Relative Atomic Mass & Relative Molecular Mass (WJEC GCSE Chemistry: Combined Science)

Relative Atomic Mass

• The symbol for relative atomic mass is A_r
• The relative atomic mass for each element can be found in the Periodic Table, along with the atomic number
  • Relative atomic mass is shown above the atomic symbol
  • It is always larger than the atomic number (except for hydrogen, where they are the same)

Key for chemical symbols on the Periodic Table

This key is given on the Periodic Table in exams and shows that the top number is the relative atomic mass.

• Atoms are too small to accurately weigh but scientists needed a way to compare the masses of atoms 
  • Carbon-12 is used as the standard atom and has a fixed mass of 12 units 
  • The mass of all other atoms are compared against carbon-12 
• The relative atomic mass of carbon is 12
  • The relative atomic mass of magnesium is 24 which means that magnesium is twice as heavy as carbon 
  • The relative atomic mass of hydrogen is 1 which means it has one-twelfth the mass of one carbon-12 atom 

Relative molecular (formula) 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
• Relative formula mass is used when referring to the total mass of an ionic compound

Relative Formula Mass Calculations Table

Substance	Atoms present	Calculation	Mr
Hydrogen H2	2 x H	(2 x 1)	2
Water H2O	(2 x H) + (1 x O)	(2 x 1) + (1 x 16)	18
Potassium carbonate K2CO3	(2 x K) + (1 x C) + (3 x O)	(2 x 39) + (1 x 12) + (3 x 16)	138
Calcium hydroxide Ca(OH)2	(1 x Ca) + (2 x O) + (2 x H)	(1 x 40) + (2 x 16) + (2 x 1)	74
Ammonium sulfate (NH4)2SO4	(2 x N) + (8 x H) + (1 x S) + (4 x O)	(2 x 14) + (8 x 1) + (1 x 32) + (4 x 16)	132

Higher tier

• Mass number is the sum of protons and neutrons present in the nucleus
  • Mass number is a whole number value
• Relative atomic mass is the ratio of the mass of an element compared to 1/12th of the mass of a carbon atom
  • Relative atomic mass can be a decimal