A-Level Trends in the elements across a period and down a group: worked solution

Question

Al3+ has an ionic radius of 0.050 nm, whereas Ti2+ has one of 0.086 nm. Explain why these two ionic radii differ.

Al3+ and Ti2+ have the same number of electron shells occupied (both are relatively small cations), but two factors make Al3+ smaller. (1) Charge: Al3+ carries a greater positive charge (3+) than Ti2+ (2+), so for a given number of electrons there is a higher effective nuclear charge pulling the electrons in more strongly. (2) Electrons/size of species: Al3+ (13 protons, 10 electrons) has a larger proton-to-electron ratio, so each electron is held more tightly, giving a smaller radius. Ti2+ has more electrons and lower charge, so its outer electrons are held less tightly and the ion is larger. Hence Al3+ (0.050 nm) is smaller than Ti2+ (0.086 nm).