Editing Electron configuration (section)

== Notation ==
{{See also|Atomic orbital}}
Physicists and chemists use a standard notation to indicate the electron configurations of atoms and molecules. For atoms, the notation consists of a sequence of atomic [[Electron shell#Subshells|subshell]] labels (e.g. for [[phosphorus]] the sequence 1s, 2s, 2p, 3s, 3p) with the number of electrons assigned to each subshell placed as a superscript. For example, [[hydrogen]] has one electron in the s-orbital of the first shell, so its configuration is written 1s<sup>1</sup>. [[Lithium]] has two electrons in the 1s-subshell and one in the (higher-energy) 2s-subshell, so its configuration is written 1s<sup>2</sup>&nbsp;2s<sup>1</sup> (pronounced "one-s-two, two-s-one"). [[Phosphorus]] ([[atomic number]] 15) is as follows: 1s<sup>2</sup>&nbsp;2s<sup>2</sup>&nbsp;2p<sup>6</sup>&nbsp;3s<sup>2</sup>&nbsp;3p<sup>3</sup>.

For atoms with many electrons, this notation can become lengthy and so an abbreviated notation is used. The electron configuration can be visualized as the [[core electron]]s, equivalent to the [[noble gas]] of the preceding [[Period (periodic table)|period]], and the [[valence electron]]s: each element in a period differs only by the last few subshells. Phosphorus, for instance, is in the third period. It differs from the second-period [[neon]], whose configuration is 1s<sup>2</sup>&nbsp;2s<sup>2</sup>&nbsp;2p<sup>6</sup>, only by the presence of a third shell. The portion of its configuration that is equivalent to neon is abbreviated as [Ne], allowing the configuration of phosphorus to be written as [Ne]&nbsp;3s<sup>2</sup>&nbsp;3p<sup>3</sup> rather than writing out the details of the configuration of neon explicitly. This convention is useful as it is the electrons in the outermost shell that most determine the chemistry of the element.

For a given configuration, the order of writing the orbitals is not completely fixed since only the orbital occupancies have physical significance. For example, the electron configuration of the [[titanium]] ground state can be written as either [Ar]&nbsp;4s<sup>2</sup>&nbsp;3d<sup>2</sup> or [Ar]&nbsp;3d<sup>2</sup>&nbsp;4s<sup>2</sup>. The first notation follows the order based on the [[Aufbau principle#Madelung energy ordering rule|Madelung rule]] for the configurations of neutral atoms; 4s is filled before 3d in the sequence Ar, K, Ca, Sc, Ti. The second notation groups all orbitals with the same value of ''n'' together, corresponding to the "spectroscopic" order of orbital energies that is the reverse of the order in which electrons are removed from a given atom to form positive ions; 3d is filled before 4s in the sequence Ti<sup>4+</sup>, Ti<sup>3+</sup>, Ti<sup>2+</sup>, Ti<sup>+</sup>, Ti.

The superscript 1 for a singly occupied subshell is not compulsory; for example [[aluminium]] may be written as either [Ne]&nbsp;3s<sup>2</sup>&nbsp;3p<sup>1</sup> or [Ne]&nbsp;3s<sup>2</sup>&nbsp;3p. In atoms where a subshell is unoccupied despite higher subshells being occupied (as is the case in some ions, as well as certain neutral atoms shown to deviate from the [[Aufbau principle#Madelung energy ordering rule|Madelung rule]]), the empty subshell is either denoted with a superscript 0 or left out altogether. For example, neutral [[palladium]] may be written as either {{nowrap|[Kr] 4d<sup>10</sup> 5s<sup>0</sup>}} or simply {{nowrap|[Kr] 4d<sup>10</sup>}}, and the [[Lanthanum|lanthanum(III)]] ion may be written as either {{nowrap|[Xe] 4f<sup>0</sup>}} or simply [Xe].<ref>{{Cite book|last1=Rayner-Canham|first1=Geoff|title=Descriptive Inorganic Chemistry|last2=Overton|first2=Tina|publisher=Macmillan Education|year=2014|isbn=978-1-319-15411-0|edition=6|location=|pages=13–15}}</ref>

It is quite common to see the letters of the orbital labels (s, p, d, f) written in an italic or slanting typeface, although the [[International Union of Pure and Applied Chemistry]] (IUPAC) recommends a normal typeface (as used here). The choice of letters originates from a now-obsolete system of categorizing [[spectral lines]] as "[[Sharp series|'''s'''harp]]", "[[Principal series (spectroscopy)|'''p'''rincipal]]", "[[Diffuse series|'''d'''iffuse]]" and "[[Fundamental series|'''f'''undamental]]" (or "'''f'''ine"), based on their observed [[fine structure]]: their modern usage indicates orbitals with an [[azimuthal quantum number]], {{mvar|l}}, of 0, 1, 2 or 3 respectively. After f, the sequence continues alphabetically g, h, i... ({{mvar|l}}&nbsp;= 4, 5, 6...), skipping j, although orbitals of these types are rarely required.<ref>{{cite web| url=http://scienceworld.wolfram.com/physics/ElectronOrbital.html|year=2007 |first=Eric W.|last= Weisstein|title=Electron Orbital|work=wolfram}}</ref><ref>{{cite book|title=General Chemistry
|first1=Darrell D. |last1=Ebbing|first2= Steven D. |last2=Gammon|url=https://books.google.com/books?id=_vRm5tiUJcsC&pg=PA284 |page=284|isbn=978-0-618-73879-3|date=2007-01-12|publisher=Cengage Learning }}</ref>

The electron configurations of molecules are written in a similar way, except that [[molecular orbital]] labels are used instead of atomic orbital labels (see below).