As important and also useful as the octet dominance is in chemical bonding, there are some well-known violations. This does not typical that the octet preeminence is useless—quite the contrary. Similar to many rules, there are exceptions, or violations.

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


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Although the O atom has actually an octet that electrons, the N atom has actually only seven electrons in that is valence shell. Although NO is a stable compound, the is an extremely chemically reactive, together are most other odd-electron compounds.

Electron-deficient moleculesA molecule with much less than eight electrons in the valence shell of an atom. Represent the 2nd violation come the octet rule. These secure compounds have less than eight electrons roughly an atom in the molecule. The most typical examples are the covalent link of beryllium and boron. Because that example, beryllium can kind two covalent bonds, leading to only four electrons in that is valence shell:


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


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The 3rd violation come the octet preeminence is uncovered in those compounds with an ext than eight electron assigned to their valence shell. These are called expanded valence shell moleculesA molecule with more than eight electrons in the valence shell of one atom.. Such compounds are formed only by main atoms in the third row the the routine table or beyond that have actually empty d orbitals in their valence shells that have the right to participate in covalent bonding. One such compound is PF5. The just reasonable Lewis electron dot diagram because that this compound has the p atom making five covalent bonds:


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Example 9

Identify every violation come the octet rule by drawing a Lewis electron period diagram.

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ClO SF6

Solution

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


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In SF6, the main S atom provides six covalent bonds come the six surrounding F atoms, so the is an broadened valence covering molecule. The Lewis electron period diagram is together follows:


Test Yourself

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

Answer


Key Takeaway

There room three violations to the octet rule: odd-electron molecules, electron-deficient molecules, and also expanded valence shell molecules.