H 1.0 amu He 4.0 amu C 12.0 amu O 16.0 amu Cu 63.5 amu Pb 207 amu U 238 amu A helium atom has a mass 4 times that of hydrogen. A carbon atom has a mass 3 times that of Helium 124. Mass N atoms of hydrogen 1.0g N atoms of helium 4.0g N atoms of Carbon 12.0g N atoms of oxygen 16.0g N atoms of Lead 207g N atoms of uranium 238g Mole: The number of atoms that has a mass equal to the atomic mass in grams A mole of an element always has the same amount of atoms, N. But a mole doesnt have to refer only to atoms. We can have compounds in which a molecule (a group of atoms) is the smallest unit. On the atomic scale: Mass 1 molecule H2O 18.0 amu 1 molecule CO 2 44.0 amu 1 molecule C6 12 6 180 amu Mass N atoms of carbon 12.0g N molecules of H2O 18.0g N molecules of CO2 44.0g N molecules C6 12 6 180g N represents the number of elementary units in a mole. For elements, the elementary unit is an atom, for compounds the elementary unit is a molecule. Definition of a mole The mole is the SI unit of amount. An atom of carbon12 has a mass of exactly 12 amu. A mole of carbon12 is the amount of carbon12 having a mass of exactly 12 grams. A mole of any element contains the same number of atoms as is contained in exactly 12 grams of carbon 12. Elementary entities may be atoms, molecules, ions, electrons, or any other appropriate unit.