- Mg2+ ION IS SMALLER THAN O2_ ION ALTHOUGH HAVE THE SAME ELECTRONIC CONFIGURATION.
- ELECTRON AIN ENTHALPY OF CL IS MORE NEGATIVE THAN THAT OF F
- Be HAS SLIGHTLY HIGHER VALUE OF IONIZATION ENTHALPY THAN THAT OF BORON <
The fact is that boron's outer electron is in a 2p orbital rather than a 2s. 2p orbitals have a slightly higher energy than the 2s orbital, and the electron is, on average, to be found further from the nucleus. This has two effects.
The increased distance results in a reduced attraction and so a reduced ionisation energy.
The 2p orbital is screened not only by the 1s2 electrons but, to some extent, by the 2s2 electrons as well. That also reduces the pull from the nucleus and so lowers the ionisation energy.