Chem Explorers

Sulfur Tetrafluoride: Defying the Octet Rule with a See-Saw Shape

Understanding the structure and properties of molecules is a crucial aspect of chemistry. The molecule SF4 is of particular interest due to its unique structure and bonding properties.

This article aims to provide a comprehensive overview of the Lewis structure, formal charge calculation, shape, and hybridization of SF4.

Lewis Structure Determination

To determine the Lewis structure of SF4, we must first understand the concept of valence electrons. Valence electrons are the outermost electrons in an atom that are involved in chemical bonding.

For the sulfur atom in SF4, the number of valence electrons is six (two in the 3s orbital and four in the 3p orbital), while each fluorine atom contributes seven valence electrons (one from the 2s orbital and six from the 2p orbital). Therefore, the total number of valence electrons in SF4 is 34 (6+4×7).

Next, we need to determine the covalent bonds between each atom. Covalent bonds are formed through the sharing of electrons between atoms.

Each fluorine atom in SF4 requires one electron to complete its octet, while the sulfur atom requires two. Therefore, four fluorine atoms bond with the sulfur atom to form SF4.

The Lewis structure of SF4 shows the arrangement of atoms and electrons in the molecule. To draw the Lewis structure, we can follow these steps:


Write the symbols of the atoms and connect them with a single bond. 2.

Distribute the valence electrons around each atom, starting with the outer atoms first. 3.

Place any remaining electrons on the central atom. The resulting Lewis structure for SF4 is shown below:






Formal Charge Calculation

Formal charge is a measure of the distribution of electrons in a molecule or ion. It helps determine which resonance structure is more stable.

To calculate formal charge, we need to subtract the number of non-bonding electrons and half the number of bonding electrons from the number of valence electrons in the free atom. The formula for formal charge is:

Formal charge = valence electrons – non-bonding electrons – 0.5 x bonding electrons

For SF4, the formal charge of sulfur is zero, while each fluorine atom has a formal charge of -1.

The total formal charge of the molecule is -4, which equals the total number of valence electrons.

Shape and Hybridization of SF4

The shape of a molecule is determined by the arrangement of its atoms and lone pairs. Lone pairs are unshared pairs of electrons that occupy the valence orbitals of an atom.

They repel the bonding electron pairs and affect the molecule’s shape. In SF4, there is one lone pair of electrons on the central sulfur atom.

The four fluorine atoms occupy the equatorial positions around the sulfur atom, while the lone pair occupies the axial position. This arrangement is known as a “see-saw” structure.

The hybridization of SF4 refers to the mixing of atomic orbitals to form new hybrid orbitals. In SF4, the sulfur atom is hybridized to form five sp3d orbitals.

The s and p orbitals from the sulfur atom combine with one d orbital to form five equivalent sp3d orbitals. Four of these orbitals are used to form sigma bonds with the fluorine atoms, while the fifth orbital contains the lone pair of electrons.


In summary, the structure and properties of SF4 can be fully understood by analyzing its Lewis structure, formal charge calculation, shape, and hybridization. The see-saw structure and sp3d hybridization of SF4 are unique and serve as excellent examples for understanding molecular properties and chemical bonding.

Octet Rule and SF4

The octet rule is a fundamental rule in chemistry that states that atoms tend to gain, lose or share electrons to obtain a noble gas configuration. The noble gases have a complete outer shell of electrons and are considered the most stable atoms.

Therefore, atoms strive to attain this stable configuration, either by transferring electrons to form ionic bonds or by sharing electrons to form covalent bonds.

Valence Shell Electron Configuration

The valence shell is the outermost shell of an atom that contains electrons. The valence electrons are the electrons in the valence shell and are involved in chemical bonding.

The valence shell electron configuration refers to the arrangement of valence electrons in an atom. In SF4, sulfur has six valence electrons, and each of the four fluorine atoms brings seven valence electrons.

When they combine, the valence shell of sulfur contains ten electrons. This means that sulfur violates the octet rule because it has more than eight electrons in its valence shell.

SF4 and Octet Rule

Despite violating the octet rule, SF4 is still a stable compound. The stability of SF4 can be understood by examining the structure and bonding of the molecule.

The Lewis structure of SF4 shows that there are two types of bonds in the molecule: sulfur-fluorine single bonds and a sulfur-lone pair bond. This arrangement induces electron pair repulsion that causes the molecule to adopt a trigonal bipyramidal shape.

This shape is not possible under the octet rule, as it does not follow the typical geometry of eight electrons in the valence shell. This deviation from the octet rule can be attributed to the concept of expanded valence shells.

Some atoms, such as elements in periods three and above, have available d orbitals that can be used to accommodate extra electrons. This allows the octet rule to be surpassed, and expand the valence shells of such atoms to accommodate extra electrons.

In the case of SF4, sulfur is an element in period three, which means it has available 3d orbitals in addition to its 3s and 3p orbitals. These orbitals hybridize to form an sp3d hybrid orbital set, which accommodates the extra electrons in the valence shell.

This hybridization allows sulfur to form five sigma bonds with the surrounding atoms, resulting in a total steric number of five. The lone pair occupies an equatorial position to minimize repulsion from the adjacent atoms, leading to the formation of the molecule’s see-saw geometry.

The expanded valence shell in SF4 may compromise the molecule’s stability to some extent, but the hybridization of orbitals on sulfur and the location of the lone pair increase the molecule’s stability. This stability is demonstrated by the fact that SF4 is a useful reagent in organic chemistry, particularly in the synthesis of fluorine-containing compounds.


In conclusion, the octet rule is a crucial concept in chemistry that explains the stability of atoms and molecules. SF4 is a molecule that violates the octet rule but is still energetically favorable and stable.

This is due to the concept of expanded valence shells, which allows some atoms, such as sulfur, to accommodate extra electrons beyond the octet rule and form an sp3d hybrid orbital set. Despite the violation of the octet rule, SF4 has a see-saw geometry that minimizes electron repulsion and increases the molecule’s stability.

Understanding the deviation from the octet rule in molecules such as SF4 is critical to understanding the concept of chemical bonding and molecular geometries. In conclusion, the article has shown how the application of the octet rule, valence shell electron configuration, Lewis structures, formal charge calculations, shape and hybridization can be effectively used to explain the unique properties of SF4.

By violating the octet rule, SF4 has expanded valence shells, which ultimately lead to a unique molecular geometry that increases its stability. The article provides insights into the fundamentals of chemical bonding, molecular geometries, and the deviance from conventional chemical rules.

SF4 is a great example, demonstrating the exceptions to the octet rule and the importance of hybridization in molecular stability.


Q: What is the octet rule?

A: The octet rule is a chemical principle stating that atoms tend to gain, lose or share electrons to obtain a stable configuration with eight electrons in the outermost shell. Q: Why does SF4 violate the octet rule?

A: SF4 has an expanded valence shell beyond eight electrons. It demonstrates that atoms in periods three and beyond can surpass the octet rule to accommodate extra electrons.

Q: Is SF4 stable? A: Although SF4 violates the octet rule, the molecule is still stable due to the hybridization of atomic orbitals that accommodate the extra electrons in the valence shell.

Q: How does hybridization contribute to the stability of SF4? A: Hybridization of sulfur’s orbitals into sp3d hybrid orbitals enables the accommodation of the extra electrons and enables the molecule to form stable bonds with the surrounding atoms.

Q: Why is SF4 useful in organic chemistry? A: SF4 is a useful reagent in organic chemistry because it can selectively add a fluorine group to an organic molecule, a valuable trait for drug discovery.

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