Chapter 2 1st year Book
(Page 7)

Factors that influence the magnitude of EA

The following factors influence the magnitude of electron affinities of the elements :

(i) Atomic size:

The value of electron affinity decreases with an increase in atomic size but it does not follow a regular trend. However, the smaller the size of an atom, the greater will be its electron affinity.

(ii) Effective nuclear charge (Zeff):

The greater the magnitude of Zeff of the elements, the stronger will be the attraction of its nucleus for an electron to be added. Thus, with an increase of the magnitude of Zeff, electron affinity also increases, It is evident on proceeding from left to right in a period (see table 2.10).

(iii) Screening effect:

On descending in a group, with the increase of atomic number. inner shells increase gradually, and this causes a greater screening effect by inner electrons Consequently, the nucleus of an atom exerts less attraction on the electron to be added Thus, with an increase in screening effect by the inner electrons, electron affinity decreases. (iv) Type of orbitals in which electron enters: The magnitude of the electron affinity of an element depends upon the type of orbital into which the electron is being added, Generally, It is highest for an electron going to enter in s-orbital and decreases for p, d, and f-orbitals.
i.e.    s > p > d > 1

Periodic Variations of EA in the periodic table :

(i) In a group:

On moving down a group with an increase of atomic number atomic size increases and therefore the effective nuclear attraction for the electron decreases. Consequently, the atom will have less tendency to pull additional electrons toward itself, Hence, electron affinity decreases from top to bottom in a group.

For examples :
(i) For the elements of IA group: H>Li>Na>Rb>Cs
(ii) For the elements viz S, Se, Te, and Po of the VIA group: S>Se> Te >Poand
(iii) For elements viz Cl, Br, and I of VIIA group: Cl>Br>1
However, electron affinity values of O and Fare are unexpectedly low than those of S and Cl respectively. Although the atomic size of S and Cl is greater. This is due to the fact that the small size of O and F atoms is compared to the size of S and Cl atoms respectively. The addition of an extra electron to O and F atoms produces high electron density around them resulting in strong electron-electron repulsion. Due to strong electron-electron repulsion O and F atoms show a lesser tendency to attract an electron to form O−and F−ions respectively. Therefore, the electron affinity values of these elements become less than those of S and Cl atoms respectively.
Thus, the actual order of electron affinity values for the elements of VIA and VIIA groups are:
VIA group : 0<S>Se>Te>Po or S>O>Se>Te>PO
VIIA group : F<Cl>Br>1>At or Cl>F>Br>1>At

(ii) In period:

On moving from left to right in a period, the atomic size decreases with a gradual increase of nuclear charge, and hence the force of attraction exerted by the nucleus on the additional electrons increases. Consequently, the atom has a tendency to attract the extra electron. Hence, electron affinity increases (with few exceptions) on passing from alkali metals to halogens in a period.
Exceptions; The electron affinity values of noble gases are zero. It is because these atoms have stable valence-shell electronic configuration ns2np8. Electron affinities of Be, Mg, and Ca are also equal to zero. This is attributed to the extra stability of the full-filled 5 -orbital
12Mg=1s2,2 s2,2p6,3s220
Similarly, N and P nave unexpected lower values of electron affinities. This is because they have hall-filled 2p and 3 p-orbitals respectively.
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