Relative masses of atoms and molecules

1 Describe relative atomic mass, Aᵣ, as the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of ¹²C

2 Define relative molecular mass, Mᵣ as the sum of the relative atomic masses. Relative formula mass, Mᵣ will be used for ionic compounds

3 Calculate reacting masses in simple proportions. Calculations will not involve the mole concept