IGCSE: Relative Masses of Atoms and molecules.

The masses of a single hydrogen atom is incredibly small when measured in grams. Mass of hydrogen atom, H = 0.000,000,000,000,000,000,000,001,7 g= 1.4 x 10^-24 g. Mass of methane, CH4 = 0.000,000,000,000,000,000,000,022,4 g = 2.24 × 10^-23g.

In practice, chemists do no use these actual masses of atoms and molecules in their calculation. They use relative masses instead. To do this, a standard atom ( an atom of the carbon - 12 isotope ) has been chosen.

The relative atomic mass of an element is the average mass of one atom of the element when compared with 1/12 of the mass of an atom of carbon 12.

The symbol for relative atomic mass is Ar. Relative atomic mass has no unit.

The relative atomic masses some elements are not whole numbers. This is because many elements are a mixture of isotopes (Atoms with the same number of protons but different numbers of neutrons)

The relative mass of chlorine is 35.5

Chlorine gas is a mixture of molecules containing 35^ CL and 37^ CL atoms.

The figure of 35.5 is not the relative atomic mass of any one atom but the average of all the atoms found in chlorine gas.