2.1 Relative masses of atoms and molecules
- Syllabus
- 9701–2028–2029
- Topic
- 2.1
- Level
- AS
Learning objectives
2.1.1Unified atomic mass unit• Define the unified atomic mass unit as one twelfth of the mass of a carbon-12 atom2.1.2Atomic mass, Ar, isotopic mass• Define relative atomic mass, Ar, relative isotopic mass, relative molecular mass and relative formula mass, both referred to as Mr, in terms of the unified atomic mass unit
The unified atomic mass unit is based on carbon-12
One unified atomic mass unit, 1 u, is defined as one twelfth of the mass of one carbon-12 atom. It provides a common mass scale for atoms and other microscopic particles.
1 u=121mass of one 12C atom
The standard is one twelfth of a carbon-12 atom's mass, not the mass of the whole atom. The unit u has dimensions of mass; by contrast, relative atomic, isotopic, molecular and formula masses are ratios and have no units.
Relative masses compare particles with the unified atomic mass unit
| Quantity | Definition relative to 1 u | Symbol |
|---|---|---|
| Relative isotopic mass | mass of one atom of a specified isotope ÷ 1 u | — |
| Relative atomic mass | weighted mean mass of an atom of an element ÷ 1 u | Aᵣ |
| Relative molecular mass | average mass of one molecule ÷ 1 u | Mᵣ |
| Relative formula mass | average mass of one formula unit ÷ 1 u | Mᵣ |
Use relative molecular mass for a substance made of discrete molecules. Use relative formula mass for an ionic or giant structure represented by a formula unit. Both use Mᵣ and are found from the Aᵣ values of all atoms shown in the formula.
All four relative quantities are dimensionless ratios, so do not attach u, g or kg. Relative isotopic mass refers to one isotope; Aᵣ averages the naturally occurring isotopes of the element and therefore need not be a whole number.