What Are Isotopes?
By the Periodixy Editorial Team · Last reviewed July 11, 2026
Isotopes are versions of the same element that differ only in neutron count. Same protons, same electrons, same chemistry — but different masses, and sometimes very different nuclear stability. Carbon-12 quietly builds your body while carbon-14 slowly decays, letting archaeologists date ancient remains.
Same element, different mass
The proton count defines the element; the neutron count can vary. Hydrogen has three natural isotopes: protium (0 neutrons), deuterium (1) and tritium (2). All behave chemically as hydrogen because chemistry is run by electrons, and all three have one electron.
| Isotope | Protons | Neutrons | Mass number | Natural share |
|---|---|---|---|---|
| Carbon-12 | 6 | 6 | 12 | ≈ 98.9% |
| Carbon-13 | 6 | 7 | 13 | ≈ 1.1% |
| Carbon-14 | 6 | 8 | 14 | trace (radioactive) |
Notation
Two standard ways to name an isotope: the hyphen form (carbon-14) and the nuclear symbol form, where the mass number rides as a superscript before the symbol: ¹⁴C. The atomic number can be added as a subscript (₆¹⁴C) but is redundant — carbon is always 6.
Why atomic masses are decimals
The periodic table shows a weighted average of natural isotopes. Chlorine (75.8% Cl-35, 24.2% Cl-37) averages to 35.45 amu. Try re-deriving it in the Isotope Abundance Calculator.
Weighted average practice
Boron is 19.9% B-10 (10.013 amu) and 80.1% B-11 (11.009 amu). Find its average atomic mass.
- (10.013 × 0.199) + (11.009 × 0.801)
- = 1.993 + 8.818
Answer: ≈ 10.81 amu — matching the periodic table.
Stable vs radioactive isotopes
Some neutron–proton combinations are stable forever; others are unstable. A radioactive isotope (or radioisotope) is an isotope whose nucleus is unstable, so it decays over time and emits radiation as it changes into a more stable nucleus. Each radioisotope decays at its own fixed rate, measured by its half-life. Those predictable decay clocks and penetrating radiation give them jobs everywhere:
- Carbon-14 — dating archaeological remains up to ~50,000 years old.
- Uranium-238 → lead — dating rocks over billions of years.
- Technetium-99m — the most common medical imaging isotope.
- Cobalt-60 — sterilising medical equipment and treating tumours.
- Americium-241 — the working heart of household smoke detectors.
Summary
- Isotopes: same protons, different neutrons, same chemistry, different mass.
- Notation: carbon-14 or ¹⁴C; mass number = protons + neutrons.
- Periodic table masses are abundance-weighted averages of isotopes.
- Radioisotopes decay predictably — useful for dating, imaging and treatment.