Relative Atomic Mass and Relative Molecular Mass

Relative Atomic Mass & Relative Molecular Mass

Relative atomic mass

• The symbol for the relative atomic mass is A_r
• This is calculated from the mass number and relative abundances of all the isotopes of a particular element

Symbol, mass number and atomic number of chlorine

Equation:

The top line of the equation can be extended to include the number of different isotopes of a particular element present.

Example for Isotopes:

The table shows information about the Isotopes in a sample of rubidium

 ( 72   x   85 )   +   ( 28   x   87 ) ÷ 100  =  85.6

Relative Atomic Mass = 85.6

Relative formula (molecular) mass

• The symbol for the relative molecular mass is M_r and it refers to the total mass of the molecule.
• To calculate the M_r of a substance, you have to add up the Relative Atomic Masses of all the atoms present in the formula

Example: