This article is about the polarity of sulfur dioxide (SO2) a highly reactive gas that belongs to the group called ‘oxides of sulfur’. It is a strong reducing agent that is commonly used as a food preservative, disinfectant, and bleaching agent.
Now coming to the main point i.e., is the SO2 molecule polar or nonpolar in nature. Well, it is a polar molecule. Let’s find out how.
Is SO2 polar or non-polar?
Sulfur dioxide (SO2) is a polar molecule. There is a difference in electronegativity between sulfur (S) and oxygen (O) atoms which makes each S=O bond in the SO2 polar. The asymmetrical, bent, or V-shaped geometrical structure of SO2 maintains the polarity of the molecule intact. Thus, SO2 is a polar molecule with a net dipole moment greater than zero.
Name of molecule
Sulfur dioxide (SO2)
Bond type
Polar covalent
Molecular geometry
Bent, angular, or V-shape
Polar or Non-polar?
Overall polar molecule
Dipole moment
Non-zero (1.62 D)
Bond angle
119°
Here we hinted at three different factors i.e., electronegativity, molecular geometry, and dipole moment that together make an SO2 molecule polar.
Below is a more in-depth discussion on the effect of each factor in controlling SO2 polarity.
The concepts of electronegativity, dipole moment, and geometrical shape are some of the most important factors in determining the polarity of a chemical bond. So let us discuss all these factors one by one.
Electronegativity
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 its atoms have an electronegativity difference between 0.5 and 1.6.
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.
A double covalent bond between an S and an O atom is formed when each atom shares two electrons. There are two S=O bonds present in an SO2 molecule. In this way, there are four bond pairs and one lone pair on the central S atom in the SO2 molecule.
The electron density is not equally shared on both sides. Oxygen is more electronegative than sulfur, so it more strongly attracts the shared electron cloud from each S=O bond. Therefore, each S=O bond is polar.
The central sulfur has a partial positive charge while each oxygen carries a partial negative charge.
Atom
Electronegativity
Electronegativity difference
Sulfur (S)
2.58
3.44 – 2.58 = 0.86
Oxygen (O)
3.44
We have shown you the Lewis structure of SO2. However, we would like you to keep in mind that this Lewis structure is a hybrid of two equivalent resonance structures.
Resonance structures are different ways of drawing a Lewis structure, based on the movement of electrons from one position to another in a molecule.
Dipole moment
Dipole moment (µ) measures the separation between opposite electrical charges. It is defined as the product of charge (Q) and bond length (r).
⇒ µ = Q. r
According to the difference in electronegativity between sulfur and oxygen atoms. Each S=O bond is polar, and it has a dipole moment equal to 0.82 Debye.
The dipole moment is a vector quantity. Its direction is from the positive to the negative center of a polar bond. Thus, in S=O, it directs from the Sδ+ atom to Oδ- atom.
Molecular geometry
According to the Valence Shell Electron Pair Repulsion (VSEPR) theory of chemical bonding, SO2 is an AB2E type molecule. Each S=O is considered one bond pair. So, there are two bonds (B) pairs and 1 lone (E) pair around the central sulfur (A) atom.
The lone pair situated on sulfur repels the bonded pair of electrons. Lone pair-bond pair repulsions are greater than bond pair-bond pair repulsions.
So, the lone pair pushes the two bond pairs closer to each other, and the bond angle in an SO2 molecule decreases.
This contributes to the bent or V-shape of the SO2 molecule. 119° bond angle is present in the polar SO2 molecule as opposed to 120° present in trigonal planar molecules with three bond pairs and no lone pair.
The overall geometry of the SO2 molecule is asymmetrical. The additive effect of the dipole moments of two S=O bonds (black arrows) is shown by a blue arrow(dipole along the bonds will add up). Thus, SO2 is an overall polar molecule with a net dipole moment approx. 1.62 D.
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 oxygen atom is more electronegative than the sulfur atom. So, dipoles occur from S to O.
The shape of SO2 is bent which is asymmetrical, hence, dipole-generated along the bond(S to O) will not cancel out, it will add up to give 1.6 D.
