What is the atomic number of an element?

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Accepted Answer

The atomic number of an element is the number of protons in the nucleus of each of its atoms, and it is what defines the element: every atom with 6 protons is carbon, and every atom with 79 is gold. It is written as Z, shown as the whole number on each tile of the periodic table, and it sets the element's order in the table, which is arranged by increasing atomic number from hydrogen (1) to oganesson (118). In a neutral atom the number of electrons equals the atomic number, so it also fixes the electron configuration and chemical behavior. Atomic number is not the same as atomic mass or mass number: mass number is protons plus neutrons, so isotopes of one element share an atomic number but differ in mass number, as with carbon-12 and carbon-14. Henry Moseley showed in 1913 that ordering by atomic number, not atomic weight, explains the table's pattern. Change the number of protons and you change the element. The atomic number also decides where an atom sits in the periodic table and how many electrons it has when neutral: hydrogen has 1 proton and 1 electron, carbon has 6 and 6, iron has 26 and 26, and uranium has 92 and 92. Ions have the same atomic number as the neutral atom, because they gain or lose electrons, not protons, so Na + is still sodium with atomic number 11. Nuclear reactions can change the atomic number: in alpha decay it drops by 2, in beta decay it rises by 1, and in nuclear fusion or fission the products are different elements. Do not confuse the atomic number (Z) with the mass number (A) or atomic mass. The mass number counts protons plus neutrons in one nucleus, and atomic mass is an average over isotopes. Chemists write isotopes with both numbers, as in carbon-14 (6 protons, 8 neutrons). To find it, read the whole number on the tile.