Shop Windows to the Universe

With Explore the Planets, investigate the planets, their moons, and understand the processes that shape them. By G. Jeffrey Taylor, Ph.D. See our DVD collection.
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.
Click on image for full size
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.

Shop Windows to the Universe Science Store!

Our online store includes issues of NESTA's quarterly journal, The Earth Scientist, full of classroom activities on different topics in Earth and space science, as well as books on science education!

Windows to the Universe Community

News

Opportunities

You might also be interested in:

Cool It! Game

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

The Periodic Table of the Elements

Everything you see around you is made of tiny particles called atoms. Atoms are so small that you canít see them but scientists have found out that there are many different types of atoms, and each type...more

Isotope

Isotopes are different "versions" of an element. All atoms of an element have the same number of protons. All hydrogen atoms have one proton, all carbon atoms have 6 protons, and all uranium atoms have...more

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...more

Carbon-14

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

Content for Climate Change Education Courses

Looking for online content that can be used for a climate change education course or module? Pages linked below can be used to support an introductory climate change education for either a unit or a full...more

Atomic Physics & Particle Physics

Did you know that a lot of what happens in space physics has to do with the interaction of really tiny particles? These particles like protons and electrons are so tiny that you can't even see them with...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