Question

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?

Answer 1

• Electron gain enthalpy of an element is the energy released when a neutral isolated gaseous atom accepts an extra electron to form the gaseous negative ion i.e. anion.
  
• The atoms of noble gases have completely filled subshell. As a result, there is no room in their valence orbitals and the additional electron has to be placed in an orbital of next higher shell. As a result, energy has to be supplied to add on additional electrons. Thus, they have a positive electron gain enthalpy.
  
• As we move down a group, both the atomic size and the nuclear charge increases. But the effect of the increase in atomic size is much more pronounced 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.

• As we move across a period from left to right the atomic size decreases and the nuclear charge increases. Both these factors tend to increase the attraction by the nucleus for the incoming electron. Hence, electron gain enthalpy becomes more and more negative in a period from left to right.