All the elements of Group 13 form trioxides also known as sesquioxides of the  formula, M₂O₃. Thallium forms stable Tl₂0 also. The basic character of oxides increases down the group with increase in the atomic number. Thus, B₂O₃ is acidic, Al₂0₃ and Ga₂0₃ are amphoteric whereas In₂0₃ and Tl₂O₃ are basic. Tl₂0 is soluble  in water and the resulting hydroxide TlOH is, in fact, as strong a base as KOH. Let us now study the oxides of boron and aluminium in a little more detail, this will be followed by a discussion of the acids of boron and the borates.