atomic mass

atomic mass, the mass of a single atom, usually expressed in atomic mass units (amu). Most of the mass of an atom is concentrated in the protons and neutrons contained in the nucleus. Each proton or neutron weighs about 1 amu, and thus the atomic mass is always very close to the mass number (total number of protons and neutrons in the nucleus). Atoms of an isotope of an element all have the same atomic mass. Atomic masses are usually determined by mass spectrography (see mass spectrograph). They have been determined with great relative accuracy, but their absolute value is less certain.

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.

More on atomic mass from Fact Monster:

  • atomic weight - atomic weight atomic weight, mean (weighted average) of the masses of all the naturally occurring ...
  • atomic weight: Effect of Isotopes in Calculating Atomic Weight - Effect of Isotopes in Calculating Atomic Weight Most naturally occurring elements have one ...
  • ATOMS - An atom is the smallest part of an element that can exist on its own. Copper, for example, is made from copper atoms, which are different to the oxyge
  • hydrogen: The Isotopes and Forms - The Isotopes and Forms Atmospheric hydrogen is a mixture of three isotopes. The most common is ...
  • phosphorus - phosphorus phosphorus [Gr.,=light-bearing], nonmetallic chemical element; symbol P; at. no. 15; at. ...

See more Encyclopedia articles on: Chemistry: General