Explain why the first ionisation energy of strontium is less than the first ionisation energy of calcium

Both of these are alkali-earth elements, this means that both elements have the same amount of outer-shell electrons (2 electrons). Strontium is a row below calcium, meaning it has an additional electron shell. This additional shell means that the outer-shell electrons in strontium are further away from the nucleus in calcium (strontium has a larger atomic radius than calcium).

As electrons are negatively charged, and the nucleus is positively charges (as it contains protons), there is an attractive force between the electrons and nucleus, which makes it harder to ionise the atom. The greater the attraction, the higher the ionisation energy. The electrons are further away from the nucleus in strontium than calcium, so the attraction is weaker and so the first ionisation energy is less. On top of this, the additional electron shells shield the outer layer from the nucleus, decreasing this attractive force further. As there are more shells in strontium, this shielding is greater than for calcium.