| A | B |
|---|
| molecule | A group of atoms bound or bonded together |
| nucleus | The center of an atom. |
| atomic number | The total of protons in the nucleus. |
| mass number | The total of protons AND neutrons in an atom. |
| isotopes | Atoms with the same number of protons but with different numbers of neutrons. |
| Particle with a negative charge. | electron |
| Particle with no charge. | neutron |
| Particle with a positive charge. | proton |
| atom | Smallest particle of an element. |
| Substance made of only one kind of atom. | element |
