Question

EXPLAIN the general trend (from left to right) and anomalies observed on the ionization energy of the second period elements in the periodic table.
The ionization energy for elements in the second period (Li to Ne) generally increases from left to right, but it decreases slightly from beryllium (899 kJ) to boron (800 kJ) and from nitrogen (1402 kJ) to oxygen (1314 kJ). Explain the general trend of increasing ionization energy from left to right and the minor decrease from beryllium to boron

Answer 1

This has the general trend of increasing across a period as the effective nuclear charge experienced by the valence electrons in each sucessive element increases, resulting in a progressively greater force being needed to remove them, i.e. a greater ionisation energy. This is not a linear trendm the ionisation energy of boron being unexpectedly less than that for beryllium, but this is due to the 2s orbital being totally filled in beryllium, whereas boron has one electron in a 2p orbital as well, and the 2s orbitalis shielded much more than the 2p orbital, which gives boron a lower ionisation energy (as its valence electron is easier to remove).

Oxygen also has an unexpectedly low ionisation energy, less than that of nitrogen. This is due to an electron being added to an already half full orbital in oxygen, which results in electron electron repulsion, which will lower the ionisation energy. Nitrogen also has the added stability of a half filled shell of electrons in the 2p shell.

This property will decrease down a group, as the valence electrons get further and further away from the nucleusas the atoms increase in size, i.e. the valence electrons are shielded by more and more inner shells of electrons as the group is desended.