Relative Atomic mass

Different types of atoms have different masses. This mass is too small to measure using a conventional scale, therefore we compare their masses to each other. A carbon atom having a mass number 12, i.e. (¹²C) is taken as standard for this comparison and its relative atomic mass is 12.

 

The relative atomic mass of an element compares the mass of atoms of the element with the ¹²C isotope.

 

It is written as A_r or R.A.M.

Some of the elements exist in nature as a mixture of their isotopes in specific proportions. The R.A.M of such elements is the average mass of the different proportions of each isotope in the mixture.

 

R.A.M. of some elements:

hydrogen	1
oxygen	16
copper	63.5
iron	55.8

 

It’s important to note that atomic number and mass number are always whole numbers because they are based on the number of sub-atomic particles, while the R.A.M. can have fractions because it is the average mass of different isotopes.

 

 

Example

A sample of chlorine gas is a mixture of 2 isotopes, chlorine-35 and chlorine-37. These isotopes occur in specific proportions in the sample i.e. 75% chlorine-35 and 25% chlorine-37. Calculate the R.A.M. of chlorine in the sample.

 

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