| Atoms | The smallest unit of an element. |
| Subatomic | A term that means 'smaller than an atom'. |
| Nucleus | The center of an atom. |
| Proton | A positively charged subatomic particle. |
| Neutron | A subatomic particle with no charge |
| Electron | A negatively charged subatomic particle. |
| Particle | a very small piece of something |
| Electron Cloud | Empty space around the nucleus of an atom where electrons travel |
| Thomson | The man who discovered the electron. |
| Rutherford | The man who discovered the nucleus of an atom. |
| Gold Foil Experiment | The experiment conducted by Ernest Rutherford to discover the nucleus of an atom. |
| Quarks | The particles that make up protons and neutrons. |
| Mass Number | The total number of protons and neutrons in the nucleus of an atom. |
| Isotopes | The term given to atoms of the same element that have different numbers of neutrons. |
| Transmutation | The process that occurs when an atom undergoes radioactive decay. |
| Half-life | In atoms that are radioactive, the time required for half of the element to decay. |
| Decay | To break into smaller pieces. |
| Atomic Number | The number of protons in an atom. |
| Cathode Ray Tube | A Piece of Equipment used by Thomson to discover the electron. |
| Bohr | The man who discovered electron energy levels. |
| Electron Cloud Model | The current model for atomic structure. |
| Beta Particle | The term given to 'loose' electrons that come from radioactive decay. |
| Alpha Particle | The term given to a particle consisting of two neutrons and two electrons, that comes from radioactive decay. |
| Element | A form of matter that cannot be separated into simpler substances by ordinary chemical methods. |