What Is an Isotope? Atoms, Mass, and Examples

Why does the periodic table list chlorine's mass as 35.5 when you can't have half a particle? The answer is isotopes — and once you understand them, a lot of small mysteries about atoms suddenly make sense.

The short answer: isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Same element, different mass.

A quick atom refresher

Every atom is built from three particles:

  • Protons — positive charge; the number of them defines which element you have (this is the atomic number).
  • Neutrons — no charge; they add mass.
  • Electrons — negative charge; they orbit the nucleus.

The key fact: protons define the element. Change the protons and you have a different element entirely.

What makes an isotope

Isotopes come from changing the neutron count while keeping the protons the same. Because the protons don't change, it's still the same element — it just weighs a little more or less.

We label isotopes by their mass number (protons + neutrons):

  • Carbon-12 has 6 protons and 6 neutrons.
  • Carbon-14 has 6 protons and 8 neutrons.

Both are carbon (6 protons), but carbon-14 is heavier because of the two extra neutrons.

Classic examples

Hydrogen has three well-known isotopes:

IsotopeProtonsNeutrons
Protium (normal hydrogen)10
Deuterium11
Tritium12

Carbon has carbon-12, carbon-13, and carbon-14 — all with 6 protons, differing only in neutrons.

Why isotopes matter

They explain those "decimal" atomic masses. The atomic mass on the periodic table is a weighted average of all an element's naturally occurring isotopes. Chlorine is about 75% chlorine-35 and 25% chlorine-37, which averages to roughly 35.5 — hence the non-whole number.

Some isotopes are radioactive. These are called radioisotopes. Carbon-14, for example, decays at a steady rate, which is the basis of carbon dating for fossils and artifacts. Other radioisotopes are used in medical imaging and cancer treatment.

Do isotopes behave differently?

Chemically, almost identically — chemistry is driven by electrons, and isotopes have the same number of those. They differ slightly in physical properties (heavier isotopes move a bit more slowly), and some are radioactive while others are stable.

Common mistakes to avoid

  • Don't confuse isotopes with ions. An isotope has a different number of neutrons. An ion has a different number of electrons (giving it a charge). Different particles, different concept.
  • Mass number isn't the same as atomic mass. Mass number is the whole-number count of protons + neutrons in one atom; atomic mass (on the table) is the averaged value across isotopes.

FAQ

Do isotopes have the same number of protons?
Yes — that's what keeps them the same element. They differ only in neutrons.

Why isn't atomic mass a whole number?
Because it's a weighted average of an element's isotopes, which have different masses (e.g., chlorine ≈ 35.5).

What is a radioisotope?
An isotope with an unstable nucleus that decays over time, releasing radiation. Carbon-14 is a famous example.

Are isotopes the same element?
Yes. Same number of protons = same element, regardless of the number of neutrons.

The takeaway

Isotopes are simply different "weights" of the same element — same protons, different neutrons. They're why periodic-table masses aren't whole numbers, and why tools like carbon dating exist.

Pair this with The Periodic Table and What Is the Mole? to see where atomic mass comes from.

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