Question

The correct order of electron affinity is:
A.Se$<$S$<$O
B.O$<$Se$<$S
C.S$<$O$<$Se
D.S$<$Se$<$O

Answer 1

Hint: To answer this question the periodic properties of the elements from the different groups should be known. The periodic properties have opposite trends between left to right and top to bottom. The electron affinity is the change of energy of a gaseous atom when an electron is added to its outermost shell to form a negative ion.

Complete step by step answer:
Most of the first electron affinities are negative because the energy is released when an electron is added to a neutral atom or molecule to create its negative ion i.e., anion, this means that it is an exothermic reaction. If a reaction is exothermic, the change in energy is negative but the electron affinity is positive.
In between sulfur and oxygen, the electrons enter in 3p and 2p orbitals respectively. Due to the small size of the 2p orbital, the electron-electron repulsion is greater in 2p than 3p. so, due to this repulsion, the electron enthalpy of oxygen is lower than sulfur. Also due to this electron-electron repulsion is 2p orbital selenium has greater electron affinity than oxygen.
So, considering all factors, the order or electron affinity is, O$<$Se$<$S.

The correct answer is, B

Note:Electron affinity also depends on the size of the atom. The shielding effect weakens the net nuclear charge on the valence electrons. But if the size of the atom is small, then the shielding effect will be much lower as compared to larger atoms, due to a decrease in the number of shielding inner electrons. This increases the net nuclear charge, and in turn, increases the electron affinity of the atom.