Isotope: definition in chemistry
Isotope: Atoms of the same element with different numbers of neutrons, such as carbon-12 and carbon-14.
Isotopes of an element have the same number of protons and electrons, so they behave almost identically in chemical reactions, but they differ in mass. Some isotopes are stable, while others are radioactive and decay over time, which makes them useful for dating fossils, medical imaging and nuclear power. An isotope is named by its mass number, the total number of protons and neutrons, as in uranium-235.
Examples
- Carbon-12 (6 neutrons) and carbon-14 (8 neutrons)
- Hydrogen-1, deuterium (hydrogen-2) and tritium (hydrogen-3)
- Uranium-235 — used as fuel in nuclear reactors
- Iodine-131 — used in medicine to treat thyroid conditions
On the periodic table
The atomic mass shown for each element is an average of its natural isotopes weighted by how common they are, which is why chlorine's is about 35.45 rather than a whole number.