Question

Why electron gain enthalpy of chlorine is more negative than fluorine.

Answer 1

Electron gain enthalpy tells us about how easily an electron can gain an electron, more negative electron shows that an electron is added to the atom easily and basically depends on the size and repulsions (among electrons) of the atom.

Complete answer:
Electron gain enthalpy is the amount of energy released when an electron is added in the shell of an atom.
The element in whose shell the electron gets easily added has more negative electron gain enthalpy.
Both fluorine and chlorine belong to group 17 of halogens (7 electrons in valence shell) and have highly negative electron gain enthalpies as they can easily gain one electron to gain a stable electronic configuration (8 electrons in valence shell)
According to periodic properties, as we move down in a group of the periodic table the electron gain enthalpy should become less negative and fluorine is placed above chlorine yet has more negative electron gain enthalpy.
This is because:
The size of fluorine is comparatively smaller due to which there exists force of repulsion between the electrons between the valence electrons and they also repel the incoming electron (do not let it enter easily) but the size of chlorine is comparatively larger and it easily attains one electron to become stable
Thus, the electron gain enthalpy of chlorine is more negative than that of fluorine.

Note: Valence electrons are those which are present in the last shell in the dot structure of an atom and a valence shell with 8 valence electrons is most stable.
Noble gases have 8 valence electrons are stable, don’t gain electrons easily and hence have positive electron gain enthalpies whereas halogens have 7 valence electrons, gain one electron easily and thus have highly negative electron gain enthalpies.
With the decrease in atomic size, the nuclear charge increases and vice – versa, this fact influences the trend in the periodic table.