Here's how to determine the number of atoms in Mg(OH₂)₃:

* Break down the formula:

* Mg: 1 magnesium atom

* (OH₂)₃: This means there are 3 water molecules (H₂O) attached to the magnesium.

* H₂O: Each water molecule contains 2 hydrogen atoms and 1 oxygen atom.

* Calculate the total:

* Magnesium: 1 atom

* Hydrogen: 3 water molecules * 2 hydrogen atoms/water molecule = 6 hydrogen atoms

* Oxygen: 3 water molecules * 1 oxygen atom/water molecule = 3 oxygen atoms

* Total atoms: 1 + 6 + 3 = 10 atoms

Therefore, there are 10 atoms in Mg(OH₂)₃.