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This picture explains the idea of "atomic mass". The carbon atom (14C) nucleus on the top has 6 protons plus 8 neutrons. It has an atomic mass of 14. Tritium (3H), an isotope of hydrogen, is shown on the bottom. It has 1 proton plus 2 neutrons in its nucleus. Tritium has an atomic mass of 3.
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Original artwork by Windows to the Universe staff (Randy Russell).

Atomic Mass

One way scientists measure the size of something is by its mass. Scientists can even measure very, very tiny things like atoms. One measure of the size of an atom is its "atomic mass". Almost all of the mass of an atom (more than 99%) is in its nucleus, so "atomic mass" is pretty much a measure of the size of the nucleus of an atom.

The nucleus of an atom is made up of protons and neutrons. Protons and neutrons are almost exactly the same size. If you add up the number of protons and neutrons in the nucleus of an atom, you get that atom's atomic mass. A simple hydrogen atom has just one proton and zero neutrons. Its atomic mass is 1. The most common kind of carbon atom has 6 neutrons and 6 protons. It has an atomic mass of 12.

All atoms of a certain element have the same number of protons. Oxygen atoms always have 8 protons; carbon atoms all have 6 protons. Most atoms come in different types called isotopes. Isotopes have different numbers of neutrons. The most common isotope of carbon has 6 neutrons and 6 protons. Its atomic mass is 12. A rare, radioactive isotope of carbon has 8 neutrons. Its atomic mass is 14 ( = 6 protons + 8 neutrons).

In chemistry, the number of protons in an atom is more important than the number of neutrons. Scientists call the number of protons the "atomic number". Normal atoms have the same number of electrons as protons. The number of electrons is the main thing that controls how atoms behave in chemical reactions. Scientists use the letter "Z" to stand for atomic number and the letter "A" to stand for atomic mass.

Last modified August 26, 2009 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF