This picture illustrates the idea of "atomic mass". The carbon atom (14C) nucleus on the top has 6 protons plus 8 neutrons, giving it 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, giving it an atomic mass of 3.
Click on image for full size
Original artwork by Windows to the Universe staff (Randy Russell).

Atomic Number

The atomic number of an atom tells us how many protons are in the nucleus of that atom. Why is that important? The chemical properties of an element are determined by the number of electrons in its atoms, and the number of electrons equals the number of protons in "normal", neutral atoms. Each element has a different number of protons in the nuclei of atoms of that element; so each element has a different atomic number.

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!

Scientists also use the concept of "atomic mass". Since the nucleus of an atom contains nearly all (more than 99%) of an atom's mass, "atomic mass" is more-or-less a description of the mass in the nucleus. The atomic mass of an atom is essentially a count of the number of neutrons plus the number of protons. Common carbon has 6 protons and 6 neutrons in each carbon atom, so its atomic mass is 12 ( = 6 + 6). Sometimes scientists use the letter "Z" to stand for atomic number and the letter "A" to stand for atomic mass.

Most elements have different "versions" with varying numbers of neutrons. The different versions are called isotopes. Carbon, for example, has isotopes with 7 neutrons and with 8, along with the standard 6-neutron variety. Scientists specify which isotope they are talking about by including the atomic mass in the name. Normal carbon is thus carbon-12, while the less common varieties are written as carbon-13 and carbon-14. Remember, however, that the different isotopes of carbon behave almost identically in most chemical reactions, for they share the same atomic number.

Last modified August 26, 2009 by Randy Russell.

You might also be interested in:

Cool It! Game

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

The Periodic Table of the Elements

Everything you see around you is made of tiny particles called atoms, but not all atoms are the same. Different combinations of protons , neutrons and electrons make different types of atoms and these...more

Element (Chemical Element)

An element (also called a "chemical element") is a substance made up entirely of atoms having the same atomic number; that is, all of the atoms have the same number of protons. Hydrogen, helium, oxygen,...more

Atomic Mass

"Atomic mass" is a term physicists use to describe the size (mass) of an atom of a specific type. Since the nucleus of an atom contains nearly all (more than 99%) of an atom's mass, "atomic mass" is more-or-less...more


Isotopes are different "versions" of a chemical element. All atoms of an element have the same number of protons. For example, all hydrogen atoms have one proton, all carbon atoms have six protons, and...more


Carbon-14 is an isotope of the element carbon. All carbon atoms have 6 protons in their nucleus. Most carbon atoms also have 6 neutrons, giving them an atomic mass of 12 ( = 6 protons + 6 neutrons). Carbon-14...more


Nitrogen is a chemical element with an atomic number of 7 (it has seven protons in its nucleus). Molecular nitrogen (N2) is a very common chemical compound in which two nitrogen atoms are tightly bound...more


Oxygen is a chemical element with an atomic number of 8 (it has eight protons in its nucleus). Oxygen forms a chemical compound (O2) of two atoms which is a colorless gas at normal temperatures and pressures....more

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