Central, positively charged core of an atom. It consists of positively charged protons and neutral neutrons, known collectively as nucleons, held together by the strong force.
The number of nucleons can range from 1 to about 270, depending on the element. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Some nuclei, especially heavier ones, are unstable, or radioactive (&see; radioactivity), emitting energy in the form of alpha rays (&see; alpha decay), beta rays (&see; beta decay), or gamma rays. The nucleus makes up nearly all the mass but only a minute fraction of the volume of the atom.
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