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| a unit of energy, the work done by a force of one newton when its point of application moves through a distance of one meter in the direction of the force |
| any of the energy levels of a physical system, such as an atom or molecule, that has higher energy than the lowest energy level |
| atomic structure | |
| —n | |
| the concept of an atom as a central positively charged nucleus consisting of protons and neutrons surrounded by a number of electrons. The number of electrons is equal to the number of protons: the whole entity is thus electrically neutral | |