Atomic number

The atomic number of a chemical element (atomic species) is the number of protons in the nucleus of the atom, and is characterized uniquely by an integral number Z. Since a proton has a positive charge e (the elementary charge), a chemical element is uniquely identified by its nuclear charge eZ. A list of the elements sorted by atomic number Z can be found in this article.

All elements can be arranged in increasing atomic number in rows, this gives the periodic system of elements. This arrangement was first thought of by the Russian chemist Dmitri Mendeleev, who discovered that chemically similar elements appear in the same columns of the periodic system. The first 94 elements, with atomic number 1 ≤ Z ≤ 94, appear in nature, although the elements with Z = 93, 94 appear only as minute traces. The elements with 95 ≤ Z ≤ 118 are man-made, and very short-lived.