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Atoms and Molecules




Bonds and Octet Rules
Chemical Equations
The Mole




Functional Groups
Periodic Table


Practice Quiz I
Practice Quiz II
Practice Quiz III


The proton is one of the three principal sub-atomic particles in the atom. Protons are positively charged particles. The magnitude of the positive charge on a proton is equal to the magnitude of the negative charge on an electron. Thus, the positive charge of one proton completely cancels out the negative charge of one electron. Protons are located in the nucleus, where they are compacted tightly against each other along with the third sub-atomic particle, neutrons.

A proton in an atom

This is an imaginary construct being used for illustrative purposes.

The specific identity of an atom depends on the number of protons in the nucleus. This number is called the atomic number (Z). If you know the number of protons in an atom, you can identify what atom you are looking at. For example, all atoms with 13 protons (Z = 13) must be aluminum, while all atoms with 92 protons (Z = 92) must be uranium. The number of electrons does not determine the atom's identity as an element. If you have two atoms that both have 13 protons, but one has 13 electrons while the other has 10 electrons, the two atoms are still both aluminum atoms (the former, a neutral aluminum atom, and the latter, a charged aluminum atom) because atoms are defined by their protons, not their electrons.

Since atoms usually have an equal number of protons and electrons, all the positive charges of the protons cancel out all the negative charges of the electrons, rendering atoms neutral. However, atoms lose and gain electrons constantly in chemical changes, and so it is possible for atoms to have an excess positive or negative charge to them.

An atom that has a charge due to unequal amounts of protons and electrons is called an ion. If there are more protons than electrons in an atom, the atom will have an excess of positive charge and is called a cation (positive ion, for example, the aluminum atom, described previously, having 13 protons and 10 electrons). If there are more electrons than protons in an atom, the atom will have an excess of negative charge and is called an anion (negative ion).

While protons and electrons have equal but opposite charges, protons are much heavier than electrons—approximately 2000 times heavier. Because of this difference, electrons do not make a significant contribution to the mass of an atom.

The masses of atoms are expressed in relative mass units called atomic mass units (amu's). One amu is defined as 1/12 of the mass of the isotope of carbon (C-12) that has six protons and six neutrons in its nucleus and whose mass is exactly 12 amu. On this relative scale, a proton has a mass of 1 amu (a neutron also has a mass of 1 amu).

Atom Mass Unit relative to a gram

The discovery of the proton occurred in 1914 when New Zealander-English physicist Ernest Rutherford used a version of Sir J.J. Thomson's cathode ray tube modified by German scientist Eugene Goldstein. Goldstein's tube was identical to Thompson's, except that in Goldstein's tube one of the metal pieces had holes in it. This modification caused the tube to produce not only electrons when exposed to electricity but also positively charged particles called protons, each nearly 2000 times heavier than the electron.