Sulfur hexafluoride (SF6) does not follow the octet rule because its central sulfur atom has an expanded octet, accommodating more than eight electrons in its valence shell. This deviation is possible due to the availability of empty d-orbitals in sulfur, allowing it to form more bonds than predicted by the simple octet rule.
Understanding the Octet Rule
The octet rule is a chemical rule of thumb that states atoms tend to form bonds in such a way that they each have eight electrons in their outermost electron shell, giving them a stable electron configuration similar to that of a noble gas. This rule primarily applies to elements in the second period of the periodic table, such as carbon, nitrogen, oxygen, and fluorine.
For more information on the octet rule, you can refer to Wikipedia's explanation of the Octet Rule.
The SF6 Exception: An Expanded Octet
In the SF6 molecule, the central sulfur atom (S) is bonded to six fluorine atoms (F). Sulfur is an element from the third period, which means it has access to empty 3d-orbitals in addition to its 3s and 3p orbitals. This allows sulfur to accommodate more than eight electrons in its valence shell, forming an expanded octet.
Specifically, in SF6:
- Sulfur's ground state electron configuration is [Ne] 3s² 3p⁴.
- To form six bonds with fluorine atoms, sulfur undergoes hybridization, utilizing one 3s, three 3p, and two 3d orbitals to form six equivalent sp³d² hybrid orbitals.
- Each of the six fluorine atoms donates one electron to form a single covalent bond with sulfur.
- This results in the central sulfur atom being surrounded by 12 valence electrons (6 bonds × 2 electrons/bond = 12 electrons).
Since the central atom in SF6 has more than eight electrons in its valence shell, it does not obey the octet rule.
Why Some Atoms Expand Their Octet
Elements from the third period and beyond (e.g., phosphorus, sulfur, chlorine, bromine, iodine, xenon) can exhibit expanded octets for several reasons:
- Availability of d-orbitals: These elements possess empty d-orbitals in their valence shell, which can be used to accommodate additional electrons beyond the s and p orbitals. This allows them to form more than four bonds.
- Larger atomic size: Larger atoms can accommodate more electron density around them without excessive electron-electron repulsion.
- Electronegativity of surrounding atoms: When bonded to highly electronegative atoms (like fluorine or oxygen), the electrons in the bonds are drawn closer to the electronegative atoms. This reduces electron-electron repulsion around the central atom, allowing it to host more bonds.
Other Examples of Molecules with Expanded Octets
SF6 is not unique in violating the octet rule. Many other compounds also exhibit expanded octets:
- PCl5 (Phosphorus pentachloride): Central phosphorus atom has 10 valence electrons.
- SO4²⁻ (Sulfate ion): Central sulfur atom can have 12 valence electrons (depending on resonance structure).
- IF7 (Iodine heptafluoride): Central iodine atom has 14 valence electrons.
- XeF4 (Xenon tetrafluoride): Central xenon atom has 12 valence electrons.
These examples further illustrate that while the octet rule is a useful guideline, it is not universally applicable, particularly for elements beyond the second period.
