Main group elements in the third period and below form compounds
that deviate from the octet rule by having more than 8 valence
electrons.
LEARNING OBJECTIVE [ edit ]
Explain why some elements can form an expanded octet
KEY POINTS [ edit ]
Main group elements that form more bonds than would be predicted by the octet rule are called
hypervalentcompounds, and have what is known as an 'expanded octet,' meaning that there are
more than eight electrons around oneatom.
The octet rule can be 'expanded' by some elements by utilizing the d­orbitals found in the third
principal energylevel and beyond. Sulfur, phosphorus, silicon, and chlorine are common
examples of elements that form an expanded octet.
Phosphorus pentachloride (PCl5 ) and sulfur hexafluoride (SF6) are examples of molecules that
deviate from the octet rule by having more than 8 electrons around the central atom.
TERMS [ edit ]
hypervalent molecule
A molecule that contains an atom from a main group elementwhich deviates from the octet rule
by sharing more than eight electrons.
expanded octet
A case where an atom shares more than eight electrons with its bonding partners.
main group element
Elements that are not part of the transition metal block in the periodic table.
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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 their valence levels as a result of bonding. Phosphorus
pentachloride (PCl5), sulfur hexafluoride (SF6), chlorine trifluoride (ClF3), and the
triiodide ion (I3−) are examples of hypervalent molecules.
For the elements in the second period 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 rule without exceptions.
Phosphorus pentachloride
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 third­period 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 3d­orbitals 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.
Sulfur hexafluoride
In the SF6 molecule, the central sulfur atom is bonded to six fluorine atoms, so sulfur has 12 bonding
electrons around it. The overall geometry of the molecule is depicted (tetragonal bipyramidal, or
octahedral), and bond angles and lengths are highlighted.
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
An illustration the Aufbau rules as they are applied to all the elements. Note how the energies of the nth d
orbitals fall between the (n+1) s and (n+1) p orbitals. For example, the 3d orbitals begin to fill after the 4s
orbital is filled, but before electrons populate the 4p orbitals. A similar relation exists with d­ and f­
orbitals.