SF4 is the chemical formula for sulfur tetrafluoride or sulfur (IV) fluoride. It is a commonly used fluorinating agent and an oil repellent. It is a colorless, corrosive gas. SF4 is also often used to make pesticides.
Considering the many important applications of this chemical compound, we must be aware of its physicochemical properties as well.
One such property is the chemical nature of SF4 i.e., its polarity. In this article, we will discuss whether SF4 is a polar or a nonpolar molecule.
Is SF4 polar or nonpolar?
Sulfur tetrafluoride (SF4) is a polar molecule. It is made up of four fluorine (F) atoms bonded to a sulfur (S) atom. Due to the electronegativity difference between S and F atoms, each S-F bond in the SF4 molecule is polar.
The asymmetric, seesaw shape of the molecule further endorses bond polarity. Thus, SF4 is a polar molecule overall with a net dipole moment (µ) value of 0.632 D.
Polar molecules have distinct positive and negative poles. But not only that, rather the poles should be asymmetric. That means the overall charge distribution in a molecule should be unbalanced for it to be called a polar molecule.
If the molecular geometry or the shape of a molecule are such that the oppositely charged poles get evenly balanced then such a molecule will be non-polar.
Electrically charged poles develop due to an electronegativity difference between the bonded atoms.
In total, there are three factors that control the polarity of molecules namely :
Electronegativity
Dipole moment
Molecular geometry or shape
How do these three factors contribute in leading to an unbalanced charge distribution in the sulfur tetrafluoride molecule and because of which SF4 is a polar molecule?
Let us find out in a detailed discussion below
Factors affecting the polarity of SF4
Electronegativity
Electronegativity is the tendency of an atom to attract a shared electron pair from a covalent bond.
A covalent bond between two identical atoms is a purely non-polar bond such as H2 or O2. Covalent bonds between two dissimilar atoms with an electronegativity difference between 0.5 to 1.6 units are called polar covalent bonds
An S-F bond in the SF4 molecule is an example of a polar covalent bond. Fluorine (E.N=3.98) is more electronegative than sulfur (E.N= 2.58). Both the atoms have an electronegativity difference of 3.98 -2.58= 1.4 units.
Therefore, according to the definition of polar covalent bonds (given above), an S-F bond is extremely polar. This is called the bond polarity of SF4.
Atom
Electronic configuration
Valence electrons
Sulfur (6S)
1s2 2s2 2p63s23p4
6
Fluorine (9F)
1s2 2s2 2p5
7
Fluorine strongly attracts the shared electron cloud from each S-F bond. Due to this electronegativity difference, charged poles develop in the SF4 molecule.
Fluorine gains a partial negative (Fδ-) charge due to a slight excess of electrons while Sulfur attains a partial positive (Sδ+) charge due to a slight electron deficiency.
Dipole moment
The dipole moment is a vector quantity. It is defined as the product of electrical charge (Q) and bond length (r).
The direction of a dipole moment arrow is from the positive pole of a covalent
bond to a negative pole.
The higher the electronegativity difference between two bonded atoms, the greater the bond polarity thus this bond will have a high dipole moment value.
As each S-F bond in the SF4 molecule is polar so S-F has a specific dipole moment value. The dipole moment of each S-F bond points from Sδ+ to Fδ- as shown in the figure below.
Now let’s see (in the next section) how these individual dipole moments are arranged in the overall SF4 molecule to claim its polarity.
Molecular geometry
As we have already seen, SF4 consists of four S-F covalent bonds. In the SF4 molecule, all the four F atoms achieve a stable octet configuration, each forming a single covalent bond with the central S atom.
As 4 valence electrons of sulfur get consumed in bond formation, there is still a lone pair present on the central S atom.
According to the Valence Shell Electron Pair Repulsion (VSEPR) theory of chemical bonding, SF4 is an AB4E- type molecule. The ideal electronic geometry of an AB5-type molecule with 5 bond pairs (B) around the central A atom is trigonal bipyramidal.
But in the presence of a lone pair (E) as in SF4, the lone pair-bond pair repulsions have a pronounced effect, and the molecule obtains a distorted tetrahedral commonly known as a seesaw shape.
The lone pair occupies an equatorial position while all the bond pairs occupy the vertices of a tetrahedron.
Each S-F bond gets slightly pushed back from the lone pair region to withstand the repulsive effect. Thus, the molecular geometry, in this case, is not a symmetrical tetrahedron rather SF4 attains an asymmetric, distorted tetrahedral/ seesaw shape.
