Atomic mass

Atomic mass is the mass of an atom. An atom is the smallest part of a chemical substance that retains its properties. Because atoms are extremely small, their mass cannot be measured in grams directly. Instead, a special unit is used called the atomic mass unit (symbol u or Da, known as the dalton).

This unit is defined as one-twelfth of the mass of a free, stable carbon-12 atom. Therefore, a carbon-12 atom has a mass of exactly 12 u. The masses of other atoms are compared against this value.

Where the mass comes from

Most atomic mass is found in the nucleus, which consists of protons and neutrons. Electrons orbiting the nucleus are extremely light by comparison—roughly one part per 1,836 relative to a proton. The atomic mass therefore closely approximates the total number of protons and neutrons.

The difference between mass and mass number

The mass number is a whole number equal to the sum of protons and neutrons. Atomic mass is a decimal quantity. They are not identical: an atom's mass is typically less than the sum of its constituent parts, because some mass has been converted to binding energy holding the nucleus together. This difference is called the mass defect.

Isotopes and relative atomic mass

Each element can have different isotopes—atoms with the same number of protons but different numbers of neutrons. Each isotope has its own atomic mass. When referring to the atomic mass of an element in its ordinary state, the reference is usually a weighted average based on the natural abundances of its isotopes. This is called the relative atomic mass of the element.