Definitions

atomic mass

the number of protons and neutrons in an atom

relative atomic mass (A_r)

the average mass of an atom compared to the mass of 1/12 of a carbon atom

relative molecular/formula mass (M_r)

the sum of relative atomic masses of the atoms in a molecule or ‘species’

mole

the amount of substance that contains the same number of particles as there are atoms in 12g of a carbon-12 atom

molar mass

the mass of one mole of a substance (g/mol)

empirical formula

the simplest whole number ratio of atoms of each element in a formula (compound)

molecular formula

the actual number of atoms of each element in the formula of a compound