Electron Affinity

This is the notes for Electron Affinity.

 Electron Affinity (E.A.)

The amount of energy released when an electron is added to an isolated gaseous atom in its ground state to form an anion is called electron affinity (or electron gain enthalpy).

A(g) + e- = A- (g) + E.A.

The greater the tendency of accepting an electron, the greater is the electron affinity i.e., larger is the energy released.

Factors affecting electron affinity 

I. Nuclear charge: The E.A. increases as the nuclear charge increases because greater the nuclear charge, greater will be the attraction to the additional electron. 

II. Size of the atom: As the atomic size increases, the distance between the nucleus and the incoming electron increases which results in less attraction for electron and hence E.A. will decrease. 

III. Electronic configuration: The elements having stable electronic configuration such as half-filled and completely filled orbital have low values of EA.

• Variation of EA across a period 

EA increases on moving from left to right in a period because the size of the atom decreases and nuclear charge increases which results in the increase in attraction for additional electron. 

• Variation of EA across a group

The EA increases on moving top to bottom in a group because the size of the atom increases due to the addition of extra shell and screening effect. As a result, the additional electron feel less attraction by the large atom.

Important trends in Electron Affinities

I. Halogens have the highest EA: The valence shell electronic configuration of halogens is ns2 np5 . They require only one additional electron to acquire stable electronic configuration. Therefore, the elements of this group have great tendency to accept additional electron and have highest EA. 

II. EA values of noble gases are zero because of their most stable electronic configuration ns2np6 and they do not have any tendency to accept electrons. 

III. E.A. of elements like Be, Mg, Ca, etc are almost zero because their valence shell electronic configuration show extra stability having completely filled 3S and 2S orbital. i.e., these elements have little tendency to accept an electron. 

IV. The EN of chlorine is greater than fluorine. This is due to the very small size of F-atom which increases the inter-electronic repulsion. Therefore, incoming electron does not feel much attraction towards the nucleus. 

Iso-electronic atoms and ions 

If the atoms and ions have same number of electrons then such atoms and ions are said to be iso-electronic.

For e.g. Al3+, O- - and Mg++ are iso-electronic because all of them have 10 electrons.

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