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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

The nucleus of an atom has protons and neutrons in it. Each element (like carbon or oxygen or gold) has a different number of protons in its atoms. Scientists have a special name for the number of protons in an atom. They call it the "atomic number".

Why is the atomic number important? Normal atoms have the same number of electrons as protons. The number of electrons is what makes each element behave a certain way in chemical reactions. So the atomic number, which is the number of protons and thus of electrons, is what makes one element different from another.

Hydrogen atoms have 1 proton, and thus 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 the same element and same atomic number can have different numbers of neutrons. All carbon atoms have 6 protons. Most carbon atoms also have 6 neutrons, but some carbon atoms have 7 or even 8 neutrons. Scientists call these different kinds of carbon atoms "isotopes" of carbon.

Scientists also talk about the "atomic mass" of an atom. The nucleus of an atom contains nearly all (more than 99%) of an atom's mass. Neutrons and protons have almost exactly the same mass. So, to calculate atomic mass, we just add up the number of protons plus the number of neutrons. A carbon atom with 6 protons and 8 neutrons has an atomic mass of 14 ( = 6 + 8). 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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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA