Periodic Table & Periodicity

1. Mercury is the only metal that exists as liquid at room temperature.

✅ Correct.

Mercury is indeed the only metal that is liquid at room temperature (~25°C). Gallium also melts just above room temperature (at about 29.7°C), but it is solid under normal room conditions.

2. Among non-metals, carbon has the highest melting point.

✅ Correct.

Carbon (in the form of diamond) has the highest melting (or sublimation) point among non-metals — around 3550°C.

3. Hydrogen is the most abundant element in the universe.

✅ Correct.

Hydrogen accounts for about 75% of the elemental mass of the universe.

4. Oxygen is the most abundant element in the Earth's crust.

✅ Correct.

Oxygen constitutes about 46% by mass of the Earth’s crust, making it the most abundant element there.

✅ Answer: Option D (1, 2, 3, 4) — All are correct.

For electronic configuration $=(n-1) d^2 n s^2$

Sum of electrons present in outer most orbital ($d$ and $s$ ) denoted group and period position is represented by principal quantum number.

$\text { e.g., }(n-1) d^2 n s^2$

Group = number of valence electrons in $d$ and $s$-orbital.

Sum of electrons in $d$ and $s$ orbital $=2+2=4$

$\therefore$ Number of electrons in $d$-orbital and valence shell $=4$

$\therefore 4$th group and 4th period.

The outermost electronic configuration of nitrogen $\left(2 s^2 2 p_x^1 2 p_y^2 2 p_z^1\right)$ is very stable because $p$-orbital is half-filled. The addition of an extra electrons to any of the $2 p$-orbitals require energy as it breaks the stability of N. Oxygen has 4 electrons in $2 p$-orbital and acquires stable configuration i.e. $2 p^3$ after removing one electron and complete octet after gaining two electrons. A nitrogen has positive electron gain enthalpy, whereas oxygen has negative, however oxygen has lower ionisation enthalpy than nitrogen. Hence, both statements 1 and 2 are correct.

In contrast element neon [$\mathrm{Ne}]$ has seven electrons in the valence shell and hence, does not i.e. in the same group as the other 3 elements i.e. $[\mathrm{Ar}],[\mathrm{Kr}],[\mathrm{Xe}]$.

In a family all elements have same outermost electronic configuration. Since, $[\mathrm{Ne}] 3 s^2 3 p^5$, chlorine belongs to halogen family while the remaining three are in same group i.e. group 12 (noble gas ).

Helium is as a safe, non-flammable gas to fill in balloons and parade balloons. It is lighter in weight. Liquid helium is used for MRI system. It is needed as are refrigerant and superconducting appliances. It in useful to carry low temperature experiments in research.

Electron gain enthalpy, (EGE) increases in period from left to right and decreases in group on moving downward.

Here, in group 17, chlorine having very high electron gain enthalpy (349 kJ/mol). Due to high electronegativity of fluorine, it has second high electron gain enthalpy (328 kJ/mol).

Order of increasing electron gain enthalpy (in kJ/mol)

$\matrix{ N & < & P & < & F & < & {Cl} \cr {(7)} & {} & {(72)} & {} & {(328)} & {} & {(349)} \cr } $

Only Xe can form compounds with fluorine among noble gases because Xe is large in size and have high atomic mass. Due to having larger atomic radius the force of attraction between the outer electrons and protons in the nucleous is weaker. Hence, they are easily available to form compound.

Across a period (left to right) in the periodic table :

Ionization enthalpy tends to increase

Electron gain enthalpy (in magnitude) tends to increase (more negative values)

Electronegativity tends to increase

Atomic radius tends to decrease

Hence, atomic radius shows the opposite trend compared to the other three properties. Therefore, the correct answer is

(D) Atomic radius.

The basic difference in approach between Mendeleev's periodic law and Modern periodic.

law is as follows :

The Mendeleev's periodic law is based on atomic weight while modern periodic law is based on atomic number. i.e. according to Mendeleev's periodic law.

If the element are arranged in order of increasing atomic weights their properties vary in definite manner from member to member of the series, but return more or less nearly to same value at certain fixed points in the series, while according to modern periodic law.

Physical and chemical properties of the elements are periodic function of their atomic numbers.

Hence, option (b) is the correct answer.

A diagonal relationship is said to exist between certain pairs of diagonally adjacent elements in the 2nd and 3rd periods.

(a) Li shows diagonal relationship with Mg. Due to comparable :

Electronegativities

$ \mathrm{Li}=1.0 \Rightarrow \mathrm{Mg}=1.20 $

(b) Atomic size, i.e. atomic radii (in pm)

$ \mathrm{H}=152 \Rightarrow \mathrm{Mg}=160 $

(c) Atomic volume

$ \begin{aligned} & \mathrm{Li}=12.97 \mathrm{~mL} \Rightarrow \mathrm{Mg} \\ = & 13.97 \mathrm{~mL} \end{aligned} $

Thus, Li-resembles Mg in many respects.

Hence, option (a) is the correct answer.

The electronic configuration of $\mathrm{Be}^{2+}$ is $\left[{ }_2 \mathrm{He}\right], \mathrm{Na}^{+}$is $\left[{ }_{10} \mathrm{Ne}\right]$, $\mathrm{Cl}^{-}$is $\left[{ }_{18} \mathrm{Ar}\right]$ and $\mathrm{Br}^{-}$is $\left[{ }_{36} \mathrm{Kr}\right]$.

We know, site of group 18 elements follow the order

$ \mathrm{He}<\mathrm{Ne}<\mathrm{Ar}<\mathrm{Kr}<\mathrm{Xe} . $

So, the order of ionic radii of the given ions will be

$ \mathrm{Br}^{-}>\mathrm{Cl}^{-}>\mathrm{Na}^{+}>\mathrm{Be}^{2+} $

The maximum number of elements for sixth period of the periodic table are 32, i.e. from atomic number $(Z)=55$ to atomic number $(Z) =86 .(Z)=55$ stands for caesium and $( Z)=86$ stands for radon. Hence, option (c) is the correct answer.

The correct option describing the trend for chemical reactivity within the group for alkali metals and halogens is Option D.

Here is the explanation for the periodic trends in reactivity for alkali metals and halogens:

Alkali Metals (Group 1 elements): For alkali metals, the chemical reactivity increases with an increase in atomic number down the group. This is because alkali metals have only one electron in their outermost shell, and the ease with which this electron is lost (their ionization energy) decreases as the size of the atoms increases down the group. Larger atoms have the valence electron further from the nucleus, which results in less electrostatic attraction between the nucleus and the electron. Therefore, the electron can be removed more easily, making the atom more reactive. As a result, cesium and francium are the most reactive alkali metals.

Halogens (Group 17 elements): For halogens, the chemical reactivity decreases with an increase in atomic number down the group. Halogens have seven electrons in their outermost shell and are one electron short of a full octet. They tend to gain an electron to achieve a stable electronic configuration. The ability to attract an electron is known as electronegativity, which typically decreases down the group as the size of atoms increase and the added inner electron shells shield the outer electrons from the pull of the nucleus. Thus, fluorine is the most reactive halogen, being able to attract electrons more easily than other halogens like chlorine, bromine, and iodine.

Therefore, Option D is correct: "In alkali metals the reactivity increases but in the halogens it decreases with increase in atomic number down the group."