Atomic mass: definition in chemistry
Atomic mass: The mass of an atom, usually given in atomic mass units (u), where one unit is one-twelfth of the mass of a carbon-12 atom. The value listed for an element is the average for its natural mix of isotopes.
Protons and neutrons each have a mass of about 1 u and electrons very little, so an atom's mass is close to its mass number. The atomic mass shown for an element, also called its relative atomic mass or atomic weight, is a weighted average of its isotopes, so it is often not a whole number. The same number in grams is the mass of one mole of the element, which makes atomic masses essential for converting between grams and moles.
Examples
- Hydrogen — 1.008, the lightest of all elements
- Carbon — 12.011, mostly carbon-12 with about 1% carbon-13
- Iron — about 55.85, mostly from iron-56
- Chlorine — about 35.45, from roughly 76% chlorine-35 and 24% chlorine-37
On the periodic table
The atomic mass is usually the decimal number in each element's box, and it increases with atomic number apart from a few exceptions such as argon and potassium.