Question

Increasing order of electron gain enthalpy is:
A) $N < O > Cl < Al$
B) $O < N < Al < Cl$
C) $Al < N < O < Cl$
D) $Cl < N < O < Al$

Answer 1

Electron gain enthalpy is defined as the energy released when an electron is added to an isolated or neutral gaseous atom. Along the periods the electron gain enthalpy of chemical elements decreases due to the increasing nuclear effect on outermost electrons.

Complete answer:
Periodic table consists of chemical elements arranged in the increasing order of atomic numbers. The chemical elements are arranged in the form of vertical columns and horizontal rows. The vertical columns were called groups and the horizontal rows were called periods.
The Boron family consists of boron, aluminum and other elements. These elements belong to the group $13$ in the periodic table. Thus, aluminium belongs to group $13$ and period $3$ .
Nitrogen is an element belonging to group $15$ and period $2$ .
Oxygen is an element with atomic number $8$ and belongs to group $16$ and period $2$ .
Chlorine is an element with atomic number $17$ and belongs to group $17$ and period $3$ .
The electron gain enthalpy increases along the periods. But, there is no regular trend of electron gain enthalpy in the periodic table.
Thus, aluminium has less electron gain enthalpy than nitrogen, oxygen and chlorine.
Chlorine is the element with higher electron gain enthalpy than the remaining other given chemical elements.
Thus, the order is $Al < N < O < Cl$
Option C is the correct one.

Note:
As chlorine is more electronegative than the other elements, it is difficult to add an electron to the neutral atom of chlorine as the nuclear effect of nucleus is more on outermost electrons. Hence more energy is released when an electron is added to chlorine to become an ion.