Question

Why do noble gases have positive electron gain enthalpy?
(A)- It is difficult to add an electron due to its small size.
(B)- It is difficult to add an electron due to high electronegativity.
(C)- It is difficult to add an electron due to stable configuration.
(D)- It is difficult to add an electron due to high electron affinity.

Answer 1

Hint: The noble gas family having complete octet is highly stable. Enthalpy due to the addition of extra electrons causes the increase in energy.

Complete step by step answer:
The noble gases present in the group 18 of periodic table, have electronic configuration $n{{s}^{2}}n{{p}^{6}}$. It consists of elements: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn).
These elements have the outermost shell completely filled with electrons, thus have the most stable configuration. Due to which the addition or removal of an electron requires a large supply of energy.
Thus, in addition to the electron, it is placed in the next higher energy shell or principal quantum number. This needs extra energy supply and produces unstable configuration. Therefore, known as the electron gain enthalpy which is defined as:
“The energy released when a neutral isolated gaseous atom accepts an extra electron to form the gaseous negative ion is called as the electron gain enthalpy”

The enthalpy is measured in electron volts per atom or kJ per mole, which determines the strength of the added bound electron. For all the noble gases, it is a positive value.
It is affected by the size of the atom which increases down the group. Thereby, making the electrostatic force of attraction holding the outermost shell electrons to the nucleus less. Thus, in case of noble gases, where it resists the addition of electrons, extra energy is required in order to force the electron to bind to the stable atom. Therefore, making the electron gain enthalpies positive.
Its tendency to accept the electron from neon to radon increases, making it less positive down the group. However, the smaller size of helium atom, with 1s orbital, has a higher tendency to easily accept the electron making it the least positive among the noble gases and neon as the highest positive electron gain enthalpy.
Therefore, the order of electron gain enthalpy, ${{\vartriangle }_{eg}}\,H$ of noble gases is as follows:
     \[\underset{\text{116}\,\text{kJ/mol}}{\mathop{\text{Neon}}}\,\underset{\text{96kJ/mol}}{\mathop{\text{ Argon }}}\,\underset{\text{96kJ/mol}}{\mathop{\text{ Krypton}}}\,\text{ }\underset{\text{77kJ/mol}}{\mathop{\text{ Xenon}}}\,\text{ }\underset{\text{48kJ/mol}}{\mathop{\text{ Helium}}}\,\]

Thus, the electron gain enthalpy is positive due to option (C)- difficulty to add electron to stable configuration.
So, the correct answer is “Option C”.

Note: The factors affecting the electron gain enthalpy generally have the same effect as the ionization enthalpy such as the size of atom, effective nuclear charge, electronic configuration etc.