Calculation of Atomic, Molecular, and Ionic Mass

In order to calculate the approximate mass of a molecule we are used to summing up integer masses of the elements encountered, e.g., for CO2 we calculate the mass by 12 u + 2 X 16 u = 44 u. The result of this simple procedure is not particularly precise but provides acceptable values for simple molecules. This is called nominal mass. [3] 

The nominal mass is defined as the integer mass of the most abundant naturally occurring stable isotope of an element. [3] The nominal mass of an element is often equal to the integer mass of the lowest mass isotope of that element, e.g., for H, C, N, O, S, Si, P, F, Cl, Br, I (Table 3.1). The nominal mass of an ion is the sum of the nominal masses of the elements in its empirical formula. 

Example To calculate the nominal mass of SnCl2, the masses of Sn and Cl have to be used, i.e., 120 u + 2 x 35 u = 190 u. While, the Cl isotope represents the most abundant as well as the lowest mass isotope of chlorine, Sn is the most abundant, but not the lowest mass isotope of tin which is Sn. 

Note When dealing with nominal mass, mass number and nominal mass both have the same numerical value. However, the mass number is dimensionless and must not be confused with nominal mass in units of u (below). 

The isotopic mass is the exact mass of an isotope. It is very close to but not equal to the nominal mass of the isotope (Table 3.1). The only exception is the carbon isotope which has an isotopic mass of 12.000000 u. The unified atomic mass 

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