Periodic Table and Periodicity

Amongst isoelectronic ions, ionic radii of anions is more than that of cations. Further size of the anion increases with increase in -ve charge and size of cation decreases with increase in +ve charge. Hence, the correct order is

O^2- > F^- > Na⁺ > Mg²⁺ > Al³⁺

Along the period, I.P. generally increases but not regularly. Be and B are exceptions. First I.P. increases in moving from left to right in a period, but I.P. of B is lower than Be.

As in case of boron, 2p¹ electron have to be removed to get B⁺(1s² 2s²) from B(1s² 2s² 2p¹), while in case of Be (1s²2s²) electron have to be removed to get Be⁺ (1s²2s¹). p electron can be removed more easily than s electron so the energy required to remove electron will be less in case of boron.

Be – 1s²2s²;

B – 1s²2s²2p¹;

C – 1s²2s²2p²;

N – 1s²2s²2p³;

O – 1s²2s²2p⁴.

IP increases along the period. But IP of Be > B. Further IP of O < N because atoms with fully or partly filled orbitals are most stable and hence have high ionisation energy.

As in case of boron, 2p¹ electron have to be removed to get B⁺(1s² 2s²) from B(1s² 2s² 2p¹), while in case of Be (1s²2s²) electron have to be removed to get Be⁺ (1s²2s¹). p electron can be removed more easily than s electron so the energy required to remove electron will be less in case of boron.

Bond energy generally decreases on moving from top to bottom along a group. It happens due to fact that size increases on moving down the group and thus, the two nuclei are far apart and less capable of holding the two atoms together.

But in case of F₂ molecule due to more repulsion in between non-bonding electron pair (2p) of two fluorines (due to small size of F-atom) in comparison to non-bonding electron pair (3p) in chlorine, the bond energy of F₂ is less than Cl₂.

BE(F₂) = 158.5 kJ/mole and

BE(Cl₂)= 242.6 kJ/mole