Atomic Mass Definition: Atomic Weight

What Is Atomic Mass?

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Atomic mass or weight is the average mass of the protons, neutrons, and electrons in an element's atoms. Science Photo Library - ANDRZEJ WOJCICKI, Getty Images

Atomic Mass or Weight Definition

Atomic mass, which is also known as atomic weight, is the average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally occurring element.

Atomic mass indicates the size of an atom. Although technically the mass is the sum of the mass of all the protons, neutrons, and electrons in an atom, the mass of an electron is so much less than that of the other particles, that the mass is simply that of the nucleus (protons and neutrons).

Examples of Atomic Mass

  • The atomic mass of carbon is 12.011. Most carbon atoms consist of six protons and six neutrons.
  • The atomic mass of hydrogen is 1.0079. Hydrogen (atomic number 1) is the element that has the lowest atomic mass. The most common isotope of hydrogen is protium, an atom that consists of a proton or a proton and an electron. Because of a small amount of deuterium (one proton and one neutron) and tritium (one proton and two neutrons), the atomic mass of hydrogen is slightly higher than 1.