(N/A) The elements of the second period ($Z=3$ to $10$) and their corresponding first ionisation enthalpies $(IE_1)$ in $kcal \, mol^{-1}$ are as follows:
$1$. $Li$ $(Z=3)$: $520$
$2$. $Be$ $(Z=4)$: $899$
$3$. $B$ $(Z=5)$: $801$
$4$. $C$ $(Z=6)$: $1086$
$5$. $N$ $(Z=7)$: $1402$
$6$. $O$ $(Z=8)$: $1314$
$7$. $F$ $(Z=9)$: $1681$
$8$. $Ne$ $(Z=10)$: $2080$
General Trend: As we move from left to right across a period,the atomic radius decreases and the effective nuclear charge increases,leading to a general increase in ionisation enthalpy.
Exceptions:
- $Be$ $(2s^2)$ has a higher $IE_1$ than $B$ $(2s^2 2p^1)$ because the $2s$ orbital is fully filled and more stable.
- $N$ $(2s^2 2p^3)$ has a higher $IE_1$ than $O$ $(2s^2 2p^4)$ because the $2p$ subshell is half-filled,providing extra stability.