atomic number

Atomic number is the number of protons in one atom of an element. Naturally occurring elements have atomic numbers ranging from 1 (hydrogen) to 92 (uranium). Atomic number (at. no.) has the symbol Z and is also known as the proton number. It is signified by a subscript number before the symbol for the element; for example, the atomic number for carbon is written 6C. Isotopes of an element all have the same atomic number but a different mass number.


The atomic number gives the following information:


  • What the element is: only atoms of the same element have the same atomic number.

  • The element's numbered position in the periodic table; the periodic table is arranged in atomic number order.

  • The number of electrons in the neutral atom.