 The isotope proportional composition of the sample used must be measured and taken into account . Silicon occurs in three stable isotopes ( 28Si , 29Si , 30Si ) , and the natural variation in their proportions is greater than other uncertainties in the measurements . The atomic weight Ar for the sample crystal can be calculated , as the relative atomic masses of the three nuclides are known with great accuracy . This , together with the measured density ρ of the sample , allows the molar volume Vm to be determined : 
