Electron Gain Enthalpy - Practice Questions & MCQ

Electron Affinity

Electron affinity is the energy change that occurs when an electron is added to a neutral atom to form a negative ion. It is also known as electron gain enthalpy.

A(g) + e →A^-(g) + ?_egH

Factors affecting electron affinity

The electron gain enthalpy or electron affinity depends upon various factors such as:

• Atomic Size

With the increase of atomic size, the distance between the nucleus and the last shell electrons also increases due to which the force of attraction between the nucleus and the incoming electron decreases. Hence, the electron gain enthalpy becomes less negative.

• Nuclear Charge

With the increase of nuclear charge, the force of attraction between the nucleus and the incoming electron increases. Thus, the electron gain enthalpy becomes more negative.

• Electronic Configuration

Elements that have half filled or completely filled orbitals are more stable than others. I these cases, energy has to be provided to add an electron. Thus, their electron gain enthalpy has large positive values. 

Variation of Electron Affinity

• The electron gain enthalpy becomes less negative in going from top to bottom in a group.

• In moving from top to bottom in a group, both the atomic size and the nuclear charge increases. But the effect of the increase in atomic size is more prevalent than the nuclear charge.

• With the increase in atomic size, the attraction of the nucleus for the incoming electron decreases. Hence, the electron gain enthalpy becomes less negative. But in moving from left to right in a period, the attraction of the nucleus and the incoming electron increases and thus electron gain enthalpy becomes more negative.

• Halogens have the most negative electron gain enthalpies. In moving down from chlorine to iodine, the electron gain enthalpies become less negative due to the increase in their atomic radii.

• Chlorine has the most negative electron gain enthalpy value than fluorine. Because fluorine is very small in size due to which there is a very strong inter-electronic repulsion for the incoming electron, thus its electron gain enthalpy is less than chlorine.

Importance of Electron Gain Enthalpy

Some properties of the elements can be predicted on the basis of the electron gain enthalpy values.

• The elements with high electron gain enthalpy values accept electrons easily and form ionic compounds. For example NaCl.

• The elements with high electron gain enthalpy values are strong oxidising agents. For example, F, Cl, O.