Encyclopedia of Science | Atomic Mass

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• Modern atomic mass tables

The atomic mass of an atom is the mass of that atom compared to some standard, such as the mass of a particular type of carbon atom. The terms atomic mass and atomic weight are often used interchangeably, although, strictly speaking, they do not mean the same thing. Mass is a measure of the total amount of matter in an object. Weight is a measure of the heaviness of an object. In general, the term atomic mass is preferred over atomic weight.

Scientists usually do not refer to the actual mass of an atom in units with which we are familiar (units such as grams and milligrams). The reason is that the numbers needed are so small. The mass of a single atom of oxygen-16, for example, is 2.657 × 10⁻²³ grams, or...