... lithium. The extra electron goes into the same 2s orbital. The increase in ionisation energy (I.E.) can be attributed to the increased nuclear charge. The ionisation energy of Boron is less than that of Beryllium because in Boron there is a complete 2s orbital. The increased shielding of the 2s orbital reduces the ionisation energy. Similarly, the I.E. of Oxygen is less than that of Nitrogen because the extra electron is shielded by the half-filled 2p orbital. The break in the graph between N-O can be explained by the increased repulsion produced when two electrons are in the same orbital. The latter seems to be preferred by examiners! Within a group the first I.E. decreases down the group as the outer electron becomes progressively further from the nucleus. Also there is more shielding because of the extra filled orbitals. The graph below shows the successive I.E. for sodium: The electronic structure for ...

See this essay or coursework document in full right now