Although the nucleus of an atom is far too small for us to see, here's one way of thinking about an atomic nucleus: as a cluster of tightly packed "balls". The red "balls" represent protons; the blue "balls" represent neutrons. The cloud of electrons that "orbit" an atom's nucleus and define the "size" of an atom is roughly 100,000 times as large as that atom's nucleus!
Click on image for full size
Original artwork by Windows to the Universe staff (Randy Russell).


A neutron is a sub-atomic (meaning it is smaller than an atom) particle. The nucleus of an atom is made up of neutrons and protons. Neutrons and protons are almost exactly the same size (a neutron has about 1/10th of one percent more mass). A neutron does not have an electrical charge, unlike protons (which have a charge of +1) and electrons (which have a charge of -1). Neutrons are much larger than electrons; the mass of a neutron is about 1,839 times that of an electron!

The number of protons in the nucleus of an atom determines what type of element the atom is. The number of protons is called the element's "atomic number". For example, hydrogen has an atomic number of one, since all hydrogen atoms have one proton in their nucleus. Carbon has 6 protons, so its atomic number is 6; oxygen has 8 protons, so its atomic number is 8. Uranium has 92 protons, so its atomic number is 92! If we count the number of protons plus neutrons, we get an atom's atomic mass. Most elements come in different versions, called "isotopes", with different numbers of neutrons. For example, the most common form of carbon is carbon-12 (12C); that isotope of carbon has 6 protons and 6 neutrons, and thus an atomic mass of twelve. Another isotope of carbon, carbon-14 (14C), has 6 protons and 8 neutrons, hence and atomic mass of fourteen. 14C is radioactive and is used to determine how old things are in a technique called "carbon dating".

Neutrons can exist outside of an atoms nucleus. There is a type of particle radiation called neutron radiation.

Neutrons are made up of even smaller particles called quarks. A neutron is made up of two down quarks and one up quark. Particles, like neutrons, made of three quarks are called baryons.

Last modified July 15, 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

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 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...more


Oxygen (O2) is a kind of gas. A lot of the air you breathe is oxygen. That's a good thing, since we need oxygen to stay alive! About 4/5ths of the air in Earth's atmosphere is nitrogen (N2). Almost all...more

Atomic Mass

One way scientists measure the size of something is by its mass. Scientists can even measure very, very tiny things like atoms. One measure of the size of an atom is its "atomic mass". Almost all of the...more


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

Particle Radiation

One main type of radiation, particle radiation, is the result of subatomic particles hurtling at tremendous speeds. Protons, cosmic rays, and alpha and beta particles are some of the most common types...more


Chemistry is the study of matter, energy, and their interactions. Chemists study the composition of substances, their properties, and how they react with each other under varying circumstances. Indeed,...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