Why do ionisation energies decrease down a group?

Ionisation energies decrease down a group because atomic size increases, reducing the nuclear attraction experienced by the outermost electrons.

In more detail, ionisation energy is the energy required to remove an electron from an atom. As you move down a group in the periodic table, the number of electron shells increases. This means that the atomic radius, or size of the atom, also increases. The outermost electrons are further away from the nucleus and are also shielded by the inner shells of electrons.

This increase in atomic size and electron shielding reduces the nuclear attraction experienced by the outermost electrons. In other words, the positive charge of the nucleus has less hold on the outermost electrons because they are further away and there are more electrons in between. This makes it easier to remove an outer electron, hence the ionisation energy decreases.

It's also worth noting that the increase in nuclear charge down a group (more protons in the nucleus) does not compensate for the increased distance and shielding. This is because the effect of distance and shielding is greater than the effect of increased nuclear charge. Therefore, the overall trend is that ionisation energies decrease down a group.

Remember, this trend is a general one and there may be exceptions due to the structure and properties of specific elements. For example, the ionisation energy of aluminium is lower than that of magnesium, even though aluminium is below magnesium in the periodic table. This is due to the particular electronic configuration of aluminium. However, in general, the trend holds true.