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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 Number

Every atom has a nucleus. The nucleus has protons and neutrons in it. Scientists have a special name for the number of protons in an atom. They call it the "atomic number".

There are almost 100 different elements, like carbon and oxygen and gold. Each element has a different atomic number. Hydrogen atoms have 1 proton, so they have an atomic number of 1. Carbon has 6 protons and an atomic number of 6; oxygen has 8 protons and thus and atomic number of 8. The atomic number of uranium is 92!

Atoms of one kind of element always have the same number of protons. They often have different numbers of neutrons, though. Most carbon atoms have 6 protons and 6 neutrons. A few carbon atoms have 6 protons and 8 neutrons. The different kinds of atoms of the same element are called "isotopes".

Sometimes scientists talk about the "atomic mass" of an atom. If we add up the number of protons plus the number of neutrons we get the atomic mass. The atomic mass of the isotope of carbon that has 8 neutrons is 14 ( = 6 protons + 8 neutrons). Sometimes 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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