Notation Main article: Orbital atomic physics and chemistry in a standard notation used to describe the electron configurations of atoms and molecules. For atoms, the notation contains the definition of atomic orbitals (in the form nl, for example 1s, 2p, 3d, 4f), indicating the number of electrons assigned to each orbital (or orbital set in the same binder) and superscript. For example, hydrogen has one electron in the s of the first layer, so its electronic configuration 1s1 type. Lithium has two electrons in the 1s subshell and one in the 2s subshell (highest energy), so its electronic configuration 1s2 2s1 type (pronounced “a-s-two, two-s-one”). For phosphorus (atomic number 15), we have: 1s2 2s2 2p6 3s2 3P3.For atoms with many electrons, this notation can be quite long as you use a shorthand notation that takes into account that the first sub-layers are equal to those of a noble gas. For example, phosphorus, differs from neon (1s2 2s2 2p6) only by the presence of the third layer. Thus, the electron configuration of phosphorus can be written on the neon as Ne 3s2 3P3. This notation is useful when you consider that most of the chemical properties of elements are determined by the outer layers. The order in which they write the orbitals is given by the relative stability of the orbitals, first write those with lower energy orbital. This means that, although it remains general guidelines, there can be exceptions. Most of the atoms follow the order given by the Madelung rule. Thus, in accordance with this rule, the electron configuration of iron is written as Ar 4s2 3d6.Another possible notation first the orbital grouping with the same quantum number n, so that the configuration of iron is expressed as Ar 3d6 4s2 (grouping the 3d orbital on 3s and 3p that are implicit in the configuration of argon). The superscript 1 of the orbitals occupied by a single electron is not required. It is quite common to see the lyrics of the orbitals written in italics or cursive. However, the International Union of Pure and Applied Chemistry (IUPAC) recommends using normal font, as is done here.