| A | B |
|---|
| Model | A schematic description of a system that accounts for its known properties |
| Nucleus | The center of an atom, containing the protons and neutrons |
| Atomic number | The number of protons in an atom |
| Mass number | The sum of the numbers of neutrons and protons in the nucleus of an atom |
| Isotopes | Atoms with the same number of protons but different numbers of neutrons |
| Element | A collection of atoms that all have the same number of protons |
| Radioactive isotope | An atom with a nucleus that is not stable |
| Half-life | The time it takes for half of the original sample of a radioactive isotope to decay |
