Isotopes Explained

Click through each step to understand the basics.

What Are Isotopes?
Isotopes are atoms of the same element — meaning they have the same number of protons (atomic number) — but with a different number of neutrons, giving them different mass numbers. Because they're the same element, their chemical properties are nearly identical, but their physical properties or radioactivity can differ because of the mass difference.
The Relationship Between Atomic Weight and Isotopes
The atomic weight shown on the periodic table is a weighted average of the masses of an element's naturally occurring isotopes, based on how abundant each one is. Carbon, for example, exists mostly as carbon-12, with small amounts of carbon-13 and trace amounts of carbon-14, which is why its average atomic weight comes out to 12.011 — a decimal, not a whole number.
Radioactive Isotopes and Half-Life
Some isotopes have unstable nuclei that emit radiation and gradually decay into a different element — these are called radioactive isotopes. Half-life is the time it takes for half of a radioactive isotope's atoms to decay, and each element has its own characteristic value.
Carbon Dating
The atmosphere contains a fixed proportion of the radioactive isotope carbon-14, which living organisms maintain at a constant level in their bodies while alive. After an organism dies, its carbon-14 gradually decreases according to its half-life (about 5,730 years), so measuring how much remains lets scientists estimate the age of artifacts or fossils.
Uses in Medicine
Radioactive isotopes are widely used in medical imaging and treatment. PET (positron emission tomography) scans use radioactive isotopes to visualize metabolic activity in the body, and some radioactive isotopes are used directly in radiation therapy to destroy cancer cells.
The Difference Between Stable and Radioactive Isotopes
Stable isotopes have nuclei stable enough that they don't decay and exist permanently, while radioactive isotopes decay into a different element over time. Hydrogen, for example, has the stable isotopes hydrogen-1 and hydrogen-2 (deuterium), as well as the radioactive isotope hydrogen-3 (tritium).
Isotope Notation
Isotopes are distinguished by writing the mass number above and to the left of the element symbol — carbon-14 is often written as ¹⁴C, or referred to by name followed by the mass number. From this notation alone, you can immediately read the number of protons from the element symbol, and the number of neutrons by subtracting the atomic number from the mass number.

Why It's Worth Knowing About Isotopes

Why the atomic weight listed for an element on the periodic table isn't a whole number, and how scientists determine the age of an old fossil, can both be explained by the concept of isotopes. Understanding that atoms of the same element can differ in mass or radioactivity depending on their neutron count helps connect the dots across chemistry, earth science, and medicine.

How Electrons Participate in Bonding Is Also Worth Exploring

While isotopes are about differences in the nucleus (protons and neutrons), how an atom connects with other atoms is determined by the behavior of its electrons — worth exploring separately as a related but distinct concept covering ionic, covalent, and metallic bonds.

Frequently Asked Questions

Do isotopes also differ in chemical properties?

Barely. Isotopes have the same number of outer-shell electrons, so they participate in chemical reactions the same way, but because their mass differs, reaction speed can vary slightly, and physical properties like boiling point can differ a bit too.

Does every element have radioactive isotopes?

Most elements have naturally occurring stable isotopes alongside radioactive ones, or radioactive isotopes can be created artificially. However, some elements with very high atomic numbers have no stable isotopes at all — every isotope of those elements is radioactive.