Mass

Because the large majority of an atom's mass comes from the protons and neutrons, the total number of these particles in an atom is called the mass number. The mass of an atom at rest is often expressed using the unified atomic mass unit (u), which is also called a Dalton (Da). This unit is defined as a twelfth of the mass of a free neutral atom of carbon-12, which is approximately 1.66 × 10⁻²⁷ kg.^[72] Hydrogen-1, the lightest isotope of hydrogen and the atom with the lowest mass, has an atomic weight of 1.007825 u.^[73] An atom has a mass approximately equal to the mass number times the atomic mass unit.^[74] The heaviest stable atom is lead-208,^[68] with a mass of 207.9766521 u.^[75]

As even the most massive atoms are far too light to work with directly, chemists instead use the unit of moles. The mole is defined such that one mole of any element will always have the same number of atoms (about 6.022 × 10²³). This number was chosen so that if an element has an atomic mass of 1 u, a mole of atoms of that element will have a mass very close to 0.001 kg, or 1 gram. Because of the definition of the unified atomic mass unit, carbon has an atomic mass of exactly 12 u, and so a mole of carbon atoms weighs exactly 0.012 kg.^[72] Other nuclides have atomic masses and molar masses very close to whole numbers in their usual units, such as hydrogen-1. However, except for carbon-12, they cannot be exactly integer numbers, because the masses of different nuclides are not exact integer ratios of each other, although they do not differ from whole number ratios by more than 1%, and often much less.^{[citation needed]}