Question

Among the following pairs
(1) Oxygen - Sulphur
(2) Nitrogen - Phosphorus
(3) Phosphorus - Arsenic
(4) Chlorine - Iodine
Those in which the first ionization energies differ by more than $300kJmol{e}^{-1}$ are

• A. (1) and (3) only
• B. (1) and (2) only
• C. (2) and (3) only
• D. (3) and (4) only

Answer 1

Answer: (B)

Ionization energy decreases on moving down in a group and difference between I.E. of two successive members also decreases as we move from top to bottom in the same group. So difference of I.E. of $2^{nd}$ and $3^{rd}$ period element of same group is higher than that of $3^{rd}$ and $4^{th}$ period element. Hence, $1^{st}I.E.$ differ by more than $300kj/mol$ for oxygen-sulphur and nitrogen-phosphorus pair.