What is an isotope

An isotope is any form of a chemical element that has the same number of protons in the nucleus, or the same atomic number, but has a different number of neutrons in the nucleus. The result is that two isotopes of the same element have different atomic weights or molecular masses.

In other words, an isotope is one or two or more nuclides that are chemically identical. Isotopes have the same number of protons but differ in their mass number. Their nuclei contain different numbers of neutrons.

For example, hydrogen has zero (0) neutrons, therefore it is defined as an atom with only one proton, with the atomic mass of 1. Hydrogen  1, or ¹H, is a boson, 1p 0n, with a spin of ½ and a parity of 1, and a gas. It has an accurate atomic weight of 1.00782503207, and an abundance of 99.9885%. This isotope is stable and has no decay products. This isotope is the most abundant chemical element or substance in the universe. Many stars are mainly composed of hydrogen in its plasma form, H. Hydrogen 1 has the electronic configuration 1s^1_.

Figure 1: Isotopes for the elements hydrogen (H), helium (He), carbon (C), nitrogen (N), oxygen (O), and fluorine (F) are shown as examples.

Isotopes are classified as “staple isotopes”  or “unstable” radioactive isotopes. Unstable isotopes are atoms that decay until they reach stability. By emitting a nuclear electron (β particle) or a helium nucleus (α particle), and radiation (γ rays) unstable isotopes disintegrate at measurable rates. Unstable or radioactive isotopes are used as tracers, and as radiation or energy sources.

Stable isotopes show no tendency to undergo radioactive decomposition and are therefore used in mass spectrometry, for example, in metabolomics and protoemics during "stable-isotope-labeling" experiments.   

Every element is known to have isotopic forms with almost identical chemical and physical properties.