The following are common mass-related definitions that students encounter when studying Chemistry:-
Relative isotopic mass
The relative isotopic mass is the ratio of the mass of an isotope in unified atomic mass unit to one unified atomic mass unit. It is a dimensionless quantity, for example, the relative isotopic mass of 1H is 1.007825.
Relative atomic mass
The relative atomic mass of an element is the weighted average of the relative isotopic masses of all its naturally occurring isotopes. It is represented by the symbol Ar and is the value usually found next to an element’s symbol in the periodic table. For example, the relative atomic mass of copper, Ar(Cu), is (62.929601 x 0.6917) + (64.927794 x 0.3083) ≈ 63.546.
| Relative isotopic mass | % Abundance | |
| 63Cu | 62.929601 | 69.17 |
| 65Cu | 64.927794 | 30.83 |
The terms ‘atomic weight’ and ‘standard atomic weight’ are sometimes used in place of relative atomic mass. Strictly speaking, standard atomic weight is only equivalent to relative atomic mass when studying the masses of elements on earth.
Relative molecular mass
The relative molecular mass of a molecule is the sum of the relative atomic masses of all the atoms in the molecule. It is represented by the symbol Mr . For example, the relative molecular mass of carbon dioxide, Mr(CO2), is 12.011 + (15.999 x 2) = 44.009.
| Relative atomic mass | |
| C | 12.011 |
| O | 15.999 |
For an ionic compound, the term relative formula mass is used instead of relative molecular mass.
Atomic mass
Atomic mass refers to the mass of an atom (i.e. the mass of a specific isotope rather than the average mass of all the isotopes of an element). It is expressed in the unified atomic mass unit, u, and can be converted to the SI base unit of mass, kg. When the atomic mass of an isotope is expressed in u, it has the same numerical value as the isotope’s relative isotopic mass. For example,
| Relative isotopic mass | Atomic mass, u | Atomic mass, kg | |
| 2H | 2.014104 | 2.014104 | 3.34450 x 10-27 |
The relationship between the unified atomic mass unit, u, and the basic SI mass unit, kg, is:
where is NA is the Avogadro constant. The value 0.001 kg mol-1 is called the molar mass constant and is usually written as 1 g mol-1. With the Avogadro constant now defined as exactly 6.02214076 x 1023 mol-1, the value of the molar mass constant deviates very slightly from the exact value of 1 g mol-1 and will need to be determined through future experiments.
Molecular mass
The molecular mass is the mass of a molecule. It is expressed in the unified atomic mass unit, u, and can be converted to the SI base unit of mass, kg. Since each atom in a molecule is a specific isotope of an element, different molecules of the same compound may have different molecular masses. For example, the molecular mass of nitrogen gas can be
| Molecular mass, u | |
| 14N14N | 14.003074 + 14.003074 = 28.006148 |
| 14N15N | 14.003074 + 15.000109 = 29.003183 |
| 15N15N | 15.000109 + 15.000109 = 30.000218 |
For ionic compounds, the term formula mass is used instead of molecular mass.
Molar mass
The molar mass of a substance is the mass of the substance per mole. It is represented by the symbol M and has the SI unit of kg mol-1. However, for practical purposes, most molar masses are expressed in g mol-1. For an isotope, the molar mass is numerically equal to its atomic mass expressed in g mol-1. For an element, it is numerically equal to its average atomic mass expressed in g mol-1. For a covalent and an ionic compound, it is numerically equal to the average molecular mass and the average formula mass, respectively, expressed in g mol-1. For example,
| M, g mol-1 | |
| 35Cl | 34.97 |
| Cl | 35.45 |
| N2 | 28.014 |
| CuSO4 | 159.602 |