Answer: It's important to note that the atomicity is different from the formula mass or molar mass which is calculated by adding the atomic mass of all the atoms in the compound.
Explanation: Mg(OH)2 is a compound composed of one magnesium (Mg) atom and two hydroxides (OH) ions. The atomicity of a compound refers to the number of atoms in one formula unit of the compound. In the case of Mg(OH)2, the formula unit contains one magnesium atom and two hydroxide ions, so the atomicity of Mg(OH)2 is 1 + 2 = 3 atoms.