1. How to Count Atoms Using subscripts and Coefficients to count the number of atoms in molecules.

2. Coefficient The LARGE number written in front tells you how many molecules you have. A single molecule of water is written: H2O Example: 10 water molecules would be written as 10H 2 O = 20 hydrogen atoms (2 x 10) & 10 oxygen atoms (10 x 1)

3. Subscript If an element has a small number after it, this is how many atoms of that specific atom there are. Example H 2 O –TWO Hydrogen atoms, and ONE Oxygen atom C 2 H 4 –TWO Carbon atoms and FOUR Hydrogen atoms

4. IF AN ATOM DOES NOT HAVE A NUMBER IN FRONT OR BEHIND, IT IS ALWAYS ONE

5. Putting the Two Together If you have coefficients AND subscripts, you need to multiply the coefficient by the subscript. Examples: 2 MnO 4 (2 x 1 =) TWO Mn manganese atoms and (2 x 4 = ) EIGHT O oxygen atoms 3 HPO 4 : (3 x 1 = ) THREE H hydrogen atoms, (3 x 1 = ) THREE P phosphorous atoms, and ( 3 x 4 = ) TWELVE O oxygen atoms

6. Brackets If there are brackets within your molecule... A) The coefficient applies to ALL atoms in the molecule, bracket or not. B) A subscript OUTSIDE the bracket applies to ALL atoms INSIDE the bracket

7. EXAMPLES Al(NO 3 ) 3 has ONE Al Aluminum atom, THREE N Nitrogen atoms, and NINE (3 x 3) O oxygen atoms 3 Au 2 (SeO 4 ) 3 has SIX Au gold atoms (2 x 3), NINE Se selenium atoms (3 x 3), and THIRTY-SIX O oxygen atoms (4 x 3 x 3).