Relative Atomic Mass and Atomic Mass Unit
3.4 Relative Atomic Mass and Atomic Mass Unit
Historical Context
- Early chemists (Dalton, Gay Lussac, Lavoisier, Avogadro, Berzelius) made initial calculations based on element combination proportions in compounds
- Direct measurement of atomic masses not possible due to the extremely small size of atoms
Standardization of Atomic Mass
- 1961: Carbon-12 isotope chosen as the standard
- Carbon-12 assigned a value of exactly 12 atomic mass units (amu)
- All other atomic masses compared to this standard
Atomic Mass Unit (amu)
Definition: One-twelfth the mass of a carbon-12 atom
Used to express atomic mass (also known as atomic weight)
Relative Atomic Mass
The mass of an atom compared to the mass of the carbon-12 standard
Expressed in atomic mass units (amu)
Examples of Relative Atomic Masses
- Hydrogen: ~1.008 amu (about 8.40% as massive as carbon-12)
- Oxygen: 15.9994 amu
- Sodium: 22.9898 amu
- Aluminum: 26.9815 amu