Question

The order of first electron affinity of O, S and Se is -

• A. O > S > Se
• B. S > Se > O
• C. Se > O > S
• D. S > O > Se

Answer 1

The correct option is B

S > Se > O

We know how atomic radius is related with electron affinity

Atomic size: In general electron gain enthalpy value becomes less negative with he increasing atomic radius because electrostatic force of attraction decreases between the electron being added and the atomic nucleus due to increase of distance between them.

Electron gain enthalpy

$\text{Electron gain enthalpy}\alpha \frac{1}{Atomicsize}$

Electron Affinity $\left(A_c\right)$ term can also be used to measure ability of an atom to gain electron. If energy is released when an electron is added to an atom, the electron affinity is taken as positive, contrary to thermodynamic convention

$So,EA\alpha \frac{1}{AtomicRadius}$

So, EA

From our knowledge of atomic radius,

We know, Se > S > O [Atomic radius]

Therefore, we can deduce

O > S > Se [EA]

But, because of small size, high electron density and increased electron repulsion makes addition of an electron to O less favourable that in case of sulphur and selenium.

So, the correct order of EA is:

S > Se > O