Oxides, oxoacids, oxoanions and hydroxides

It is a general observation that, within the p-block, basic character increases down a group. Thus:

• boron oxides are exclusively acidic;
• aluminium and gallium oxides are amphoteric;
• indium and thallium oxides are exclusively basic.

   Thallium(I) oxide is soluble in water and the resulting hydroxide is as strong a base as KOH.