Sulfur hexafluoride is a colorless gas with the chemical formula SF6. Hexa means six, so in a sulfur hexafluoride molecule, six fluorine atoms are chemically bonded to a sulfur atom.
SF6 is popular as an insulating gas due to its high dielectric strength. It has the unique quality of being able to withstand a strong electric field without becoming conductive.
Is there any role of the chemical properties of SF6 in making it a non-conductor of electricity?
Well, that question can be addressed only if first, we have some know-how of the chemical nature of SF6 such as its polarity.
Is SF6 polar or nonpolar?
Each S-F bond in the SF6 molecule is polar due to an electronegativity difference between sulfur and fluorine atoms. But Sulfur hexafluoride (SF6) is a non-polar molecule overall.
This is because the individual S-F dipole moments get canceled in the symmetric, octahedral SF6 molecule. Thus, SF6 is a non-polar molecule overall with a net dipole moment µ=0.
A covalent bond is polar if it is composed of two dissimilar atoms that have an electronegativity difference greater than 0.5 units, at least. All polar bonds have a specific dipole moment value.
If the dipole moments of individual polar bonds get canceled due to the symmetrical shape and geometry of the molecule then the molecule will be overall non-polar such as SF6.
The charge distribution is equally balanced in a non-polar molecule.
So, there are the following three factors that influence the polarity of covalently bonded molecules:
Electronegativity
Dipole moment
Molecular geometry or shape
In the next section, we will discuss each of the three factors and their relevance in making sulfur hexafluoride (SF6) non-polar. So, continue reading for more insightful information.
Factors affecting the polarity of SF6
Electronegativity
Electronegativity is the tendency of an atom to attract a shared electron pair from a covalent bond.
Polar covalent bonds usually have an electronegativity difference of 0.5 to 1.6 units between the bonded atoms respectively.
Fluorine (F) belongs to group VII A (or group 17) of the Periodic Table. It is a halogen with 7 valence electrons. It is short of a single electron only to achieve a stable octet electronic configuration.
Thus, F has a strong tendency to attract electrons, meaning it has a high electronegativity.
Sulfur (S) belongs to group VI A (or group 16) of the Periodic Table. It has 6 valence electrons. A sulfur atom uses all six of its valence electrons to form six single covalent bonds with fluorine atoms in the SF6 molecule.
Fluorine (E.N =3.98) is more electronegative than sulfur (E.N = 2.58). An electronegativity difference of 1.4 units is present between the two bonded atoms.
So, F strongly attracts the shared electron cloud from each S-F bond in the SF6 molecule. Oppositely charged poles develop in the molecule as each F atom gains a partial negative (Fδ-) charge while the central S atom attains a dense partial positive (Sδ+) charge.
Atom
Electronic configuration
Valence electrons
Sulfur (6S)
1s2 2s2 2p63s23p4
6
Fluorine (9F)
1s2 2s2 2p5
7
The F atoms pull electrons away from each S-F bond. Consequently, each S-F bond in the SF6 molecule is polar. This is called the bond polarity of SF6.
Dipole moment
Dipole moment, represented by a Greek symbol µ is a vector quantity. It is mathematically defined as the product of electrical charge (Q) and charge separation (r).
The charge separation is measured from the center of the positive pole to the center of the negative pole.
The dipole moment arrow points from +Q to -Q as shown in the figure below.
As each S-F bond in the SF6 molecule is polar so all S-F bonds have a specific dipole moment value as well. The dipole moment of each S-F bond points from the Sδ+ to Fδ-.
Molecular geometry
According to the Valence Shell Electron Pair Repulsion (VSEPR) theory of chemical bonding, SF6 is an AB6-type molecule.
To one atom of sulfur in the center, six bond pairs are attached and there is no lone pair on the central atom. The geometry of the molecule is octahedral.
Although there are six fluorine atoms present but these atoms are so aligned that the molecule attains eight (octa means eight) vertices arrangement.
S lies at the center of this regular octahedron while F atoms occupy the corners.
