Editing Octet rule (section)

== Example: sodium chloride (NaCl) ==
[[File:Ionic bonding animation.gif|right]]

The octet rule is simplest in the case of [[ionic bonding]] between two atoms, one a [[metal]] of low [[electronegativity]] and the other a [[Nonmetal (chemistry)|nonmetal]] of high electronegativity.  For example, [[sodium metal]] and [[chlorine gas]] combine to form [[sodium chloride]], a [[crystal lattice]] composed of alternating sodium and chlorine [[atomic nucleus|nuclei]].  Electron density inside this lattice forms clumps at the atomic scale, as follows.  

An isolated chlorine atom (Cl) has two and eight electrons in its [[principal quantum number|first and second]] electron shells, located near the nucleus.  However, it has only seven electrons in the third and [[valence shell|outermost electron shell]].  One additional electron would completely fill the outer electron shell with eight electrons, a situation the octet rule commends.  Indeed, adding an electron to the produce the [[chloride ion]] (Cl<sup>&minus;</sup>) [[electron affinity|releases]] 3.62&nbsp;[[electron-volt|eV]] of energy.<ref>{{cite book |last1=Housecroft |first1=Catherine E. |last2=Sharpe |first2=Alan G. |title=Inorganic Chemistry |date=2005 |publisher=Pearson Education Limited |isbn=0130-39913-2 |page=883 |edition=2nd}}  Per source, the enthalpy change for
:Cl&nbsp;+ {{e-}}&nbsp;→ Cl<sup>-</sup>
is -349&nbsp;kJ/mol.  Unit conversion performed using [[WolframAlpha|Wolfram{{!}}Alpha]] database, 13 April 2025.</ref>  Conversely, another surplus electron cannot fit in the same shell, instead beginning the fourth electron shell around the nucleus.  Thus the octet rule proscribes formation of a hypothetical Cl<sup>2&minus;</sup> [[ion]], and indeed the latter has only been observed as a [[plasma (physics)|plasma]] under extreme conditions.  

A sodium atom (Na) has a single electron in its outermost electron shell, the first and second shells again being full with two and eight electrons respectively. The octet rule favors removal of this outermost electron to form the Na<sup>+</sup> ion, which [[isoelectronic series|has the exact same electron configuration]] as Cl<sup>&minus;</sup>.  Indeed, sodium is observed to transfer one electron to chlorine during the formation of sodium chloride, such that the resulting lattice is best considered as a periodic array of Na<sup>+</sup> and Cl<sup>&minus;</sup> ions.  

To remove the outermost Na electron and return to an "octet-approved" state [[ionization energy|requires]] a small amount of energy: 5.14&nbsp;eV.<ref>{{harvnb|Housecroft|Sharpe|2005}}, p.&nbsp;880.  Source gives ionization energy of +495.8&nbsp;kJ/mol.  Unit conversion performed using [[WolframAlpha|Wolfram{{!}}Alpha]] database, 13 April 2025.</ref>  This energy is provided from the 3.62&nbsp;eV released during chloride formation, and the [[electrostatic attraction]] between positively-charged Na<sup>+</sup> and negatively-charged Cl<sup>&minus;</sup> ions, which releases a 8.12&nbsp;eV [[lattice energy]].<ref>{{harvnb|Housecroft|Sharpe|2005}}, p.&nbsp;156.  Source gives lattice energy of 783&nbsp;kJ/mol.  Unit conversion performed using [[WolframAlpha|Wolfram{{!}}Alpha]] database, 13 April 2025.</ref>  By contrast, any further electrons removed from Na would reside in the deeper second electron shell, and produce an octet-violating Na<sup>2+</sup> ion.  Consequently, the second ionization energy required for the next removal is much larger&nbsp;&mdash; 47.28&nbsp;eV<ref>{{harvnb|Housecroft|Sharpe|2005}}, p.&nbsp;880.  Source gives ionization energy of +4562&nbsp;kJ/mol.  Unit conversion performed using [[WolframAlpha|Wolfram{{!}}Alpha]] database, 13 April 2025.</ref>&nbsp;&mdash; and the corresponding ion is only observed under extreme conditions.