Because of some permanent net dipole moment in the molecule, the overall SO2 molecule is polar in nature.
“SO2 is polar in nature because of the difference in electronegativity between sulfur and oxygen atoms. The greater the difference in electronegativity more will be the polarity of the molecule.”
What is the shape of SO2 and how does it affect the polarity?
SO2 has a trigonal planar electronic geometry but the lone pair present on the sulfur central atom repels the S=O bond pairs, therefore, the O-S-O bond angle reduces, as a result, SO2 diverts from the ideal trigonal planar geometry and attains an angular, bent, or V-shape.
So, the shape of SO2 is bent which is asymmetrical, therefore, dipoles will not cancel each other, thus, there will be some permanent net dipole moment in the molecule, hence, SO2 is a polar molecule.
Why is SO2 polar covalent compound?
SO2 is a polar covalent compound. Because the electronegativity difference between sulfur and an oxygen atom is 0.82 which is in the range of the Pauling scale of the polar compound from 0.4 to 1.7.
Also, the unequal sharing of electrons occurs in SO2 molecules because oxygen(3.4) is higher electronegative than a sulfur atom(2.58), hence, it will pull more electrons towards itself, hence, the oxygen atom induced a negative charge on it and sulfur atoms get a slightly positive charge.
The separation of charges generates the dipole moment directed from positive to negative, hence, due to all these a bond is formed between sulfur and oxygen, which is called a polar bond.
What is the hybridization of sulfur dioxide?
Sulfur dioxide (SO2) has sp2 hybridization. Two sp2 hybrid orbitals of S form two sigma bonds with the p orbitals of oxygen while one sp2 hybrid orbital is situated as a lone pair on the S atom.
Two sp2 hybrid orbitals contain a single unpaired electron while the third contains two electrons. Two sp2 hybrid orbitals thus form a sigma bond with one p orbital of oxygen on each side of S=O.
The third hybrid orbital stays as a lone pair on sulfur. The unbonded electrons present in the unhybridized 3p and a 3d orbital of sulfur however form a pi (π) bond with oxygen on each side of S=O.
Why is SO2 polar but CO2 not polar?
Both SO2 and CO2 have polar bonds present in their molecules. SO2 has a lone pair of electrons. It has an asymmetric, bent, or V-shape therefore, the dipole moments of its polar S=O bonds do not get canceled rather they add up to make SO2 a polar molecule.
CO2 has a linear molecular geometry. There is no lone pair of electrons present on central carbon. The dipole moments of its polar C=O bonds get canceled in opposite directions, so it is a non-polar molecule.
Contrarily the asymmetric, bent shape of SO2 molecules keeps their polarity intact.
Which one is polar between SO2 and H2O?
Both SO2 and H2O have polar bonds present and both molecules are polar. The difference in electronegativity between S and O or between O and H atoms makes S=O and O-H bonds polar respectively.
Both molecules (H2O and SO2) have an asymmetric bent shape, hence, a net dipole moment > 0 in both, and both molecules are polar.
The bond angle in SO2 however is 119° while that in H2O is 104.5° because there are two lone pairs on central O in H2O while only one lone pair on central S in SO2.
The valence bond theory (VBT) of chemical bonding tells us that the Lone pair – lone pair repulsions > lone pair-bond pair repulsions > bond pair-bond pair repulsions.
Summary
Sulfur dioxide (SO2) is a polar molecule.
SO2 has polar bonds present due to the difference in electronegativity between S and O atoms which leads to an unequal charge distribution in each S=O bond.
The asymmetric, V or bent shape of the SO2 molecule leads to a net dipole moment greater than zero thus SO2 is a polar molecule overall.
The central sulfur atom in the SO2 molecule is sp2 hybridized. There are two bond pairs and one lone pair around S.
The lone pair repels the bond pairs therefore SO2 obtains an angular shape with a bond angle of less than 120°. S=O bond length in SO2 is 143.1 pm. The molecule has a net dipole moment of 1.62 D.
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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