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Atomic number
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The atomic number of an atom is the number of protons in the nucleus of the atom.[1][2] The atomic number of an atom defines which element it is. In a neutral atom, the atomic number is equal to the number of electrons orbiting the nucleus.[1] The elements of the periodic table are listed in order of increasing atomic number.[2]
Atomic number is not the same as:
- atomic mass (symbol: ma), which is the mass of a single atom, commonly expressed in unified atomic mass units
- mass number (symbol: A), which is the sum of the number of protons and number of neutrons in the nucleus of an atom
- relative atomic mass (also called atomic weight; symbol: Ar, ), which is the ratio of the average mass per atom of an element from a given sample to 1/12 the mass of a carbon-12 atom.
Atomic number is not changed by the number of electrons or neutrons; it is simply the proton number.
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