Generally the ionization potential in a period increases, but there are some exceptions. The one which is not an exception is

$\mathrm{Be}(\mathrm{Z}=4)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^2; \mathrm{B}(\mathrm{z}=5)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^1$

Therefore, I.E of Be > I.E of B. is completely filled and is incompletely filled. So, IE of Be > B

$\mathrm{N}(\mathrm{Z}=7)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^3; \mathrm{Half} \mathrm{filled} \mathrm{configuration}$

$\mathrm{O}(\mathrm{Z}=8) \to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^4; \mathrm{Hence},\mathrm{I}.\mathrm{E} \mathrm{of} \mathrm{N} > \mathrm{I}.\mathrm{E} \mathrm{of} \mathrm{O}$

$\mathrm{Mg}(\mathrm{Z}=12)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^2; \mathrm{inert} \mathrm{pair} \mathrm{effect};$

$\mathrm{Al}(\mathrm{Z}=13)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^{2}3{\mathrm{p}}^1; \mathrm{So} \mathrm{that} \mathrm{IE} \mathrm{of} \mathrm{Mg}>\mathrm{I}.\mathrm{E} \mathrm{of} \mathrm{Al}$

$\mathrm{Na} (\mathrm{Z}=11)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^1; \mathrm{Mg} (\mathrm{Z}=12)\to 1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^{6}3{\mathrm{s}}^2$

Note: Here I.E of Na < I.E of Mg