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 electron affinity of an atom or a molecule is defined as the amount of energy released when an electron is attached to a neutral atom or molecule in the gaseous state to form a negative ion.First electron affinity is the energy released when 1 mole of gaseous atoms each acquire an electron to form 1 mole of gaseous 1 – ion. If the energy released when this change happens.

Complete step by step answer:
The first electron affinity of oxygen is $-142KJmo{{l}^{-1}}$
The first electron affinity of sulphur is $-200KJmo{{l}^{-1}}$
The first electron affinity of selenium is $-195KJmo{{l}^{-1}}$

The first electron of affinity of oxygen is smaller than that of sulphur for the reason that oxygen is a small atom and when a new electron is put in the region (which already is crowded with electrons), there is a significant amount of repulsion. This repulsion lessens the attraction electrons feel and hence, reasons the electron affinity. The size of an oxygen atom. Hence, the charge density is low, thus its electron affinity increases as a compound to the affinity of oxygen.
Hence, order of electron affinity is So, the correct answer is “Option B”.

Note: 1. Electron affinity increases upward for the groups and from left to right across the period of a periodic table because the electrons added to energy levels become closer to the nucleus, thus a stronger attraction is seen between the nucleus and its electron.
2. First electron affinity is usually exothermic as the fist energy released when the nucleus attracts the additional electron is larger than the energy absorbed to overcome inter – electron repulsion.