Atomic mass
Atomic mass can be expressed in grams. However, because each atom has a very small mass, this is not easy to do. Instead, atomic mass is expressed in unified atomic mass units (unit symbol: u). 1 atomic mass unit is defined as 1/12 of the mass of a single carbon-12 atom. 1 u has a value of 1.660 539 066 60(50) × 10−27 kg.