Relative Atomic Masses
Because the mass of an atom is very small, it is convenient to define a special unit that avoids large negative exponents. This unit, called the atomic mass unit and designated by the symbol u (some authors use the abbreviation amu), is defined as exactly 1/12 the mass of a ¹²C atom. Thus the mass of a ¹²C atom is exactly 12 u. The masses and abundances of some other nuclides are listed in Table 1-1.

Naturally occurring silicon is 92.23% ²⁸Si, 4.67% ²⁹Si, and 3.10% ³⁰Si. For chemical purposes, it is sufficient to know the average mass of a silicon atom in this isotopic mixture. These average masses are designated by Ar (E), where E is the symbol for the particular element. For example, the average mass of silicon atoms is

The term atomic mass will be understood to mean average atomic mass; nuclidic mass refers to one particular isotope of an element. Atomic masses are used in nearly all chemical calculations.

Table 1-1. Some Nuclidic Masses in Atomic Mass Units