Slater's rules give an effective nuclear charge of 3.50 for the $3p$ electron of $\mathrm{Al}$ and 2.85 for the $3s$ electron of $\mathrm{Mg}$. Even so, the first ionisation enthalpy of $\mathrm{Al}$ (577) is lower than that of $\mathrm{Mg}$ ($738\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$). The correct reason is that

1. A$\mathrm{Al}$ has a half-filled $3p$ subshell
2. Bthe atomic radius of $\mathrm{Al}$ is much larger than that of $\mathrm{Mg}$ because of an extra shell
3. CZeff for $\mathrm{Al}$ is in fact smaller than for $\mathrm{Mg}$ when computed correctly
4. Da $3p$ electron is less penetrating and lies at higher energy than a $3s$ electron, so it is held less tightly despite the larger Zeff