Explain the trend in ionisation energies for the group one metals?

The first ionisation energy of an element is the minimum energy required to remove a single electron from one mole of an element in its gaseous state. As you go down the group the first electron becomes easier to remove, and therefore less energy is required to remove it, this is for a number of reasons. The first reason is the radius of the atom. As you go down the group, the outer most negative electron is further from the positive central nucleus, this means that the attraction between the electron and the nucleus is decreased, and therefore less energy is needed to remove it. Also, the shielding from inner electrons changes the ionisation energy. As you go down the group, the elements have a greater number of electron shells, this results in there being more electrons blocking the outer electron, so the electrostatic attraction between the nucleus and outer electron is decreased, hence the lower ionisation energy.