Among the following the third ionization energy is highest for (AMU 2010)

Ionization energy is the amount of energy required to remove an electron from an isolated gaseous atom. Whereas third ionization is the energy required to remove the third outermost electron. As magnesium 2 2 6 2 1s 2s 2p 3s and beryllium 2 2 1 1s 2s 2p form stable octet configuration after second ionization and it has the highest ionization energy. Again, the third ionization energy of beryllium is higher than magnesium due to its small size.