Question

In Calcium Hydroxide why OH is in bracket Ca (OH)

Answer 1

Here Calcium metal have 2 unit of positive charge...And OH is a polyatomic ion...

Charge on Oxygen => -2
Charge on Hyndrogen => +1

OH have net charge = ( -2 ) + ( +1) = -2 + 1 = -1
OH have One unit of negative charge...

Now when Calcium Oxide React with water it forms Calcium Hydroxide whose chemical formula is Ca ( OH )2

Here , We cannot write (OH)2 as OH2
Both Notations are different and convey different meaning...
(OH)2 means 2 atoms of Oxygen and 2 atoms of Hydrogen.
OH2 means One atom of Oxygen and 2 atoms of Hydrogen...

Now to form a compound like Calcium Hydroxide
calcium have 2 positive charge , Here two unit of OH ions are needed...see snap...

( ) is generally used for Polyatomic ions like Co3 ( Carbonate ) , So2 ( Sulphate ) , OH ( Hydroxide) etc...