Home > Encyclopedia of Science > Isotope

• Representing isotopes
• Radioactive isotopes
• Applications

Isotopes are two forms of an element with the same atomic number but different mass number. The existence of isotopes can be understood by reviewing the structure of atoms.

All atoms contain three kinds of basic particles: protons, neutrons, and electrons. (Hydrogen is the only exception to this statement; most hydrogen atoms contain no neutrons.) The protons and neutrons in an atom are found in the atomic nucleus, while the electrons are found in the space around the nucleus.

The number of protons in a nucleus defines an atom. Hydrogen atoms all have one proton in their nucleus; helium atoms all have two protons in their nucleus; lithium atoms all have three protons in their nucleus; and so on. The number of protons in an atom's nucleus...