Atomic Mass Unit
A mass unit defined as exactly one twelfth the mass of a free, unbound carbon-12 atom in its ground state.
Formula / equation
1 u = 1.66053907 × 10⁻²⁷ kg
Unit
u (Da)
In depth
Also written as the dalton (Da), the unit lets atomic and molecular masses be quoted as convenient numbers close to the nucleon count. Because binding energy slightly reduces nuclear mass, isotopic masses are never exact integers — a discrepancy called the mass defect that underlies nuclear energy release.
Examples in the real world
A proton is 1.00728 u, a ¹²C atom is exactly 12 u, and a haemoglobin molecule is roughly 64,500 Da.