Hence atomic number = 18 Number of protons + neutrons = 18 + 22 = 40. Hence mass number = 40The chemical ... the atoms it contains to write down its formula. For example, the formula for carbon ...
if you know the relative mass formula (M r) of the compound. Add up the atomic masses of the atoms in the empirical formula. The empirical formula of a hydrocarbon is CH 2 and its M r is 42.