Thallium, a neurotoxin, is the heaviest member of Group 13 of the periodic table and is found most usually in the +1 oxidation state. Aluminum, which causes anaemia and dementia, is also a member of the group but its chemical properties are dominated by the +3 oxidation state. Examine this issue by plotting the first, second, and third ionization energies for the Group 13 elements against atomic number. Explain the trends you observe Hints. The third ionization energy, 13, is the minimum energy needed to remove an electron from the doubly charged cation: E2+ (g) → 1 E3+ (g) + e- (g), 13= E (E3+) – E (E2+). For data, see the links to databases of atomic properties provided in the text's web site.