The octahedral molecular geometry of SF6 is perfectly symmetrical. The individual dipole moments of polar S-F bonds get canceled in opposite directions. Thus, SF6 is a non-polar molecule with a zero net dipole moment.
There are two different bond angles present: 180° along the F-S-F linear arrangement at the middle (see the figure below) and 90° where the F atoms are situated at a right angle to the central S.
All the S-F bonds in SF6 are equivalent i.e., of equal length (156 pm).
Difference between polar and non-polar molecules
Polar molecule
Non-polar molecule
Atoms have 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 sulfur hexafluoride (SF6).
Why is SF6 a nonpolar molecule even though it has polar bonds present?
The individual S-F bonds in the SF6 molecule are polar due to a specific electronegativity difference between S and F atoms.
But the molecule (SF6) overall is non-polar because it has a symmetric octahedral geometry.
The individual dipole moments get canceled in opposite directions so net µ becomes zero in the molecule.
Between SF6 and SF4, which one is polar?
SF6 is a non-polar molecule while SF4 is polar. Both molecules contain polar S-F bonds.
But there is a lone pair present on the central sulfur atom in the SF4 molecule. Lone pair-bond pair repulsions lead to an asymmetric distorted shape.
Therefore, SF4 is a polar molecule with a net dipole moment value of 0.632 D.
On the other hand, there is no such lone pair present on the central sulfur atom in SF6. All the valence electrons of S are involved in covalent bonding. As a result, the molecule obtains a symmetric octahedral shape.
The individual dipole moments get canceled making SF6 a non-polar molecule (µ=0) overall.
Which among the following is a polar molecule BF3, NF3, C2H6, and SF6?
Out of all these, only NF3 is a polar molecule. All the other three are non-polar.
NF3 has three bond pairs and a lone pair on nitrogen. NF3 forms an asymmetric trigonal pyramidal shape. The dipole moment effect of individual N-F bonds is not completely canceled so NF3 is a polar molecule with a net dipole moment µ value of 0.24 D.
BF3 has a symmetric trigonal planar shape. There is no lone pair on boron (B). The individual B-F dipole moments get canceled in the molecule overall, so it is non-polar.
The C-C covalent bond has no electronegativity difference in C2H6. Also, there is a small electronegativity difference between C and H atoms in each of its C-H bonds, so these are only weakly polar.
The symmetrical tetrahedral arrangement of all these bonds in the C2H6 molecule further cancels the dipole moment effect so it is also non-polar.
Why there are two different bond angles in SF6?
Four of the six hybrid orbitals of sulfur in SF6 lie in one plane, inclined to each other at 90° during covalent bond formation with the p orbitals of fluorine.
The remaining two are directed perpendicularly, above and below this plane thus there is another bond angle in SF6 i.e., 180°.
During chemical bond formation, one 3s electron of sulfur shifts to its empty 3d orbital.
Similarly, a paired 3p electron gets unpaired and is shifted to another 3d orbital in the excited state.
The half-filled 3s, three 3p, and two 3d orbitals mix to produce 6 sp3d2 hybrid orbitals.
Each of the 6 hybrid orbitals contains one electron each. Each sp3d2 hybrid orbital thus forms a sigma bond with the half-filled p orbital of a fluorine atom.
Summary
Sulfur hexafluoride (SF6) is a non-polar molecule.
The six individual S-F bonds present in the molecule are polar due to an electronegativity difference of 1.4 units between the bonded atoms.
It is due to the symmetric octahedral molecular geometry or shape of SF6 that the individual S-F dipole moments get canceled in opposite directions.
In conclusion, SF6 is non-polar with net µ =0.
The F-S-F bond angles are 180° and 90°.
All the S-F bonds are equivalent in the octahedral structure of SF6, so each S-F bond length is 156 pm.
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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There are two different bond angles present: 180° along the F-S-F linear arrangement at the middle (see the figure below) and 90° where the F atoms are situated at a right angle to the central S.
