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 Atom | (physics and chemistry) the smallest component of an element having the chemical properties of the element |
| Nucleus | The center of an atom, made of protons and neutrons |
| Proton | positively charged particle in the nucleus of an atom |
| Neutron | a subatomic particle that has no charge and that is located in the nucleus of an atom |
| Electron | negatively charged particle; located outside the atomic nucleus |
| Atomic Number | the number of protons in the nucleus of an atom |
| Atomic Mass | the mass of an atom expressed in atomic mass units |
| Shells | The orbit followed by the electrons around the nucleus. Each shell can contain only a fixed number of electrons. |
| Electron Cloud | The space outside of the nucleus. |
| Valence Electrons | electrons on the outermost energy level of an atom |
| Element | Consists of only one type of atom;cannot be broken down in to a simpler type of matter;can exist as either atoms or molecules |
| Molecule | Made up of two or more atoms joined tightly together. |