Polyatomic ions

Some atoms have a nett minus one charge, they are named "Monatomic Ions", those include Na+, O2-, and Cl-.

Other grouped Ions are Polyatomic Ions, which have a nett charge and are covalently bonded together.

Some commonly found Polyatomic atoms are:
Hydroxide (OH-), Acetate (C2H3O2-), ammonium (NH4+).

If you need to specify more than one Polyatomic atom, you use parentheses and put the subscript outside.
Let's take an element, Magnesium or Mg, to make it an action, it would need to lose 2 electrons to form a +2 cation. When Magnesium bonds with Hydroxide (OH-), two hydroxide ions are necessary for the magnesium ion to make a neutral compound.
To make the formula it would be {Mg(OH)2}.

Another example is Potassium sulfate, which consists of Potassium Ions, and Sulfate Ions that are bonded together.
Potassium is from group one of the periodic table meaning that it will make Monoatomic Ions with a +1 charge.
While Sulfate is a Polyatomic ion with a -2 charge.
So to make a neutral compound, you need 2 K+ ions for every SO-2 ion.
So the formula is {K2SO4}.

Another example is Sodium Nitrate, which is made of strontium ions and nitrate ions bonded together. 
Strontium is in group 2 so it forms Sr 2+ ions. While a Nitrate ion is a Polyatomic ion with NO3 -.
So the formula would be {SR(NO)3)2}.