

(B) Nuclear forces do not obey inverse square law.

Nuclear forces, specifically strong nuclear forces, do act over short ranges within the nucleus but do not obey the inverse square law, which is characteristic of the electromagnetic and gravitational forces. Thus, statement (B) is correct.

(C) Nuclear forces are spin dependent.

The strength of the nuclear force can depend on the spin alignment of the nucleons. This is why some isotopes are more stable than others depending on spin-related factors. Thus, statement (C) is correct.

(D) Nuclear forces are central and charge independent.

The strong nuclear force is indeed charge independent, meaning it is the same regardless of the types of nucleons involved (neutrons or protons). However, nuclear forces are not always central; they can be tensor forces too, which involve more complex interactions that are not purely central. Therefore, the entirety of statement (D) is not correct; the statement should specify that nuclear forces are charge independent but may not always be central.

(E) Stability of nucleus is inversely proportional to the value of packing fraction.

This statement (E) is correct.

Based on the above explanations, the correct statements are (A), (B), (C) and (E). Hence, the correct answer is:

The energy $E_n$ of an electron in the $n$th energy level of a hydrogen atom is given by the formula:

$ E_n = -\frac{13.6 \, \text{eV}}{n^2} $

where:

• $E_n$ is the energy in electron volts (eV),


• $13.6 \, \text{eV}$ is the ionization energy of hydrogen (the energy required to remove the electron from the ground state, $n=1$),


• $n$ is the principal quantum number (an integer starting from 1).

The Balmer Series

The Balmer series is a specific set of spectral lines of hydrogen that are visible to the naked eye. It results from the electron making transitions between energy levels, specifically from levels with $n \geq 3$ (i.e., from higher energy states) down to $n = 2$. These transitions release energy in the form of electromagnetic radiation, which we observe as the Balmer series.

1. Ground State Energy ($E_1$): The energy of the electron in the ground state ($n=1$) of hydrogen is:

$ E_1 = -\frac{13.6 \, \text{eV}}{1^2} = -13.6 \, \text{eV} $

This represents the electron's energy level when it's closest to the nucleus.

1. Energy for the $n=3$ Level ($E_3$): For an electron in the $n=3$ level, its energy is:

$ E_3 = -\frac{13.6 \, \text{eV}}{3^2} = -1.51 \, \text{eV} $

1. Minimum Energy for Balmer Series Transition: The minimum energy transition within the Balmer series is from $n=3$ to $n=2$, but to understand the total energy required for an electron to emit radiation in the Balmer series, starting from the ground state, we consider the energy needed to first excite the electron from $n=1$ to $n=3$.


2. Energy Difference ($\Delta E$): The energy emitted as radiation for the electron to transition from the ground state to a state where it can participate in the Balmer series is the difference between the ground state energy and the energy of the $n=3$ state:

$ \Delta E = E_3 - E_1 = -1.51 \, \text{eV} - (-13.6 \, \text{eV}) = 12.09 \, \text{eV} $

Note

This calculation shows that the minimum energy required by a hydrogen atom in the ground state to emit radiation in the Balmer series is approximately 12.1 eV. The energy difference essentially represents the energy that must be supplied to an electron to excite it from the ground state ($n=1$) to an excited state ($n=3$) from which it can then transition to $n=2$, emitting radiation observable as part of the Balmer series.

$\begin{aligned} & \mathrm{R}_1=\frac{\mathrm{R}_2}{2} \\ & \mathrm{R}_0\left(\mathrm{~A}_1\right)^{1 / 3}=\frac{\mathrm{R}_0}{2}\left(\mathrm{~A}_2\right)^{1 / 3} \\ & \mathrm{~A}_1=\frac{1}{8} \mathrm{~A}_2 \\ & \mathrm{~A}_1=\frac{192}{8}=24 \end{aligned}$

$\begin{aligned} & \frac{1}{\lambda}=\frac{13.6 \mathrm{z}^2}{\mathrm{hc}}\left[\frac{1}{1^2}-\frac{1}{2^2}\right] ...... \text{(i)}\\ & \frac{1}{\lambda^{\prime}}=\frac{13.6 \mathrm{z}^2}{\mathrm{hc}}\left[\frac{1}{1^2}-\frac{1}{3^2}\right] ...... \text{(ii)} \end{aligned}$

On dividing (i) & (ii)

$\lambda^{\prime}=\frac{27}{32} \lambda$