Atomic number

Isotopes of carbon, nitrogen and oxygen ordered according to atomic number Z{displaystyle Z} and neutral number N{displaystyle N}

In physics and chemistry, the atomic number of a chemical element is the total number of protons each atom of that element has. The conventional symbol and its representation "Z" comes from the German word Atomzahl (atomic number).

It is placed as a subscript to the left of the corresponding element symbol. For example, all the atoms of the element hydrogen have one proton and its Z=1, those of helium have two protons and Z=2, those of lithium three protons and Z=3.

Atoms of different elements have different numbers of electrons and protons. An atom in its natural state is neutral and has an equal number of electrons and protons. A sodium (Na) atom has an atomic number of 11; It has eleven electrons and eleven protons. A magnesium (Mg) atom has an atomic number of 12; It has twelve electrons and twelve protons, and a uranium (U) atom, which has an atomic number of 92, has 92 electrons and 92 protons.

In 1913 Henry Moseley demonstrated the regularity between the values of the wavelengths of X-rays emitted by different metals after being bombarded with electrons, and the atomic numbers of these metallic elements. This fact allowed to classify the elements in the periodic table by increasing atomic order.

Atomic numbers of elements

List of elements in alphabetical order with their atomic numbers:

Atomic numbers on the periodic table

Periodic table with the list of chemical elements (from 1 to 118) and their corresponding atomic numbers:

Chemical properties

Each element has a specific set of chemical properties as a consequence of the number of electrons present in the neutral atom, which is "Z" (the atomic number). The configuration of these electrons is derived from the principles of quantum mechanics. The number of electrons in each element's electron shell, particularly the outermost valence shell, is the primary factor in determining its chemical bonding behavior. Therefore, it is only the atomic number that determines the chemical properties of an element; and it is for this reason that an element can be defined as consisting of "any" mixture of atoms with a given atomic number.