Atoms of the same element have the same number of protons in common, this is call isotope. The number of protons in an atom is used to determine which element the atom comes from. The mass number of an element equals the number of protons plus the number of neutrons. The number written above the chemical symbol is the atomic number. Mole is the SI base unit used to measure the amount of a substance and the unified atomic mass unit is a unit of mass that describes the mass of an atom or molecule.
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