Shop Windows to the Universe

Our Glaciers: Then and Now activity kit helps you see the changes taking place in glaciers around the world. See all our activity kits and classroom activities.
Hydrogen has three isotopes. The nucleus of a "normal" hydrogen atom has one proton (red) but no neutrons (blue). Hydrogen's other isotopes are deuterium (1 proton + 1 neutron) and tritium (1 proton + 2 neutrons).
Click on image for full size
Original artwork by Windows to the Universe staff (Randy Russell).


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 92 protons. However, atoms of an element can have different numbers of neutrons. Most carbon atoms have 6 neutrons, but some have 7 neutrons.

Scientists use special "codes" to write the names of isotopes. One isotope of carbon has 8 neutrons. It has an atomic mass of 14 (6 protons + 8 neutrons). The "code" for this isotope is carbon-14 or 14C. Different isotopes of the same element behave almost exactly the same way in chemical reactions. For example, most oxygen is the isotope oxygen-16. Oxygen-18 is a rare isotope. However, adding two hydrogen atoms to one oxygen atom still makes water (H2O), even if we use 18O instead of 16O. Some isotopes are radioactive, but others are not. Radioactive isotopes can "decay" by giving off radiation.

Where do different isotopes come from? Astronomers think the only elements created in the Big Bang were various isotopes of hydrogen, helium, and probably lithium, beryllium and boron. Supernova explosions created the rest of the elements, including most of their isotopes. Some isotopes form when high-energy cosmic rays crash into atoms in our atmosphere.

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, ranging from seismology, rocks and minerals, oceanography, and Earth system science to astronomy!

Windows to the Universe Community



You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

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

Radioactive Decay

Some materials are radioactive. They give off radiation. When an atom of a radioactive substance gives off radiation, it becomes a new type of atom. This change is called radioactive decay. There are two...more


The text for this level hasn't been written yet. Please check the "Intermediate" or "Advanced" level of this page (click on the bar near the top of this page)....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

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

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...more

Carbon-14 Dating

Carbon-14 dating (also called "radiocarbon dating") is used to determine the age of materials that contain carbon that was originally in living things. It is often used in archeology and some...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