As important and useful as the octet dominion is in chemistry bonding, there are some well-known violations. This go not average that the octet dominance is useless—quite the contrary. As with many rules, there space exceptions, or violations.

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There space three violations to the octet rule. Odd-electron moleculesA molecule v an odd number of electrons in the valence shell of one atom. Stand for the first violation to the octet rule. Although they space few, some secure compounds have an odd number of electrons in their valence shells. V an odd number of electrons, at the very least one atom in the molecule will need to violate the octet rule. Examples of stable odd-electron molecules space NO, NO2, and ClO2. The Lewis electron period diagram because that NO is as follows:


Although the O atom has an octet that electrons, the N atom has only seven electrons in that is valence shell. Back NO is a secure compound, it is very chemically reactive, as are most other odd-electron compounds.

Electron-deficient moleculesA molecule with less than eight electron in the valence shell of one atom. Represent the second violation to the octet rule. These steady compounds have less than eight electrons about an atom in the molecule. The most typical examples room the covalent compounds of beryllium and also boron. Because that example, beryllium can kind two covalent bonds, leading to only four electrons in its valence shell:


Boron frequently makes only three covalent bonds, leading to only six valence electrons approximately the B atom. A well-known example is BF3:


The third violation come the octet ascendancy is discovered in those compound with much more than eight electrons assigned to their valence shell. These are called expanded valence covering moleculesA molecule with more than eight electrons in the valence covering of an atom.. Such compounds are formed only by central atoms in the third row of the regular table or past that have actually empty d orbitals in your valence shells that deserve to participate in covalent bonding. One such compound is PF5. The only reasonable Lewis electron dot diagram for this compound has actually the ns atom making 5 covalent bonds:


Example 9

Identify every violation to the octet ascendancy by illustration a Lewis electron period diagram.

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With one Cl atom and also one O atom, this molecule has 6 + 7 = 13 valence electrons, so that is one odd-electron molecule. A Lewis electron period diagram for this molecule is together follows:


In SF6, the main S atom provides six covalent bonds to the six bordering F atoms, so it is an expanded valence shell molecule. Its Lewis electron period diagram is together follows:

Test Yourself

Identify the violation come the octet rule in XeF2 by illustration a Lewis electron dot diagram.


Key Takeaway

There are three violations come the octet rule: odd-electron molecules, electron-deficient molecules, and expanded valence shell molecules.