What are the trends in ionisation energies of halogens?

The ionisation energies of halogens increase as you move up the group from astatine to fluorine.

In the periodic table, halogens are found in Group 7. They include fluorine, chlorine, bromine, iodine and astatine. The ionisation energy of these elements, which is the energy required to remove an electron from a gaseous atom, generally increases as you move up the group. This means that fluorine has the highest ionisation energy, while astatine has the lowest.

This trend can be explained by considering the atomic structure of halogens. As you move up the group, the atomic radius decreases. This means that the outermost electrons are closer to the nucleus and are therefore more strongly attracted to it. As a result, more energy is required to remove these electrons, leading to higher ionisation energies.

In addition, the number of electron shells also decreases as you move up the group. This reduces the shielding effect, which is the ability of inner electrons to shield outer electrons from the attractive force of the nucleus. With less shielding, the outer electrons are more strongly attracted to the nucleus, again leading to higher ionisation energies.

It's also worth noting that all halogens have seven electrons in their outermost shell. This makes them highly reactive, as they tend to gain an electron to achieve a stable electron configuration. However, the reactivity of halogens decreases down the group, which is the opposite trend to ionisation energy. This is because elements with lower ionisation energies, like astatine, find it easier to lose an electron than to gain one, making them less reactive.

In summary, the ionisation energies of halogens increase as you move up the group due to decreasing atomic radius and shielding effect. This trend is important in understanding the reactivity of halogens and their tendency to form ions.