Define first ionisation energy: There is a general increase in first ionisation energy across a period of the periodic table - Leaving Cert Chemistry - Question b - 2004

1. Increase in nuclear charge (atomic number), which leads to a stronger attraction between the nucleus and the electrons.
2. Decrease in atomic radius, where the electrons are drawn closer to the nucleus, further increasing the attraction.

The decrease can be attributed to oxygen having a higher electron-electron repulsion due to its electronic configuration. In oxygen, the addition of an electron causes it to occupy the same 2p orbital, which results in increased repulsion compared to nitrogen, where the electrons are distributed more evenly across different orbitals.

The first ionisation energy of sodium involves removing an electron from its outermost shell (3s), while the second ionisation energy involves removing an electron from a full inner shell (2s). This results in a significant increase in energy required due to the loss of the stable noble gas configuration (Neon configuration) after the first electron removal. The strong attraction between the remaining electrons and the nucleus in the inner shells contributes to the high second ionisation energy.

In neon, both the first and second electrons are removed from the same full inner shell (2p). The stability of the filled 2p subshell means that the removal of the second electron does not require as much energy as removing an electron from an already less stable configuration, as in the case of sodium.