Why the first ionisation energy of carbon atom is greater than that of boron atom whereas, the reverse is true for the second ionisation energy.

Explanation

.The first ionization energy of carbon `(1s^(2)2s^(2)2p_(x)^(s)2p_(y)^(1))` is higher than boron `(1s^(2) 2s^(2) 2p_(x)^(s))` as the nuclear charge in carbon is higher than that of boron. The second ionization energy of carbon is less than that of boron, because in carbon the second electron is to be removed from 2p orbital, while in boron the second electron is to be removed from paired 2s orbital which requires higher energy in comparison to singly occupied p-orbital.