Mg = 1
O = 2
H = 2
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In one formula unit of Mg(OH)2, there is one magnesium (Mg) atom, two oxygen (O) atoms, and two hydrogen (H) atoms.
The chemical formula shows you this. The subscripted number next to each element shows how many atoms are present in a molecule or formula unit. If no number is shown, then only one atom of that element is present.
You can determine the number of atoms of each element in a compound by using the chemical formula of the compound. The subscripts in the formula indicate the number of atoms of each element. For example, in H2O (water), there are 2 hydrogen atoms and 1 oxygen atom.
There are 4 calcium atoms, 8 chlorine atoms, and 2 carbon atoms in 4CaCl2 (calcium chloride).
There are 9 nitrogen atoms, 3 phosphorus atoms, 12 hydrogen atoms, and 4 oxygen atoms in (NH4)3PO4.
In 4CaCO3, there are a total of 4 calcium (Ca) atoms, 4 carbon (C) atoms, and 12 oxygen (O) atoms.