| Quanta: | Fixed amounts; usually referring to fixed amounts of energy absorbed or emitted by matter. |
| Quantum limit: | The shortest wavelength, present in a continuous x-ray spectrum. |
| Quantum mechanics: | Model of the atom based on the wave nature of subatomic particles, the mechanics of electron waves; also called wave mechanics. |
| Quantum numbers: | Numbers that describe energy states of an electron. |
| Quark: | One of the hypothetical basic particles, having charges whose magnitudes are one-third or two-third of the charge on an electron. |
| Q unit: | A unit of energy, used in measuring the heat energy of fuel reserves, equal to 1018 British thermal units, or approximately 1.055x1021 joules. |
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