Explain why the second ionisation energy of boron is higher than the first ionisation energy of boron.

Boron is the 5th element in the periodic table, and has an electron configuration 1s²2s²2p¹. The first ionization energy of Boron is the energy required to remove the single 2p electron, while the second ionization energy of boron is the energy required to remove one of the two 2s electron. If we already have removed the 2p electron, then we are left with a positively charged Boron ion, and it is harder to remove another electron from an already positively charged species. Moreover, the 2s electrons are closer to the nucleus and are held in the atom with a greater force. That is why the second ionization energy of Boron is higher.