Atoms consist of protons (positive), neutrons (neutral), and electrons (negative). Protons and neutrons have a relative mass of 1, while electrons have negligible mass. The atomic number is the number of protons, and the mass number is protons plus neutrons. Isotopes are atoms of the same element with different numbers of neutrons. Relative atomic mass (Ar) is the average mass of isotopes, and relative isotopic mass is the mass of a specific isotope relative to carbon-12.
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