What are isotopes?
Isotopes are different atoms of one element whose nuclei contain the same number of protons but different numbers of neutrons.
The unchanged proton number means that all isotopes of an element have the same atomic number. Their neutron numbers differ, so their mass numbers differ.
Neutral isotopes of one element also contain the same number of electrons and therefore have the same electronic structure.
Each hydrogen isotope contains one proton. Moving from protium to deuterium to tritium increases the neutron count from 0 to 2, giving mass numbers 1, 2 and 3.
An isotope can also be named by joining the element name to its mass number, such as carbon-12 or carbon-14.
Chemical and physical properties
Changing the neutron number does not alter the electron arrangement of a neutral atom, so isotopes of an element show the same chemical behaviour.
Neutrons add to the atomic mass without adding charge. Different isotopes can therefore show small differences in physical properties such as mass and density.
| Feature | Same or different? | Reason |
|---|---|---|
| Number of protons | Same | They belong to the same element |
| Number of neutrons | Different | The neutron count distinguishes the isotopes |
| Electronic structure of neutral atoms | Same | The proton and electron numbers remain equal |
| Chemical properties | Same | The electron arrangements are unchanged |
| Mass | Different | The nuclei contain different numbers of neutrons |