Question

why have he brackets around the (OH)2

and not just OH2

it ask to lsit each element and the number of atoms present.

Answer 1

If it was just OH2 then it would be a differnt element, water actually. The brackets are there to indicate that it is together, and to show you there is 2 of each, the 2 just at the end as you put it would indicate only 1 oxygen atom and 2 hydrogen atoms. Because it is a compound it is placed together in brackets.

Answer 2

See my response to Sarah's original question.

Answer 3

I should have said to look at Sam's original post and my response.