Sulfur dioxide (SO2) and sulfur trioxide (SO3) are both commonly known oxides of sulfur, extremely valuable in the chemical industry. Both belong to the same group. Both consist of oxygen atoms bonded to a sulfur atom. Does that mean SO2 and SO3 possess similar properties as well? Definitely not.
One major contradiction that boggles a chemist’s mind is that SO2 is a polar molecule while SO3 even though having polar bonds present is still non-polar in nature. Why is that and what factors make sulfur trioxide (SO3) non-polar. Let’s find out through this article.
Is SO3 polar or nonpolar?
Sulfur trioxide (SO3) has polar bonds present but the symmetric, trigonal planar shape of SO3 lead to the cancelation of bond polarities in opposite directions, thus, SO3 isan overall non-polar molecule. It has a net dipole moment equal to zero.
Name of molecule
Sulfur trioxide (SO3)
Bond type
Polar covalent
Molecular geometry
Trigonal planar
Polar or Non-polar?
Overall Nonpolar molecule
Dipole moment
Zero
Bond angle
120º
Now, let’s discuss the three factors that affect the polarity of a molecule with relevance to SO3. These three factors are:
Electronegativity is defined as the ability of an atom to attract a shared pair of electrons from a covalent bond. A covalent bond is polar if the bonded atoms have an electronegativity difference between 0.5 and 1.6.
Both sulfur (S) and oxygen (O) atoms belong to group VII-A is also known as group 16 of the Periodic Table. There are six valence electrons present in both atoms.
A sulfur trioxide molecule is formed when one sulfur atom forms three double covalent bonds with 3 oxygen atoms on each side. The electronegativity of sulfur and oxygen is 2.58 and 3.44 respectively.
This makes a difference in electronegativity of 0.86 units between the two atoms. The electronegativity difference is greater than 0.5, thus, each S=O bond in the SO3 molecule is polar. This is called the bond polarity of SO3.
As we mentioned already, both sulfur and oxygen atoms belong to group VII-A of the Periodic Table. The electronic configuration of sulfur is 1s22s22p63s23p4, so it has six valence electrons. On the other hand, the electronic configuration of oxygen is 1s22s22p4, thus it also has six valence electrons available for bonding.
Atom
Electronic configuration
Valence electrons
Sulfur (16S)
1s2 2s2 2p6 3s2 3p4
6
Oxygen (8O)
1s2 2s2 2p4
6
Dipole moment
The dipole moment is a vector quantity that represents the polarity of a bond and/or a molecule.
It has a symbol µ and is defined as the product of electrical charge (Q) and charge separation i.e., bond length (r). Debye (D) is the symbol used for dipole moment.
⇒ µ= Q.r
Due to the electronegativity difference between sulfur and an oxygen atom, oxygen has greater control over the electron cloud shared between S and O in a S=O bond.
It strongly attracts the shared electron cloud; thus, it gains a partial negative (Oδ-) charge while sulfur develops a partial positive (Sδ+) due to a slight electron deficiency. As a result, the dipole moment of each S=O bond points from Sδ+ to Oδ-.
Each S=O bond in the SO3 molecule is polar with a net dipole moment of 0.86 D.
Molecular geometry
Now that we know each S=O bond in the SO3 molecule is polar with a definite net dipole moment, the question that arises is how is SO3 molecule non-polar overall. Well, this is because of the symmetrical arrangement of atoms in the sulfur trioxide molecule that makes it non-polar.
According to the Valence Shell Electron Pair (VSEPR) theory of chemical bonding, SO3 is an AB3 type molecule. It has a symmetrical trigonal planar molecular geometry with mutual bond angles of 120°.
Three bonds (B) pairs surround the central (A) sulfur atom and there is no lone pair present on sulfur. Due to this symmetrical arrangement, the individual dipole moments of each S=O bond get canceled in opposite direction.
The combined polarity effect of two S=O pointing downwards (blue arrow) gets canceled by the polarity of one S=O bond pointing upwards (red arrow) thus a net dipole moment equal to zero and SO3 is an overall non-polar molecule.
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), ethanol (CH3CH2OH), ammonia (NH3) and sulfur dioxide (SO2), etc.,
Examples include oxygen (O2), nitrogen (N2), methane (CH4), and sulfur trioxide (SO3).
