On moving from left to right in a period, with increase in the atomic number, the ionisation enthalpy increases.

However there are few exceptions

$(1)$ Ionization energy of $B\,<\,$ ionization energy of $B e$. This is because less amount of energy is required to remove a p electron than s electron from the same principal quantum shell.

$(2)$ Ionization energy of $O\, <\,$ ionization energy of $N$.

Nitrogen has half filled $2 p$ subshell which is very stable. More energy is required for removal of electron from half filled, stable subshell.