1.16 calculate relative formula masses (Mr ) from relative atomic masses (Ar)

Relative formula masses

Relative formula mass is given the symbol, Mr.

To calculate the Mr, of a substance, all you have to do is add up the relative atomic masses of all the atoms present in the formula.

Water- H₂0 
Atoms present:(2 x H) + (1 x O) 
Mr= (2 x 1) + 16= 18

There is 1 electron per hydrogen atom, and 2 hydrogen atoms, so that is 2 electrons. There is also 1 oxygen atom, which has 16 electrons, 2 + 16= 18