Given their places on the periodic table I'd assume Aluminium has a higher ionization energy, because it has fewer energy levels, and is on a "righter" row on the periodic table, but in reality it is the opposite. Does anyone here know why?
Ionization energy is the minimum amount of energy needed to remove most loosely bound electron (valence electron).
The atomic number of Calcium is 20. So it's orbital configuration is 1s2 2s2 2p6 3s2 3p6 4s2. This is the arrangement of electrons in the shells. No free electron is present in the above configuration, all the electrons are bonded. So for removal of an electron from the outermost shell higher amount of energy is required.
In case of Aluminium, the atomic number is 13. It's orbital configuration is 1s2 2s2 2p6 3s2 3p1. One single electron is present unbonded in outermost shell, so it can be easily removed as compared to Calcium.
So, ionization energy of Calcium is higher than the ionization energy of Aluminium.