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| nucleus | the dense, positively charged mass located in the center of an atom |
| Dalton's Theory | Dalton proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided. |
| Thomson's model of the atom, | the negative charges were evenly scattered throughout an atom filled with a positively charged mass of matter. The model is called the “plum pudding” model, after a traditional English dessert. |
| Rutherford's model, | all of an atom's positive charge is concentrated in its nucleus |
| proton | a positively charged subatomic particle that is found in the nucleus of an atom |
| electron | a negatively charged subatomic particle that is found in the space outside the nucleus of an atom |
| neutron | neutral subatomic particle that is found in the nucleus of an atom |
| atomic number | a unique number for each element that equals the number of protons in an atom of that element |
| mass number | the sum of the number of protons and neutrons in the nucleus of an atom |
| isotopes | atoms of a given element that have different numbers of neutrons and different mass numbers |
| energy levels | the possible energies that electrons in an atom can have |
| electron cloud | visual model of the most likely locations for the electrons in an atom |
| orbital | a region of space around the nucleus where an electron is likely to be found |
| electron configuration | the arrangement of electrons in the orbitals of an atom |
| ground state | a state in which all the electrons in an atom have the lowest possible energies |
