How would you explain the fact that the first ionisation enthalpy of sodium is lower than that of magnesium,but its second ionisation enthalpy is higher than that of magnesium?

(N/A) The electronic configuration of $Na$ is $[Ne] 3s^1$ and $Mg$ is $[Ne] 3s^2$.
Since $Na$ has only one electron in its outermost shell,it is easier to remove the first electron from $Na$ than from $Mg$,which has a stable fully-filled $3s^2$ configuration. Thus,the first ionisation enthalpy of $Na$ is lower than that of $Mg$.
After the removal of the first electron,$Na^+$ attains the stable noble gas configuration of $Ne$ $(1s^2 2s^2 2p^6)$. Removing the second electron from $Na^+$ requires breaking this stable octet,which is very difficult.
In contrast,$Mg^+$ has the configuration $[Ne] 3s^1$. Removing the second electron from $Mg^+$ is easier because it leads to the stable noble gas configuration of $Ne$. Therefore,the second ionisation enthalpy of $Na$ is significantly higher than that of $Mg$.