Relative Atomic Mass and Relative Molecular Mass

Relative Atomic Mass
As you can imagine the mass of an atom would be significantly small. It is for this reason that we do not use the actual mass of an atom in calculations, instead we use the Relative Atomic Mass (R.A.M). RAM is the mass of an atom as it compares to the mass of a carbon atom and has no units. Since it is a ratio, the units cancel out themselves. Carbon was chosen as the standard to which other atoms are measured because it is found in a large number of compounds.

Carbon has a given mass of 12g
The relative molecular mass of each element is listed in the periodic table of elements. For example the RAM of hydrogen, sodium and chlorine is 1, 23 and 35.5 respectively.

Relative Molecular Mass
Molecular compounds contain more than one type of atom, example MgO. So the relatively molecular mass of a compound is the addition of all the atomic masses of each atom in the formula. Some compounds are made up of ions rather than atoms, hence we use the term Relative Formula Mass (R.F.M) which incorporates atomic, molecular and ionic compounds.

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