Atomic mass mass of an atom in atomic mass units An atomic mass unit is equal to? One twelth the mass of one carbon-12 atom The average atomic mass is equal to? atomic mass on the periodic table Isotopes are atoms of the same element with a different number of ___ and so they have different ___ 1) neutron 2) masses Mole Is a counter and relates to mass.* It is similar to a dozen. *It can count the number atoms/molecules/particles. Molar Mass *is the mass of one mole of the substance. *g/mol = atomic mass of the element *molar mass of an element contains mols of atoms Avogadro's number particles/atoms/molecueles > moles *6.022x10^23 Percent Composition Mass of element/ mass of compound x 100 Tell how much an element contributes to the mass of the compound