Question

The first ionisation enthalpy of Mg is higher than AI. Why?

Answer 1

Why does aluminium have a lower ionisation energy than magnesium? Its outer electron is in the 3p-orbital which is slightly further from the nucleus and is shielded by the 3s-orbital and is therefore requires less energy to remove than the magnesium's outer electron which is closer and not as shielded.

Answer 2

The easiest way to explain it is that AlAlhas one unpaired electron in it's highest energy orbital (3p3p), and MgMg's highest energy orbital (3s3s) the electrons are paired. It is energetically favorable for allthe electrons in an orbital to be paired, which means that breaking up this pair would require more energy.