What are Isotopes and Nuclides?

Casually understanding the concept of isotopes and nuclides is vital to understanding nuclear energy. Here we present a quick and simple review/preview of your high-school 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. Associated with each element are its isotopes. All isotopes of a particular element act just like that element, in terms of chemistry. Recognize the image below as the periodic table of the elements. Each element listed has many (between 2 and 20+) isotopes. Find Iron (Fe) and then proceed to the image showing the isotopes of Iron.

Periodic table of elements (from wikipedia)


Isotopes of Iron

Isotopes of Uranium

One particularly relevant set of isotopes acting chemically similar but neutronically different are those of Uranium, shown below.

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!