The ionisation energy of an element represents the energy required to remove the outermost electron from the neutral atom. It is a measure of how tightly the outermost electron is bound to the atom.

As we move down the group 2 elements (alkaline earth metals) from Be to Ba, the atomic radii increase due to the addition of new electron shells. The larger atomic radii result in a decrease in the effective nuclear charge experienced by the outermost electrons. This means the electrons are less strongly attracted to the nucleus and are more easily removed, resulting in lower ionization energies.

In addition, as the atomic radii increase down the group the outermost electrons are located further from the nucleus, making it easier to remove them.

Therefore, the ionisation energies of alkaline earth metals decrease down the group from Be to Ba.