The center of an atom, known as the nucleus, contains two types of particles: protons and neutrons.
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Protons are positively charged particles. The number of protons in an atom's nucleus determines the element and is referred to as the atomic number.
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Neutrons have no charge (they are neutral) and contribute to the mass of the nucleus along with protons. The number of neutrons, combined with protons, gives the atomic mass of the atom.
Together, protons and neutrons are known as nucleons. Electrons, which are negatively charged particles, orbit the nucleus but are not found in the nucleus itself.