Discuss the trend of the first ionisation enthalpy in the same period and explain why it occurs.

(N/A) In a period,the first ionisation enthalpy generally increases from left to right. This is because the atomic radius decreases and the effective nuclear charge increases as we move across a period,making it harder to remove an electron.
However,there are exceptions due to electronic configurations:
$1$. The ionisation enthalpy of $Be$ $(1s^2 2s^2)$ is higher than that of $B$ $(1s^2 2s^2 2p^1)$ because $Be$ has a fully filled $2s$ orbital,which is more stable.
$2$. The ionisation enthalpy of $N$ $(1s^2 2s^2 2p^3)$ is higher than that of $O$ $(1s^2 2s^2 2p^4)$ because $N$ has a half-filled $2p$ subshell,which provides extra stability.