atomic number
Sign in to savenumber of protons found in the nucleus of an atom
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The atomic number is simply the count of protons in an atom's nucleus, and it's the fundamental property that defines what chemical element an atom is. This number matters because it determines an atom's chemical behavior and its place on the periodic table—atoms with the same atomic number are always the same element, whether they have different numbers of neutrons or not.
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Article
The atomic number or nuclear charge number (symbol Z, from the German Zahl, "number") of a chemical element is the charge number of its atomic nucleus. For ordinary nuclei composed of protons and neutrons, this is equal to the proton number (np) or the number of protons found in the nucleus of every atom of that element. The atomic number can be used to uniquely identify ordinary chemical elements. In an ordinary uncharged atom, the atomic number is also equal to the number of electrons.
For an ordinary atom which contains protons, neutrons and electrons, the sum of the atomic number Z and the neutron number N gives the atom's atomic mass number A. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of the nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in daltons (making a quantity called the "relative isotopic mass"), is within 1% of the whole number A.