Question

How do you determine the formula for aluminium hydroxide?

Answer 1

"Criss-cross" the charges of each atom because it is an ionic compound. This will get you `Al(OH)_"3"`.

Explanation:

Aluminum has a chemical symbol of `Al`, with a charge of 3+.
Hydroxide is a molecule consisting of two elements - hydrogen and oxygen. The chemical formula for hydroxide is `OH^(1-)`.

Aluminum is the cation, while hydroxide is the anion, thus we get `Al(OH)_"3"`. This is because we still "criss-cross" the charges as it is an ionic compound - electrons are transferred - so aluminum's charge of 3+ goes to hydroxide while its charge of 1- goes to aluminum.

The 1 is redundant so we don't write it in.

Hope this helps :)