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Deviations from the Octet Rule

A hypervalent molecule is a molecule that contains one or more main group elements that bear more

than eight electrons in theirvalence levels as a result of bonding. Phosphorus pentachloride (PCl5), sulfur

hexafluoride (SF6), chlorine trifluoride (ClF3), and the triiodideion (I3−) are examples of hypervalent

molecules.

For the elements in the secondperiod of the periodic table (principal energy level n=2), the s2p6 electrons

comprise the octet, and no d sublevel exists. As a result, the second period elements (more specifically,

the nonmetals C, N, O, F) obey the octet rulewithoutexceptions.

Phosphorus pentachloride

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In the PCl5 molecule, the central phosphorus atom is bonded to five Cl atoms, thus

having 10 bonding electrons and violating the octet rule. The overall geometry of the

molecule is depicted (trigonal bipyramidal), and bond angles and lengths are

highlighted.

However, some of the third-period elements (Si, P, S, and Cl) have been observed to bond to more than

four other atoms, and thus need to involve more than the four pairs of electrons available in an s2p6 octet.

This is possible because for n=3, the d sublevel exists, and it has five d orbitals. Although the energy of

empty 3d-orbitals is ordinarily higher than that of the 4s orbital, that difference is small and the additional d

orbitals can accommodate more electrons. Therefore, the d orbitals participate in bonding with other

atoms and an expanded octet is produced. Examples of molecules in which a third period central atom

contains an expanded octet are the phosphorus pentahalides and sulfur hexafluoride.

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Sulfur hexafluoride

In the !6 molecule, the central sulfur atom is bonded to si" fluorine atoms, so sulfur

has 1# bonding electrons around it. The overall geometry of the molecule is depicted

(tetragonal bipyramidal, or octahedral), and bond angles and lengths are highlighted.

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For atoms in the fourth period and beyond, higher d orbitals can be used to accommodate additional

shared pairs beyond the octet. The relative energies of the different kinds ofatomic orbital reveal that

energy gaps become smaller as the principal energy level quantum number (n) increases, and the

energetic cost of using these higher orbitals to accommodate bonding electrons becomes smaller.

Energies of the highest occupied orbitals of the elements

 $n illustration the $ufbau rules as they are applied to all the elements. %ote ho& the

energies of the nth  d orbitals fall bet&een the (n'1) s and (n'1) p orbitals. !or e"ample,

the d orbitals begin to fill after the s orbital is filled, but before electrons populate the

p orbitals. $ similar relation e"ists &ith d* and f*orbitals.

Source: Boundless. “The Expanded Octet.”Boundless Chemistry. Boundless, 21 Jul. 2015. Retrieved 12

Nov. 2015 from https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/basic-concepts-of-chemical-bonding-9/exceptions-to-the-octet-rule-76/the-expanded-octet-349-1512/