| What three things make up an Atom? | Protons, Neutrons, and Electrons. |
| 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. |
