ATOMIC, MOLECULAR, AND FORMULA MASSES

1. The atomic mass unit

The ATOMIC MASS UNIT (amu) is the unit by means of which the mass of atoms and molecules are expressed. The atomic mass unit is sometimes called the dalton, Da.

By definition, the atomic mass of the carbon isotope 12C (in other words, a carbon atom whose nucleus consists of 6 protons and 6 neutrons) is exactly 12 atomic mass units.

It follows that 1 atomic mass unit = 1/12 x mass of one 12C atom. Since one atom has a mass of 1.9926 x 10-26 kg, it follows that 1 amu = 1.9926 x 10-26/12 = 1.6605 x 10-27 kg.

This quantity is known as the ATOMIC MASS CONSTANT, mu.

2. Relative atomic masses

The relative atomic mass of an element is the RATIO OF THE ABSOLUTE MASS OF AN ATOM OF THAT ELEMENT TO THAT OF THE ATOMIC MASS UNIT. (It is given the symbol Ar). For example, the Ar for potassium (K) is 39.102 (note that THE RELATIVE ATOMIC MASS DOES NOT HAVE ANY UNITS).

This means that on the average, potassium atoms are 39.102 times heavier than the atomic mass constant. The actual mass of the average potassium atom is therefore

3. Relative molecular masses

For those compounds existing as discrete molecules, the RELATIVE MOLECULAR (MOLAR) MASS, Mr is the sum of the relative atomic masses of all the atoms which make up the molecule.

For example, sulphuric acid has the formula H2SO4. The relative atomic masses of the constituent atoms are H = 1.008, S = 32.064, O = 16.000.

The relative molecular mass Mr for sulphuric acid is:

The MOLAR MASS, M of a pure substance is the mass (in grams) of ONE MOLE of that substance. Referring to the above, the molar mass M of sulphuric acid is 98.080 g·mol.

4. Relative formula masses

If the compound does not form discrete molecules, as is the case for ionic compounds (), the simplest molecular formula, known as the EMPIRICAL FORMULA is taken to describe the molecule, and we talk then about the RELATIVE FORMULA MASS of the substance.

5. Additional questions