Define electron gain enthalpy. Explain why electron gain enthalpies of some elements are positive? How does electron gain enthalpy vary in the group and in a period?

We already know that electron gain enthalpy is a periodic property and it follows a general trend in the periodic properties. Let us look at the definition of electron gain enthalpy. Electron gain enthalpy is defined as the amount of energy released when an electron is added to an isolated gaseous atom. During the addition of an electron, energy can either be released or absorbed.

Let us look at this electron gain enthalpies in two metals Magnesium and Sodium. Metals lose to get the inert gas configuration. Therefore, Sodium and Magnesium atoms will not add electrons easily. Some external energy is needed to add electrons in their atoms. Hence, the electron gain enthalpy for metals will be positive. Magnesium atoms are small in size, so the attractive nuclear force will be more on the electrons whereas the size of sodium atoms is comparatively larger than Magnesium atoms. Therefore, force from the nucleus applied on the electrons in the case of sodium will be less.

 The electron gain enthalpy becomes less negative as we go down the group. Also, electron gain enthalpy becomes more and more negative as we move from left to right in a periodic table. This is because of the variation in the effective nuclear charge.