An atom is built from three bricks: protons and neutrons in the nucleus, electrons around it. The number of protons — the atomic number Z — defines the element: 6 protons is carbon, always; 79 is gold. But the number of neutrons can vary. Two carbon atoms may have 6, 7, or 8 neutrons. These are isotopes: same element, different masses.
A one-sentence definition
Isotopes are atoms of the same element (same Z) that differ in their number of neutrons (hence their mass number A = Z + N).
We write them by giving the mass number: ¹²C, ¹³C, ¹⁴C read as "carbon 12, 13, 14." All have 6 protons; they have 6, 7, and 8 neutrons respectively. The word comes from Greek isos (same) and topos (place): the same box in the periodic table.
Chemically twins, physically distinct
This is the key point. Since isotopes have the same number of electrons and thus the same electron configuration, their chemical properties are nearly identical: they form the same bonds, the same compounds, the same reactions. Carbon-14 slots into CO₂ and glucose exactly like carbon-12.
They do, however, differ physically: mass (hence speed, diffusion — which is what enables enrichment), and above all nuclear stability. Some isotopes are stable, others radioactive.
Three examples that change everything
- Hydrogen. ¹H (protium, 0 neutrons) makes up 99.98 % of hydrogen. ²H (deuterium, 1 neutron) is stable and used to make the heavy water (D₂O) of nuclear reactors. ³H (tritium, 2 neutrons) is radioactive (half-life 12.3 years). It's the only element whose isotopes bear proper names, because their relative masses differ so much (one to threefold).
- Uranium. ²³⁸U (99.27 %) is barely fissile; ²³⁵U (0.72 %) is the fissile isotope that powers reactors and weapons. The entire difficulty of the nuclear industry lies in separating these two chemical twins — enrichment — to raise the ²³⁵U content from 0.72 % to 3–5 %.
- Chlorine. It exists as two stable forms: ³⁵Cl (≈ 76 %) and ³⁷Cl (≈ 24 %). That's why chlorine's atomic mass is not a whole number but 35.45: it's the weighted average of the isotopes' masses by their natural abundance. Every "decimal" atomic mass in the periodic table is explained this way.
What are isotopes used for?
- Dating. Carbon-14, produced continuously in the upper atmosphere, decays with a half-life of 5,730 years. By measuring how much remains in an organic relic, we recover its date of death (Libby's method, Nobel Prize 1960). Uranium-lead couples date rocks over billions of years.
- Medicine. Technetium-99m is the most widely used imaging tracer in the world; iodine-131 treats the thyroid.
- Analytical chemistry. Mass spectrometry separates and identifies molecules by the mass of their isotopes; deuterated solvents (CDCl₃) are indispensable to NMR.
Why it matters
The isotope is the concept that reconciles the periodic table (organized by Z, the chemistry) with the nuclear world (organized by N, the physics of the nucleus). Understanding that an element is not a single mass but a family of isotopes is the gateway to radioactivity, nuclear energy, dating, and isotopic medicine.