What are Isotopes and Nuclides?

Having a basic understanding of isotopes and nuclides is vital to understanding many aspects of nuclear energy. Here we present a quick and simple review (or preview!) of science class.


Elements are your basic chemical building blocks. They include things like hydrogen, oxygen, sodium, magnesium, iron, titanium..., anything on the periodic table of the elements. Each element on the periodic table has a different number of protons in its atomic nucleus (its dense center). Each element has a few varieties with the same number of protons, but different numbers of neutrons in the nucleus. All isotopes of a particular element act chemically-identically to each other. Figure 1 shows the periodic table of the elements. Each element listed has many (between 2 and 20+) isotopes.

Periodic table of elements (from wikipedia)

Figure 1. The Periodic Table of the Elements

The arrow points to Iron (Fe), which has a few isotopes shown in Figure 2.

Isotopes of Iron

Figure 2.The isotopes of Iron

Make sense? Great. One particularly relevant set of isotopes acting chemically similar but neutronically different are those of the element Uranium, shown below in Figure 3.

Isotopes of Uranium

Figure 3.The isotopes of Uranium

Grammar alert: Isotopes vs. Nuclides

Isotope and nuclide are closely related terms. When one speaks of isotopes, they are referring to the set of nuclides that have the same number of protons. Nuclide is a more general term, referring to a nuclear species that may or may not be isotopes of a single element. Examples:

  • "U-235 is my favorite isotope of Uranium."
  • "Cm-244, Pu-241, and Am-242m are lesser known fissile nuclides."

Many people use them interchangeably, including experts in the field. Just read the MCNP manual!

See Also

  1. Complete Table of Nuclides [wikipedia.org]
  2. High quality periodic table
  3. Table of Nuclides with very much data from the National Nuclear Data Center
  4. Isotope [wikipedia.org]