 Atomic number, the atomic number or proton number symbol Z of a chemical element is the number of protons found in the nucleus of an atom. It is identical to the charge number of the nucleus. The atomic number uniquely identifies the chemical element. 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, then, gives the mass number E of an atom. Since protons and neutrons have approximately the same mass, and the mass of the electrons is negligible for many purposes, and the mass defect of nuclei on binding is always small compared to the nuclei on mass, the atomic mass of any atom, when expressed in unified atomic mass units making A. Quantity called the relative isotopic mass is within 1% of the whole number A. Atoms with the same atomic number Z but different neutron numbers N, and hence different atomic masses, are known as isotopes. A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes. C-monk was a topic elements and the average isotopic mass of an isotopic mixture for an element called the relative atomic mass in a defined environment on Earth, determines the elements standard atomic weight. Historically, it was these atomic weights of elements in comparison to hydrogen that were the quantities measurable by chemists in the 19th century. The conventional symbol Z comes from the German words all meaning number, which, before the modern synthesis of ideas from chemistry and physics, merely denoted N elements numerical place in the periodic table, whose order is approximately, but not completely, consistent with the order of the elements by atomic weights. Only after 1915, with the suggestion and evidence that this Z number was also the nuclear charge and a physical characteristic of atoms, did the word atoms all and its English equivalent atomic number come into common use in this context.