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Q.
lonisation energy is highest in
AMUAMU 2002
Solution:
In a period from left to right ionisation energy of element generally increases and in a group from top to bottom ionisation energy of element decreases.
The ionisation energy of $V$ A group is greater than $VI$ A group due to extra stabilility of half-filled p-subshell.
$[Ne] 3 s^{1}$ is present in $IA$ group and third period.
$[Ne] 3 s^{2} 3 p^{3}$ is present in $V$ A group and third period.
$[Ne] 3 d^{10}, 4 s^{2} 4 p^{3}$ is present in $V$ A group and fourth period.
$[Ne] 3 s^{2} 3 p^{4}$ is present in $VI$ A group and third period.
We know that $IE$ of$ V$ A group is greater than $VI $ A group.
So, the $IE$ of $[Ne] 3 s^{2} 3 p^{3}$ is greater than $[Ne] 3 s^{2} 3 p^{4}$.
Further,$ [Ne] 3 d^{10} 4 s^{2} 4 p^{3}$ is also placed in $V$ A group but it is placed below the $[Ne] 3 s^{2} 3 p^{3}$.
As the IE in a group from top to bottom decreases therefore, the ionisation energy of $[Ne] 3 s ^{2} 3 p^{3}$ is greatest in given elements due to extra stability of half-filled $p$-sub-shell.