Define electron gain enthalpy. How it varies in a group and in a period ? Why is the electron gain enthalpy of O or F is less negative than that of succeeding element in the group ?

Explanation

The amount of energy released when an electron is added to a neutral isolated gaseous atom is called Electron affinity .
`X_(g) + e ^(-) rarr X ^(-) (g) + "energy"`
As a result of adding up an electron to neutral atom , it gets converted into unive ion . It is difficult to add up another electron to this uni-ve ion , because of the replusions between the electron to be added . Hence, in order to add up another electron energy is to be given to overcome the repulasive force. That is why second electron affinity values are always +ve,
`X_(g)^(-) + e^(-) rarr X_(g)^(--)-"energy"`
In a period as we go from left to right, the electron affinity values graduallay decreases.Electron affinity values of inter gases is '0' . The element with highest electron affinity value is chloride .
Oxygen an fluorine atoms are smaller than their succeeding elements sulphur and chlorine. In oxygen and fluorine atoms the outermost orbit is second orbit when an electron is added to these small second orbit there will be electron-electron repulsions. To overcome these repulsions some energy is consumed from the energy liberated due to attraction on the added electro and nucleus. This results in the liberation of less energy or less negative electron gain enthalpies in O and F than that of the succeeding elements in the group.