Counting Atoms in Chemical Formulas

​2H₂ + O₂ → 2H₂O

subscript

• indicates the number of atoms of an element present in a molecule, or compound

• there are 2 hydrogen atoms in H₂O

coefficient

• represents the number of molecules of a substance are involved in a reaction

• there are 2 water molecules represented by 2H₂O

​Essentially, subscripts tell you the composition of a molecule, while coefficients tell you the quantity of molecules involved in a reaction.

​the subscript 2 tells us that there are 2 K atoms in every molecule of K₂S

​the coefficient 3 tells us that there are 3 molecules of K₂S

When we count atoms of each element in a compound, we multiply the subscript times the coefficient.

2H₂O

H = 2 x 2 = 4
O = 1 x 2 = 2

5H₂O

H = 2 x 5 = 10
O = 1 x 5 = 5

H₂O

H = 2
O = 1