Thallium is the first element in group 13 where the reduction of the +3 oxidation state to the +1 oxidation state is spontaneous under standard conditions. Since bond energies decrease down the group, with thallium, the energy released in forming two additional bonds and attaining the +3 state is not always enough to outweigh the energy needed to involve the 6s-electrons.Greenwood and Earnshaw, pp. 224–7 Accordingly, thallium(I) oxide and hydroxide are more basic and thallium(III) oxide and hydroxide are more acidic, showing that thallium conforms to the general rule of elements being more electropositive in their lower oxidation states.
Thallium is the first element in group 13 where the reduction of the +3 oxidation state to the +1 oxidation state is spontaneous under standard conditions Since bond energies decrease down the group with thallium the energy released in forming two additional bonds and attaining the +3 state is not always enough to outweigh the energy needed to involve the 6s-electronsGreenwood and Earnshaw pp 224–7 Accordingly thallium(I) oxide and hydroxide are more basic and thallium(III) oxide and hydroxide are more acidic showing that thallium conforms to the general rule of elements being more electropositive in their lower oxidation states