Question

write the atomicity of Mg(OH) 2.

Answer 1

Mg(OH)2= no. of atom of Mg+O+H
=1+2+2
=5u

Answer 2

Well, the atomicity of  Mg(OH)2 is "5"

This because the atomicity is actually about the number of atom present in the molecule present in the compound, in short the number of atoms in a compound can be represent as the atomicity.
So in the case of Mg(OH)2 there are 1 atom of magnesium while 2 atoms are there for each of oxygen and hydrogen so collectively =1Mg+ 2H+  2O =1+2+2= 5 so  the answer is 5u