Isotope

The atoms of a chemical element can exist in different types that have very similar behavior, but weigh different amounts. These are called isotopes of the element. Atoms of the same element have the same number of protons, but different isotopes have different numbers of neutrons. Different isotopes of the same element have different masses because they have a different number of neutrons. Mass is the word for how much substance (or matter) something has. Things with different masses also have different weights.

Some isotopes are not stable so they change to another isotope or element by radioactive decay. These are called radioactive isotopes. Others are not radioactive. These are called stable isotopes.

Atoms of the same element have the same number of protons, which is called the atomic number. Different isotopes of an element have different numbers of neutrons. As a result, they also have different mass numbers, which is the number of protons plus the number of neutrons. An isotope is usually named by giving the element and the mass number. For example, the symbol for carbon is C and all carbon atoms have 6 protons. The most common isotope of carbon also has 6 neutrons, giving a mass number of 12, and it is written carbon-12 or ¹²C. The isotope of carbon that has 8 neutrons is written carbon-14 or ¹⁴C.

The word "isotope", meaning "at the same place", comes from the fact that isotopes of the same element are at the same place on the periodic table.