e formula for a compound indicates the elements that make up the compound and the number of atoms of each element present in the compound.  The numbers of atoms are indicated by the use of small numbers called subscripts. 
Sometimes groups of atoms act as a single atom.  Such a group of atoms is called a polyatomic ion.  If a polyatomic ion is used in a formula more than once, it is put in parentheses and the subscript appears outside of the parentheses.  When a subscript appears outside the parentheses, it indicates that all the elements inside the parentheses should be multiplied by that subscript.  For example, the formula Fe(OH)3 indicates the combination of 1 atom of iron (Fe), 3 atoms of oxygen (O) and 3 atoms of hydrogen (H).
Name 
Use
Formula
Atoms in Formula
Calcium Carbonate
Limestone
CaCO3
Ca= calcium 1
C  = carbon 1
O = oxygen 3
Aspirin
Pain reliever
C9H8O4
Magnesium
Hydroxide
Found in milk of magnesia
Mg(OH)2
Paradichlorobenzene
Moth crystals
C6H4Cl2
Acetic acid
Found in vinegar
C2H4O2
Trinitrotoluene (TNT)
Explosive
C7H5(NO2)3
Calcium dihydrogen
Phosphate
Fertilizer
Ca(H2PO4)2
Name
Use
Formula
Atoms in formula
Pyrite
Fool’s gold
FeS2
Sucrose
Sugar
C12H22O11
Pentane
A component in gasoline
C5H12
Sulfuric Acid
Used in car batteries
H2SO4
Cellulose
Found in wood products like paper /pencil
C6H7O2(OH)3
Asbestos
Insulator
H4Mg3Si2O9
Dichlorodiphenyl-trichloroethane
(DDT)
Banned pesticide
C14H9Cl5
Silicon dioxide
Sand
SiO2
Iron oxide
Rust
Fe2O3
Butane
Lighter fluid
C4H10