Why are there exceptions to trends in ionisation energies in the periodic table?

Exceptions to trends in ionisation energies occur due to electron configuration and shielding effect variations across the periodic table.

Ionisation energy is the energy required to remove an electron from an atom. The general trend in the periodic table is that ionisation energy increases across a period from left to right and decreases down a group. However, there are exceptions to these trends due to the variations in electron configuration and the shielding effect.

Electron configuration refers to the arrangement of electrons in the energy levels, sublevels and orbitals of an atom. When an electron is removed from an atom, it is usually taken from the highest energy level first. However, in some cases, an electron from a lower energy level may be easier to remove due to its specific location in an orbital. For example, the ionisation energy of aluminium is lower than that of magnesium, contrary to the general trend. This is because the outer electron in aluminium is in a 3p orbital, which is higher in energy and further from the nucleus than the 3s orbital where the outer electron of magnesium resides. Therefore, it requires less energy to remove this electron from aluminium.

The shielding effect also plays a significant role in these exceptions. The shielding effect refers to the reduction in effective nuclear charge on an electron caused by other electrons in the atom. Electrons in inner energy levels shield outer electrons from the full attraction of the nucleus, making them easier to remove. For example, the ionisation energy of sulfur is lower than that of phosphorus, which contradicts the general trend. This is because sulfur's outer electron is paired in a 3p orbital, and the repulsion between these paired electrons reduces the effective nuclear charge, making it easier to remove an electron.

In conclusion, while there are general trends in ionisation energies across the periodic table, exceptions occur due to variations in electron configuration and the shielding effect. Understanding these exceptions can provide deeper insights into the structure and behaviour of atoms.