Editing Valence electron (section)

==Valence shell==
The valence shell is the set of [[atomic orbital|orbital]]s which are energetically accessible for accepting electrons to form [[chemical bond]]s.

For main-group elements, the valence shell consists of the ''n''s and ''n''p orbitals in the outermost [[electron shell]]. For [[transition metal]]s the orbitals of the incomplete (''n''−1)d subshell are included, and for [[lanthanide]]s and [[actinide]]s incomplete (''n''−2)f and (''n''−1)d subshells. The orbitals involved can be in an inner electron shell and do not all correspond to the same electron shell or principal quantum number ''n'' in a given element, but they are all at similar energies.<ref name=KW/>

{| class="wikitable"
|-
!Element type||[[Hydrogen]] and [[helium]]||s- and p-blocks<br />([[main-group element]]s)||d-block<br />([[Transition metal]]s)||f-block<br />([[Lanthanide]]s and [[actinide]]s)
|-
!Valence orbitals<ref>{{cite journal
| title = Octacarbonyl Ion Complexes of Actinides [An(CO)8]+/− (An=Th, U) and the Role of f Orbitals in Metal–Ligand Bonding
| first1= Chaoxian |last1=Chi |first2=Sudip |last2=Pan | first3= Jiaye |last3=Jin |first4=Luyan |last4=Meng | first5= Mingbiao |last5=Luo |first6=Lili |last6=Zhao |first7=Mingfei |last7=Zhou |first8=Gernot |last8=Frenking
| journal = [[Chemistry: A European Journal|Chem. Eur. J.]]
| year = 2019
| volume = 25
| issue = 50
| pages = 11772–11784
| doi = 10.1002/chem.201902625
| pmid= 31276242 | pmc= 6772027 |doi-access=free }}</ref>
|
* 1s
|
* ''n''s
* ''n''p
|
* ''n''s
* (''n''−1)d
* ''n''p
|
* ''n''s
* (''n''−2)f
* (''n''−1)d
* ''n''p
|-
![[Electron counting]] rules
|Duet/Duplet rule
|[[Octet rule]]
|[[18-electron rule]]
|32-electron rule
|}

As a general rule, a [[main-group element]] (except hydrogen or helium) tends to react to form a s<sup>2</sup>p<sup>6</sup> [[electron configuration]]. This tendency is called the [[octet rule]], because each bonded atom has 8 valence electrons including shared electrons. Similarly, a transition metal tends to react to form a d<sup>10</sup>s<sup>2</sup>p<sup>6</sup> electron configuration. This tendency is called the [[18-electron rule]], because each bonded atom has 18 valence electrons including shared electrons.

The heavy group 2 elements calcium, strontium, and barium can use the (''n''−1)d subshell as well, giving them some similarities to transition metals.<ref>{{Greenwood&Earnshaw2nd|page=117}}</ref><ref>{{cite journal |last1=Zhou |first1=Mingfei |last2=Frenking |first2=Gernot |date=2021 |title=Transition-Metal Chemistry of the Heavier Alkaline Earth Atoms Ca, Sr, and Ba |url= |journal=Accounts of Chemical Research |volume=54 |issue=15 |pages=3071–3082 |doi=10.1021/acs.accounts.1c00277 |pmid=34264062 |s2cid=235908113 |access-date=}}</ref><ref>{{cite journal |last1=Fernández |first1=Israel |last2=Holzmann |first2=Nicole |last3=Frenking |first3=Gernot |date=2020 |title=The Valence Orbitals of the Alkaline-Earth Atoms |journal=Chemistry: A European Journal |volume=26 |issue=62 |pages=14194–14210 |doi=10.1002/chem.202002986 |pmid=32666598 |pmc=7702052 |doi-access=free }}</ref>