Group 2 first ionisation energies decrease down the group as the elements get more reactive. This is because the atomic radius increases down the group as:• the number of filled inner shells increases so the shielding (the force of repulsion of the inner shell electrons) increases• the energy of the outermost electron increases because its distance from the nucleus increases and the force of attraction from the nucleus is lessThe nuclear charge increases but this is outweighed by the effect of the shielding and the increased distance of the electron from the nucleus.The first ionisation energy of magnesium is: Mg (g) → Mg+ (g) + e-
Group 2 First Ionisation Energies