Question

Second electron gain enthalpy:
(A) is always negative
(B) is always positive
(C) can be positive or negative
(D) is always zero

Answer 1

Hint: Electron gain Enthalpy is the amount of energy released when an electron is added to an isolated neutral gaseous atom. It is measured in ev/atom or KJ/mole. We need to remember one thing when understanding the concept that when energy is absorbed the process is endothermic and when energy is released the process is exothermic.

Complete answer:
Consider a reaction-
$A + {e^ - } \to {A^ - } + \Delta {H_1}$ (First Electron gain Enthalpy)
Here $\Delta H$ is called Electron gain enthalpy. In this reaction energy is released i.e. the process is Exothermic. The energy is released hence the stability of the system increases.
Now we add another electron to the atom-
${A^ - } + {e^ - } \to {A^{2 - }}$ + $\Delta {H_2}$ (Second Electron gain Enthalpy)
In this reaction, energy needs to be provided i.e. it is an example of an endothermic reaction. The reason is the interelectronic repulsion or columbic repulsion will take place when we try to force another electron into the Anion formed recently.
For example- The value of the first electron gain enthalpy of oxygen is -141KJ/mole while the second electron gain Enthalpy is $+780KJ/mol.$
Chlorine has the highest value of Electron gain Enthalpy due to its tendency to accept electron while Noble gases have positive values because they are already stable and hence energy has to be supplied from outside to give it an electron.
Hence Option (B) is correct.

Additional Information-
(1)Factors Affecting Electron gain enthalpy-
(a) Atomic size-Electron gain Enthalpy decreases with increase in Atomic size. Since as the size increases the distance between nucleus and electrons decreases and the nucleus is no longer able to attract electrons towards itself.
(b) Nuclear charge-Electron gain enthalpy increases as we increase the nuclear charge on the atom. Since nuclear charge means the Number of protons and when the positively charged protons will increase it will attract electrons easily.
(c) Electronic configuration-When any atom has a half-filled or full-filled Electronic configuration it is highly stable and hence its Electron gain Enthalpy will be very less. For example, the value of Electron gain enthalpy of Beryllium (Be) is less than the Boron (B) due to its full-filled electronic configuration.
$Be - 1{s^2}2{s^2}$ (Full filled so no need to accept electron)
$B - 1{s^2}2{s^2}2{p^1}$ (Whether the atom accepts an electron or not it does not make any difference)
(2)Trends in Electron gain Enthalpy-
It increases from left to right in a period due to increased nuclear charge and it decreases from top to bottom in a group because of the increase in size.

Note:
Fluorine was expected to have the highest value of Electron gain enthalpy however it is Chlorine that has the highest value. The reason is that Fluorine has 2p orbital and is of very small size hence cannot accept electrons due to little space but Chlorine has 3p orbital and hence can easily accept an electron.