In
chemistry and
physics, the
atomic number (also known as the
proton number) is the number of
protons found in the
nucleus of an
atom and therefore identical to the
charge number of the nucleus. It is conventionally represented by the symbol
Z. The atomic number uniquely identifies a
chemical element. In an atom of
neutral charge, atomic number is equal to the number of
electrons.
The atomic number, Z, should not be confused with the mass number, A, which is the total number of protons and neutrons in the nucleus of an atom. The number of neutrons, N, is known as the neutron number of the atom; thus, A = Z + N. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes), the atomic mass of an atom is roughly equal to A.
Atoms having the same atomic number Z but different neutron number N, and hence different atomic masses, are known as isotopes. Most naturally occurring elements exist as a mixture of isotopes, and the average atomic mass of this mixture determines the element's atomic weight. The current standard for the atomic mass unit (amu), also termed the dalton (Da) is defined to be exactly 1/12 of the mass of a free (unbound) neutral 12C atom in its ground (lowest-energy) state.[1]
Loosely speaking, the existence of a periodic table creates an ordering for the elements. Such an ordering is not necessarily a numbering, but can be used to construct a numbering by fiat. Dmitri Mendeleev claimed he arranged his tables in order of atomic weight ("Atomgewicht")[2] However, in deference to the observed chemical properties, he violated his own rule and placed tellurium (atomic weight 127.6) ahead of iodine (atomic weight 126.9).[2][3] This placement is consistent with the modern practice of ordering the elements by proton number, Z, but this number was not known or suspected at the time.
