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

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)

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

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

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

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

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).