Silicon dioxide is an important oxide of silicon represented by the chemical formula SiO2. It is transparent to grey solid at room temperature in various crystalline and amorphous forms. SiO2 is also known as silica. It is extensively used in the glass and construction industry.
Many chemists have doubts about the polarity of silicon dioxide (SiO2). So, is SiO2 polar or non-polar?
Continue reading to find out.
Is SiO2 polar or non-polar?
Silicon dioxide (SiO2) is a non-polar molecule. Each Si=O bond in the SiO2 molecule is polar due to an electronegativity difference between the bonded Si and O atoms.
The electronegativity of the oxygen (O) atom is greater than the silicon (Si) atom. The O atoms strongly attract the shared electron pairs in each Si=O bond.
Thus, each Si=O bond in the SiO2 molecule is polar and possesses a dipole moment value.
However, the individual Si=O dipole moments get canceled in the symmetric, linear SiO2molecule. Thus, SiO2 is a non-polar molecule with a net dipole moment equal to 0.
In chemistry, a molecule with an equal charge distribution between different centers of bonded atoms is a non-polar molecule.
It is formed by the covalent bond between two same atoms leading to a symmetric electron density in the molecule overall.
In the case of symmetric molecules, If the dipole moments of individually polar bonds are canceled, the molecule will be non-polar, such as SiO2.
Hence a non-polar molecule has an equal distribution of the electronic charge. Contrarily, if the electronic charge is not evenly distributed over the molecule, in that case, it will be a polar molecule.
The following three factors influence the polarity of any covalent molecule:
Electronegativity.
Dipole moment.
Molecular geometry or shape.
How do these factors influence the non-polar nature of the SiO2 molecule? Continue reading to find out.
Factors affecting the polarity of SiO2
Electronegativity
It is defined as the ability of an atom to attract a shared pair of electrons from a covalent chemical bond.
Electronegativity decreases down the group in the Periodic Table of elements while it increases across a period.
Greater the electronegativity difference between the bonded atoms in a molecule, the higher the bond polarity.
Oxygen belongs to group VI-A (or 16) of the Periodic Table. The electronic configuration of oxygen is 1s22s22p4, so it has six valence electrons for bonding.
On the other hand, silicon belongs to group IV-A (or 14) of the Periodic Table. The electronic configuration of silicon is 1s22s22p63s23p2, so it has four valence electrons.
The two O atoms thus form two double covalent bonds with the central Si atom to complete their stable valence shell configuration.
Two valence electrons consumed in bonding out of the six initially available leaves behind four valence electrons, i.e., two lone pairs of electrons on each O atom in the SiO2 molecule.
Atom
Electronic configuration
Valence electrons
Oxygen (8O)
1s22s22p4
6
Silicon (14Si)
1s22s22p63s23p2
4
Oxygen (E.N = 3.44) is more electronegative than silicon (E.N = 1.9). There is an electronegativity difference of 1.54 units between these two atoms.
Due to this electronegativity difference, the oxygen atom strongly attracts the shared electron cloud from the Si=O bond. The bonded electrons are held close to the oxygen atom in the SiO2 molecule.
The O-atom thus gains a partial negative (Oδ-) charge, while the silicon atom, less electronegative, obtains a partial positive (Siδ+) charge. In this way, oppositely charged poles develop in the SiO2 molecule.
Consequently, each Si=O bond in the SiO2 molecule is polar. This is called the bond polarity of SiO2.
Dipole Moment
The dipole moment is the product of electrical charge (Q) and bond length (r) between two bonded atoms. It is a vector quantity expressed in Debye (D) units.
It is represented by a Greek symbol µ and measures the polarity of a bond.
The dipole moment of any molecule depends on the difference in electronegativity between the bonded atoms. The greater the electronegativity difference, the higher the bond polarity, resulting in a high dipole moment value.
It points from the partial positive (δ+) center to the partial negative (δ–) center of a bond or molecule.
As each Si=O bond in the SiO2 molecule is polar, both bonds have a specific dipole moment value.
The dipole moment of each Si=O bond points from the Siδ+ to Oδ-.
Molecular geometry
As discussed earlier, a silicon dioxide (SiO2) molecule consists of two double Si=O covalent bonds. The overall molecule consists of a total of 16 valence electrons. There are two lone pairs of electrons on each oxygen atom.
According to the Valence Shell Electron Pair Repulsion Theory (VSEPR) theory of chemical bonding, SiO2 is an AX2-type molecule.
To minimize the electronic repulsions between the bonded atoms, the SiO2 molecule has a linear shape with a mutual bond angle of 180°.
As a result, the dipole moment generated on both sides along the Si=O bond will cancel out each other, making it a non-polar molecule.
In conclusion, SiO2 is a non-polar molecule with a net dipole moment value equal to zero.
Difference between polar and nonpolar?
Polar molecule
Non-polar molecule
Atoms must 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 zero
Net dipole moment equals to zero
Examples include water (H2O), ethanol (CH3CH2OH), ammonia (NH3), sulfur dioxide (SO2), bromine trifluoride (BrF3), nitric oxide (NO), bromine pentafluoride (BrF5), etc.
Why is silicon dioxide (SiO2) a non-polar molecule even though it has polar bonds?
The individual Si=O bonds in the SiO2 molecule are polar due to a great electronegativity difference between Si and O atoms.
But the molecule (SiO2) overall is non-polar because it has a symmetric linear geometry.
The individual dipole moments get cancelled in opposite directions. So, net µ becomes zero in the molecule.
Thus, SiO2 is a non-polar molecule with a zero net dipole moment.
Compare the polarities of silicon dioxide (SiO2) and sulfur dioxide (SO2).
SiO2 is a non-polar molecule because the dipole moments of the individual Si=O polar bonds get canceled in the symmetrical linear molecule. So, the net dipole moment becomes zero in the molecule.
Thus, SiO2 is non-polar, while SO2 is a polar molecule.
Is silicon dioxide (SiO2) soluble in water?
It is a general solubility principle that “Like dissolves like”.
Water is a polar molecule, while silicon dioxide is a non-polar molecule.
Water molecules are attracted to other molecules through hydrogen bonding.
Non-polar molecules like silicon dioxide (SiO2) do not have a dipole moment and therefore do not engage in hydrogen bonding. As a result, they are not soluble in water.
Besides, the interaction between the two molecules is insufficient to overcome the strong covalent forces in the giant SiO2 molecule.
Is there a formal charge on the bonded atoms in the SiO2 molecule?
Formal charge of an atom = [ valence electrons – non-bonding electrons- ½ (bonding electrons)]
For oxygen atoms
Valence electrons = 6
Bonding electrons = 4
Non-bonding electrons = 4
∴ The formal charge on the oxygen atoms = 6-4-4/2 = 2-2 = 0
For silicon atom
Valence electrons = 4
Bonding electrons = 8
Non-bonding electrons = 0
∴ The formal charge on the silicon atom = 4-0-8/2 = 4-4 = 0
Thus, no formal charge is present on the silicon dioxide (SiO2) molecule.
Summary
Silicon dioxide (SiO2) is a non-polar molecule.
The individual Si=O bonds are polar due to an electronegativity difference of 1.54 units between the bonded atoms.
The difference in electronegativity between the bonded atoms leads to the formation of dipoles pointing from Siδ+ to Oδ-.
Due to the symmetric linear molecular geometry of SiO2, the individual Si=O dipole moments get canceled in opposite directions.
In conclusion, SiO2 is a non-polar molecule with net µ =0.
The O-Si-O bond angle is 180°.
All the Si=O bonds are equivalent in the linear structure of SiO2, so each Si=O bond length is 161 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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