What is the Relative Atomic Mass?
The mass of an atom is measured in atomic mass units, where one unit is 1/12th of the mass of one atom of C-12. The relative isotopic mass of an isotope is the ratio of the mass of one atom of that isotope to 1/12th of the mass of one atom of C-12. It is usually very close to a whole number ratio. However, the masses of protons and neutrons vary slightly from isotope to isotope, so the relative isotopic mass is not exactly a whole number:
Isotope |
Mass Number |
Relative Atomic Mass |
H-1 |
1 |
1.006 |
He-4 |
4 |
4.003 |
Be-9 |
9 |
9.012 |
Al-27 |
27 |
26.919 |
Co-59 |
59 |
58.933 |
The RAM is the average mass of all isotopes, and is often not close to a whole number:
Element |
Common Mass Numbers |
Relative Atomic Mass |
Mg |
24, 25, 26 |
24.32 |
Cl |
35. 37 |
35.45 |
Br |
79, 81 |
79.91 |
Ba |
134, 135, 136, 137, 138 |
137.33 |
Relative Molecular Mass (Mr):
Some elements and compounds exist as molecules; these also have a characteristic mass:
The relative molecular mass of a molecule is the ratio of the average mass of that molecule to 1/12th of the mass of one atom of C-12. The relative molecular mass of a molecule is the sum of the relative atomic masses of its constituent atoms.
The relative molecular mass of a molecule is the ratio of the average mass of that molecule to 1/12th of the mass of one atom of C-12. The relative molecular mass of a molecule is the sum of the relative atomic masses of its constituent atoms.
Example:
The Relative Molecular Mass of CO2 = 12.0 + 16.0 + 16.0 = 44.0
Therefore Mr(CO2) = 44.0
The Relative Molecular Mass of CO2 = 12.0 + 16.0 + 16.0 = 44.0
Therefore Mr(CO2) = 44.0