What is atomic number? The charge number of an atomic nucleus is represented by the atomic number or nuclear charge number (sign Z) of a chemical element. This is equivalent to the proton number (np), or the number of protons found in the nucleus of every atom of that element, for conventional nuclei. Ordinary chemical elements can be identified by their atomic number. The atomic number of an ordinary uncharged atom is also equivalent to the number of electrons.
The atomic mass number A for a normal atom is equal to the sum of the atomic number Z and the neutron number N. Because protons and neutrons have roughly the same mass (and the mass of electrons is statistically insignificant for many purposes), and the mass deficiency of nucleon binding is always small in comparison to the nucleon mass, any atom’s atomic mass, when expressed in unified atomic mass units (creating a quantity called “relative isotopic mass”), is within 1% of the whole number A.
Isotopes are atoms with about the same number of atoms but distinct neutron numbers, and therefore different mass numbers. The ordinary isotopic mass of an isotopic combination for an element (called the relative atomic mass) in a specified environment on Earth defines the element’s standard atomic weight. Some more than three-quarters of found natural elements occur as a combination of isotopes (see mono-isotopic elements), and the ordinary isotopic mass of an isotopic combination for an element (called the relative atomic mass) in a described environment on Earth identifies the element’s basic atomic mass. These atomic weights of elements (in contrast to hydrogen) were historically the quantities that scientists could measure in the 19th century.
Before the present synthesis of concepts from chemistry and physics, the customary sign Z simply indicated an element’s numerical position in the periodic table, whose order was then generally, but not totally, comparable with the sequence of the elements by atomic mass. The term Atomzahl (and its English counterpart atomic number) became widely used in this sense only after 1915 when it was suggested and proven that this Z number was both the nuclear charge and a physical feature of atoms.