The overall electron cloud is not evenly balanced in this asymmetric seesaw shape. Dipole moments of individual S-F bonds do not get canceled uniformly which makes sulfur tetrafluoride (SF4) a polar molecule with a net dipole moment value of 0.632 D.
There are three different bond angles in the SF4 molecule i.e., 101.6°,173.1° and 187°.
Difference between polar and non-polar molecules
Polar molecule
Non-polar molecule
Atoms having a difference in
electronegativity
Atoms may have the same or different electronegativity values
Unequal charge distribution overall
Equal charge distribution overall
Net dipole moment greater than 0
Net dipole moment equals to zero
Examples include water (H2O), hydrogen sulfide (H2S), ammonia (NH3) and sulfur tetrafluoride (SF4), etc.,
Examples include oxygen (O2), nitrogen (N2), methane (CH4), and carbon tetrachloride (CCl4).
SF4 has polar bonds present because of an electronegativity difference between bonded S and F atoms.
It is due to the asymmetric seesaw shape of SF4 that bond polarities do not get canceled.
Thus, SF4 is a polar molecule overall with a net dipole moment > 0.
Why is SF4 a polar molecule while XeF4 is nonpolar?
One shortcut to determine whether an AB4En molecule is polar or non-polar is to check the lone pairs present on the central atom.
If an odd number of lone pairs are present, the molecules are generally polar. If an even number of lone pairs are present, the molecule is usually a non-polar
molecule.
SF4 has an asymmetric shape with 1 lone pair on sulfur. The dipole moments do not get canceled which makes SF4 a polar molecule overall.
SF4 is a polar molecule while BF3 is nonpolar. In SF4, four F atoms are bonded to an S atom. S uses 4 of its valence electrons for bonding but still has one lone pair.
The lone pair-bond pair repulsions lead to an asymmetric distorted shape of the SF4 molecule. So, it is polar (µ =0.632 D).
Dipole moments of individually polar B-F bonds get canceled in opposite directions thus it (BF3) is non-polar.
All the bonds in SF4 are not equivalent. Why?
The lone pair present on S in SF4 occupies an equatorial position. The 4 bond pairs occupy both equatorial and axial positions in the molecule.
All the bond pairs are pushed back, away from the lone pair. The axial bond pairs are pushed slightly more than the bond pairs lying at equatorial positions.
So, all the S-F bonds are not equivalent. There are different S-F bond lengths and different bond angles present in the SF4 molecule.
Between SF2 and SF4, which one is polar?
Both SF2 and SF4 have polar S-F bonds present and both molecules are overall polar. There are four S-F bond pairs and 1 lone pair on S in the SF4 molecule.
The asymmetric seesaw shape of the molecule leads to distinctly charged poles thus SF4 is polar (µ=0.632 D).
In the SF2 molecule, there are two bond pairs and 2 lone pairs on S. SF2 has an asymmetric bent shape due to lone pair-lone pair and lone pair-bond pair repulsions.
Thus, SF2 is also polar (µ=0.52 D) as the electron cloud distribution in the molecule is not balanced overall.
One 3s electron of sulfur shifts to the empty 3d orbital. The 3s orbital then hybridizes with three 3p orbitals and one 3d orbital to produce five sp3d hybrid orbitals.
These hybrid orbitals are not all equivalent.
There is one set of 2 equivalent orbitals and another second set of 3 equivalent orbitals. One sp3d has two paired electrons which are situated as a lone pair on S while the other four sp3d hybrid orbitals have 1 electron each to form a covalent bond with the F atoms.
The electronic configuration of Fluorine is 1s22s22p5. Fluorine uses its unpaired p orbital to form an S-F sigma bond with sulfur by sp3d-p overlap.
Summary
Sulfur tetrafluoride (SF4) is a polar molecule.
It consists of polar S-F bonds due to an electronegativity difference of 1.4 units between the bonded atoms.
The asymmetric distorted tetrahedron or seesaw shape of the molecule leads to an overall unequal charge distribution.
Dipole moments do not get canceled in the molecule overall.
So SF4 is a polar molecule with a net dipole moment value of 0.632 D.
There are different bond lengths and bond angles at different positions of the distorted SF4 molecule.
Vishal Goyal is the founder of Topblogtenz, a comprehensive resource for students seeking guidance and support in their chemistry studies. He holds a degree in B.Tech (Chemical Engineering) and has four years of experience as a chemistry tutor. The team at Topblogtenz includes experts like experienced researchers, professors, and educators, with the goal of making complex subjects like chemistry accessible and understandable for all. A passion for sharing knowledge and a love for chemistry and science drives the team behind the website. Let's connect through LinkedIn: https://www.linkedin.com/in/vishal-goyal-2926a122b/
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