Question

(oh)2 =

open bracket

Answer 1

Mg²+ + (OH)- = Mg(OH)2

We use bracket because OH is a anion.

Answer 2

There are two good reasons why the OH is in brackets:

1) In Ca(OH)2 it becomes ambigues and unclear if we instead say CaOH2 is that [1 calcium 1 oxygen and two hydrogens] or [1 calcium and 2 hydroxides]. We put the brackets around to describe that we have 2 OH not 2 of something else.

2) When you put brackets around the OH it infers that the OH is actually the hydroxide ion. It quickly informs the reader that you are dealing with (H-O)-Ca-(O-H) instead of H2-Ca-O2. It also infers that the OH bond is covalent instead of ionic.

Hope that helped, If your still confused I can try to explain it better. It would also help if we knew what level your understanding of chemistry is at.