It can also be defined as atoms of certain elements with the same proton numbers but with different nucleon numbers.
| Properties of Isotope | |
| Number of proton | equal | 
| Number of neutron | difference | 
| Chemical properties | same | 
| Physical properties | difference | 
Example:
| Element | Name | Proton Number | Nucleon Number | 
| Hydrogen | Hydrogen | 1 | 1 | 
| Deuterium | 1 | 12 | |
| Tritium | 1 | 23 | |
| Oxygen | Oxygen-16 | 8 | 16 | 
| Oxygen-17 | 8 | 17 | |
| Oxygen-18 | 8 | 18 | |
| Carbon | Carbon-12 | 6 | 12 | 
| Carbon-13 | 6 | 13 | |
| Carbon-14 | 6 | 14 | |
| Chlorine | Chlorine-35 | 17 | 35 | 
| Chlorine-37 | 17 | 37 | |
| Sodium | Sodium-23 | 11 | 23 | 
| Sodium-24 | 11 | 24 |