The bonds in SO3 are polar because the oxygen atom is more electronegative than sulfur.
The shape of SO3 is symmetrical trigonal planar geometry, therefore, the polarity of three S=O bonds gets canceled.
This lead to a net dipole moment in SO3 equal to zero, therefore, the overall SO3 molecule is nonpolar in nature although the bonds in it are polar.
What is the molecular geometry of SO3 and how does it affect its polarity?
The molecular geometry of SO3 is trigonal planar which is symmetrical, the three S=O bonds lie in the same plane each 120 degrees away from the others.
The trigonal planar geometry results in the cancellation of the polarities along the bond, and a non-polar SO3 molecule is formed.
What are the dipole moment and formal charge in SO3?
SO3 has a net dipole moment of zero. No formal charge is present on an SO3 molecule.
The oxidation state or valency of oxygen in SO3 is -2 while that of sulfur is +6. Three oxygen atoms = 3(-2) = -6. This -6 gets canceled with +6 so no formal charge overall.
Which of the following is non-polar? a) CO2 b) SO2 c) SO3 and d) NH3
Both CO2 and SO3 are non-polar. These have polar bonds present, but the bond polarities get canceled overall due to a symmetric arrangement of atoms. CO2 has a symmetric, linear shape while SO3 has a symmetric trigonal planar shape and geometry.
On the other end, both SO2 and NH3 are polar molecules due to their asymmetric bent and triangular pyramidal shapes respectively leading to a net dipole moment greater than zero.
Both SO2 and SO3 are oxides of sulfur with polar bonds present in their structures. It is because of the asymmetric, bent shape of SO2 caused by the presence of lone pair around the central S atom that SO2 is a polar molecule.
On the other hand, SO3 is a typical trigonal planar molecule that is symmetrical. The individual bond polarities get canceled in this symmetrical structure thus SO3 is a non-polar molecule.
Differences between SO2 and SO3 polarity?
Sulfur dioxide (SO2)
Sulfur trioxide (SO3)
Polar molecule
Non-Polar molecule
sp2 hybridization, trigonal planar electronic geometry but a bent shape
sp2 hybridization, trigonal planar geometry, and shape
One lone pair and two bond pairs
Three bond pairs and no lone pair
Asymmetric structure
Symmetric structure
119° bond angle
120° bond angle
Net dipole moment > 0
Net dipole moment =0
Why does sulfur disobey the octet rule when forming molecules such as SO2 or SO3?
Sulfur (S) belongs to period 3 of the Periodic Table. There is a small energy difference between 3p and 3d orbitals of sulfur. Electrons from 3s and 3p orbitals can easily migrate to 3d.
In this way, sulfur can expand its octet and can accommodate a maximum of 10 to 12 valence electrons.
There are ten electrons present around sulfur in the Lewis structure of SO2 while the Lewis structure of SO3 shows a total of twelve electrons present around sulfur.
Define lewis’s structure of SO3?
The Lewis structure of SO3 is shown below. Here, we can see that all six electrons of the central sulfur and each oxygen are consumed in covalent bond formation. The total number of valence electrons in an SO3 molecule is 24.
Each oxygen has a complete octet while sulfur disobeys the octet rule as it has a total of 12 valence electrons surrounding it in the SO3 molecule.
Whose bond length is greater – SO2 or SO3?
The O-S-O bond length is slightly greater in SO3 as compared to SO2. This is because the bond order of SO2 (1.5) is higher than the bond order of SO3 (1.33).
Bond length is inversely proportional to bond order. In simple terms, bond order is referred to here as the strength of a S=O bond. Greater the bond strength, the shorter the bond length.
Summary
SO3 is an overall non-polar molecule with polar bonds present in it.
Due to the electronegativity difference between S and O atoms, each S=O bond in the SO3 molecule is polar with a dipole moment value of 0.86 D.
The symmetric molecular geometry of SO3 leads to the cancelation of bond polarities in opposite directions making SO3 a non-polar molecule overall with a zero net dipole moment.
SO3 has an sp2 hybridization and a trigonal planar geometry with mutual bond angles of 120°.
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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Each S=O bond in the SO3 molecule is polar with a net dipole moment of 0.86 D.
