The atomic number, or proton number, of a chemical element is the number of protons found in the nucleus of every atom of that element. The atomic number uniquely identifies a chemical element. It is identical to the charge number of the nucleus. In an uncharged atom, the atomic number is also equal to the number of electrons.
The sum of the atomic number Z and the number of neutrons N gives the mass number A of an atom. Since protons and neutrons have approximately the same mass, the atomic mass of any atom, when expressed in unified atomic mass units is within 1% of the whole number A.
After 1915, the atomic numbers of all known elements from hydrogen to uranium (Z = 92) were examined. But there were seven elements (with Z less than 92) which were not found and therefore identified as still undiscovered, corresponding to atomic numbers 43, 61, 72, 75, 85, 87 and 91. From 1918 to 1947, all seven of these missing elements were discovered.