NCI Dictionary of Cancer Terms
The NCI Dictionary of Cancer Terms features 8,105 terms related to cancer and medicine.
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- listen (A-tum)
- The smallest part of a substance that cannot be broken down chemically. Each atom has a nucleus (center) made up of protons (positive particles) and neutrons (particles with no charge). Electrons (negative particles) move around the nucleus. Atoms of different elements contain different numbers of protons, neutrons, and electrons.