Isotopes are atoms of the same element that have the same number of protons (atomic number) but a different number of neutrons, meaning that their mass number varies. They are distinct nuclear species of the same chemical element. Isotopes have nearly identical chemical and physical properties, but their nuclear properties vary, making some invaluable for mankind, while others have no practical value. Some isotopes are referred to as "stable," meaning their nuclear makeup does not change with time, while others are "radioactive" and have a probability of spontaneously changing (radioactively decaying) over time.

Isotopes are important in many fields, including nuclear medicine, oil and gas exploration, basic research, and national security. Isotope analysis is the determination of isotopic signature, the relative abundances of isotopes of a given element in a particular sample. Isotope analysis is frequently done by isotope ratio mass spectrometry. For biogenic substances in particular, significant variations of isotopes of C, N, and O can occur. Analysis of such variations has a wide range of applications, such as the detection of adulteration in food products or the geographic origins of products using isoscapes.

Some key points about isotopes include:

• Isotopes are forms of a chemical element with specific properties.
• Isotopes have the same atomic number and position in the periodic table, but differ in nucleon numbers due to different numbers of neutrons in their nuclei.
• All elements have isotopes.
• There are two main types of isotopes: stable and unstable (radioactive) .
• Isotopes can either form spontaneously (naturally) through radioactive decay of a nucleus or artificially by bombarding a stable nucleus with charged particles via accelerators or neutrons in a nuclear reactor.
• Some elements can only exist in an unstable form.
• Hydrogen is the only element whose isotopes have unique names: deuterium for hydrogen with one neutron and tritium for hydrogen with two neutrons.
• The total number of neutrons and protons (symbol A), or mass number, of the nucleus gives approximately the mass measured on the atomic-mass-unit (amu) scale.