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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. Mass is sort of like weight. 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. "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 hydrogen atom has just one proton and zero neutrons. Its atomic mass is 1. Normal carbon atoms have 6 neutrons and 6 protons. They have an atomic mass of 12.

All atoms of an element have the same number of protons. Oxygen atoms always have 8 protons. Carbon atoms all have 6 protons. Scientists call the number of protons the "atomic number". Scientists use the letter "Z" to stand for atomic number and the letter "A" to stand for atomic mass.

Most atoms come in different types called isotopes. Isotopes have different numbers of neutrons. Normal carbon atoms have 6 neutrons and 6 protons. Normal carbon atoms have an atomic mass of 12. A rare, radioactive isotope of carbon has 8 neutrons. Its atomic mass is 14 ( = 6 protons + 8 neutrons).

Last modified August 26, 2009 by Randy Russell.

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