What Are Isotopes? Definition, Notation and Examples
Atoms of the same element always share the same number of protons (atomic number), but the neutron count can differ — such atoms are isotopes of each other. Their chemistry is nearly identical; they just differ in mass, and some are stable while others are radioactive.
Definition and notation
Isotopes are atoms of one element with the same protons but different neutrons. They are written as mass numberSymbol, where mass number = protons + neutrons. For example carbon's isotopes ¹²C and ¹⁴C both have 6 protons, with 6 and 8 neutrons respectively.
Common examples
| Element | Isotopes | Note |
|---|---|---|
| Hydrogen | ¹H protium, ²H deuterium (D), ³H tritium (T) | Deuterium → heavy water; tritium is radioactive |
| Carbon | ¹²C, ¹³C, ¹⁴C | ¹⁴C is radioactive, used for radiocarbon dating |
| Uranium | ²³⁵U, ²³⁸U | ²³⁵U is fissile |
Link to relative atomic mass
An element's relative atomic mass is the abundance-weighted average of its natural isotopes, which is why most atomic masses are not whole numbers (e.g. Cl≈35.45 from a mix of ³⁵Cl and ³⁷Cl). That is also why molar mass values carry decimals.
Uses
- Radiocarbon dating of ancient remains.
- Medical tracers and radiotherapy: iodine-131, cobalt-60.
- Nuclear energy: uranium-235 fission.
Which elements are mainly radioactive? See the radioactive elements list.