Atomic structure in three lines

An atom consists of a central nucleus surrounded by electrons. The nucleus contains protons and neutrons. The number of protons determines the chemical element: an atom with 6 protons is always carbon, and one with 82 protons is always lead. The number of electrons determines chemical properties and the ability to form bonds.

The number of neutrons, however, can vary for the same element. Two carbon atoms can have 6 neutrons (carbon-12) or 7 neutrons (carbon-13). Chemically, they behave identically—forming the same bonds and participating in the same reactions. Physically, they differ by mass: carbon-13 is slightly heavier than carbon-12.

These variants of the same element that differ in their number of neutrons are called isotopes.

Stable vs. Radioactive: A matter of nuclear stability

Not all isotopes are equal. Some combinations of protons and neutrons form a stable nucleus that can exist indefinitely without changing. Other combinations are unstable; the nucleus seeks to reorganize by emitting particles or radiation. This is radioactivity.

A radioactive isotope decays spontaneously over a period of time—determined by its half-life—to produce a different "daughter" isotope. Carbon-14, for example, is radioactive: it gradually transforms into nitrogen-14 with a half-life of about 5,700 years. Carbon-12 and carbon-13, on the other hand, are stable: they do not change.

Stable
Exist indefinitely.
No radiation.
Used in traceability.
¹²C · ¹³C · ²⁰⁶Pb · ¹²¹Sb
Radioactive
Decay over time.
Emit radiation.
Used in dating.
¹⁴C · ²³⁸U · ²³²Th · ²¹⁰Pb
Distinction between stable and radioactive isotopes. Both families are useful in geochemistry, but for different purposes.

How many stable isotopes exist?

Nature produces about 254 stable isotopes, distributed across 80 chemical elements. Some elements have only one stable isotope, such as arsenic (⁷⁵As), fluorine (¹⁹F), or phosphorus (³¹P). Others have several: lead has four (²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb), antimony has two (¹²¹Sb, ¹²³Sb), and iron has four (⁵⁴Fe, ⁵⁶Fe, ⁵⁷Fe, ⁵⁸Fe).

The more stable isotopes an element possesses with varying relative abundances in nature, the more possibilities it offers for isotopic traceability. This is why lead is the most widely used tracer element in environmental geochemistry.

Radioactive isotopes in geochemistry: Dating

Radioactive isotopes are not without their uses. They are primarily used for dating. Since a radioactive isotope decays at a constant and well-known rate, measuring the ratio between the parent isotope and the daughter isotope in a sample allows researchers to calculate how long the system has been closed. This is how rocks, polar ice, lake sediments, and organic matter are dated.

In environmental traceability, certain short-lived radioactive isotopes like lead-210 (half-life: 22 years) are used to date sediment layers and reconstruct the history of metallic deposits with a resolution of a few years.

Key Takeaways
  • An isotope is a variant of a chemical element that differs by its number of neutrons.
  • Stable isotopes do not change over time and present no radiological risk.
  • Radioactive isotopes decay spontaneously and are primarily used for dating.
  • Metal traceability relies almost exclusively on stable isotopes.
  • The number of stable isotopes an element has determines its utility in traceability.