| ionised atom | <chemistry> An atom that possesses an electrostatic charge as a result of loss or gain of electrons; e.g., H+, Calcium, Cl-, O2-. (05 Mar 2000) |
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| excited atom | An atom possessing more than normal energy as a result of input of energy. See: excited state. Synonym: excited atom. (05 Mar 2000) |
| labelled atom | A radioactive atom, or a stable but rare one, which by its presence in a molecule helps localization or measurement of that molecule. Synonym: tagged atom. (05 Mar 2000) |