Question

Which of the following has highest electron gain enthalpy?
A. $F$
B. $Cl$
C. $Br$
D. $I$

Answer 1

Electron gain enthalpy is defined as the amount of energy required by a neutral gaseous isolated atom to gain an electron to get converted into a negative charged anion. The lower the electron gain enthalpy of a reaction, the easier the gain of the electron by the neutral gaseous isolated atom would be.

Complete step by step answer:

The electron gain enthalpy can also be defined as the reverse of the ionization enthalpy of a reaction. The amount of energy releases when an electron gets attached to the valence shell of a neutral gaseous isolated atom in order to become an anion. The electron gain enthalpy of an atom depends on several factors. They are:
(i) Size of the atom or atomic radius: The greater is the atomic radius, the lesser will be the value of electron gain enthalpy. Thus, the smallest atom will have the higher value of electron gain enthalpy.
(ii) Nuclear charge: The greater the value of effective nuclear charge, the lesser the value of the electron gain enthalpy.
(iii) Electronic configuration: The repulsive forces between the valence electrons and the incoming electron play an important role in the determination of the electron gain enthalpy.
(iv) Electronegativity of the atom: The greater the electronegativity of the atom, the easier the electron can get attached to it and hence, the lesser the value of electron gain enthalpy.
Among the 4 halogen atoms mentioned above, the atom element which will have a greater value of electron gain enthalpy will be chlorine. Although fluorine is the smallest atom of the halogen series, due to its higher electronegativity and high electron density, it can’t accept an electron easily. But, chlorine is large in size and has comparatively low value of electronegativity than fluorine. Thus, it accepts an electron easily and has the highest electron gain enthalpy value.
Thus, the correct option is B. $Cl$ .

Note:
The misconception about the higher value of electron gain enthalpy is that since it is a numeric value with a negative sign, thus higher value of electron gain enthalpy means a low numeric value with a negative sign in front of it. For example, suppose an atom has electron gain enthalpy value equal to 10kJ/mol and another atom has a value of 23 kJ/mol, this means that their actual values are -10 and -23 respectively.