The melting point is different for different polymorphs. It is pretty common in the earth’s crust and is even found in the green leafy vegetables we eat. The regular Glass that is used in windows, crockery is majorly SiO2. It is white or colorless, odorless, and tasteless. In this article, we will look at the polarity of SiO2, discuss the factors affecting it. We will also see the difference between polar and non-polar molecules and the properties, uses, and hazards of silicon dioxide. So, is SiO2 polar or nonpolar? SiO2 is a non-polar compound because of its linear and symmetrical shape. The bonds in the molecule are polar because the oxygen atom is more electronegative than the silicon atom but due to linear and opposite directions of both bonds, the dipoles of both bonds in SiO2 cancel out each other. Hence, the net dipole moment comes out to be zero, and SiO2 is non-polar. In this, Silicon exists in the +4 oxidation state and O in +2 oxidation. Si is a metalloid, and O is a non-metal. Hence, the bond between them is of covalent type. The molar mass is 60.084 grams.
Difference Between Polar and Non-Polar Molecules
Regarding polar molecules, I have also written an article on BF3. Check out the polarity of BF3.
Factors Affecting Polarity of a Compound
Polar molecules are the molecules that have a positive as well as a negative end which can either be induced or inherent. This definition can sometimes prove to be ambiguous, and hence the concept of dipole moment for determining polarity is more reliable.
- Dipole moment (µ): It is a vector quantity, and it is the product of the charge and distance between the two poles of a dipole. Its SI unit is Debye. It arises due to the difference in electronegativity. µ (Cm) = Q (C) * r (m) where µ is dipole moment, Q is the magnitude of charges, and r is the separation distance between two charges.
- Charge Separation: As seen in the formula of dipole moment, it depends on the charge and charges separation. A partial charge can be developed by induction. The relation between dipole moment and charge separation is linear.
For polar molecules, charge distribution is non-uniform, while for non-polar, it is uniform. One such example of a nonpolar compound is SiH4. Read out the article on the polarity of SiH4. 3. Electronegativity Difference: More electronegative atoms develop a partial negative charge as they attract the shared pair of electrons towards themselves, and lesser electronegative atoms develop a partial positive charge. So, the magnitude of these charges depends on the difference between electronegativities. Higher is the difference; more is the dipole moment. Thus, the electronegativity difference between two atoms is an important factor for the development of dipole moment. 4. The Geometry of Molecules- This also plays a vital role for molecules with more than two atoms. Diatomic molecules are always linear, and just the difference in electronegativity leads to a polar bond and hence a polar molecule. For bigger molecules, the vector sum of bonds is considered. If in SiO2, central atom Si had lone pairs (hypothetical), the shape would have been bent, and the net dipole moment would not have been zero. Symmetric arrangement cancels out the dipole moment.
Why is SiO2 Non-Polar?
The polarity of a compound is determined by the presence or absence of a net dipole moment. As we know, that dipole moment is a vector quantity, i.e., it is affected by both magnitude and direction. The bonds are polar and covalent as the electronegativity for Silicon and oxygen is 1.9 and 3.44, respectively. The compound is ionic if the electronegativity difference is more than 2 and covalent if less than 2. Over here, the difference is 1.54, and therefore, SiO2 is a covalent compound (the difference is less than 2). The direction of dipole moment vectors is from the less electronegative atom to the more electronegative atom. Thus, In SiO2, the vectors are in the opposite direction and due to the symmetry of molecular geometry, the resultant dipole moment comes out to be zero.
Molecular Structure of SiO2
The formula SiO2 is an empirical formula, and it does not describe the actual structure. It is a macromolecular solid. • SiO2 belongs to a class of compounds called silicates. In silicates, the basic unit is a SiO4 tetrahedral molecule. • For SiO2, all the four O-atoms of SiO4 are shared with another Si as seen in the figure, and Si-O-Si bridges are formed. • Even in this molecular structure, the dipole moment of the bonds cancels out each other because of the symmetry, and the structure as a whole is non-polar. • Thus, the Lewis structure of SiO2, in the beginning, is sufficient for explaining various properties, but that is not an accurate picture. • Si-O bonds are stronger than Si=O bonds, and it rarely forms double bonds. • The Lewis structure for the molecular solid is shown.
• All the crystalline forms of silica have this basic structure of SiO4 present, and those polymorphs (one compound existing in multiple crystalline forms) are interconvertible according to the following flowchart.
• Quartz is the most stable polymorph of silica. While quartz is crystalline, quartz glass is amorphous. • The liquid silica cools to form an amorphous glass with a 3-D structure consisting of SiO4 in a random manner (and hence non-crystalline). SiO2 is one of the oxides that can form Glass as it satisfies the necessary conditions.
SiO4 Properties
- Sources- Sand, quartz purification, food items (green beans, bananas, brown rice, etc.)
- Preparation- SiO2 can be prepared by exposure of elemental Si to O2. Amorphous silica is produced in labs by the wet method, which is shown by the equation below. Na2Si3O7 + H2SO4 ⟶ 3SiO2 + Na2SO4 + H2O
- Density- 2.648 g cm-3(much more than water)
- Melting and Boiling point- The temperature required for boiling (1713°C) and melting (2950°C) is relatively high due to silica’s strong network and high stability.
- Solubility – “like dissolves like.” Polar and non-polar molecules can be dissolved in polar and non-polar solvents, respectively. SiO2 is not soluble in water under normal conditions. The network is so strong that bonds do not break for dissolving in water.
- Reaction with HF – HF is a strong mineral acid and can break the strong Si-O bonds. It can dissolve silica. Silica content in a sample is estimated by gravimetric analysis using HF. SiO2 + 4HF ⟶ SiF4 + 2H2O 3SiF4 + 4H2O ⟶ 2H2SiF6 + H4SiO4
- Conducting properties – Dielectric strength of silica is high, and it is used as an insulator in microelectronics. It is a very stable compound.
- The silica nanoparticles are prepared and are very useful as the delivery system of genes, plasmid cells, etc., because of their non-toxic behavior.
SiO2 Uses
Silicon dioxide is one of the most abundantly present oxides on this planet. It occurs naturally in various forms and is very important industrially as well. It is used in-
- Pharmaceutical industry – sedatives, tablets
- Food industry – flavor enhancer, anti-caking agent (its function is to make powdered ingredients less sticky) defoaming, conditioning, fining and chill proofing agent in malt beverages, packaging material, filtration
- Construction – manufacturing concrete, cement, enamel, ceramic, glass, etc.
- Other uses – hydraulic fracturing, production of elemental Silicon, additives in toothpaste. 5. Microelectronics
- Optical fibers in telecommunication
- Chemical industry – manufacture of adhesives, adsorbents, corrosion inhibitors, sealants, porcelain, paint, and dye additives.
- The presence of silica in water lowers the possibility of dementia. Silica supplements are taken to tackle weak bones, heart disease, hair loss, digestive issues, etc.
Fun Fact
It is often seen that old window glass becomes milky. The glass contains silica (quartz polymorph) as one of the major components. Due to continuous exposure to the sun, it gets heated up and exposed to a cool atmosphere at night, it gets converted to a partly crystalline form, and it appears milky.
Conclusion
• SiO2 is a macromolecular solid having SiO4 as the basic unit. It is a simple molecule but appears in various crystalline and amorphous forms with varying properties. • In all the forms, the covalent bonds between Si and O are polar due to the electronegativity difference between them. • The compound as a whole is non-polar because of the cancellation of dipole moment in the opposite direction. • SiO2 is a useful and stable compound and finds its applications in various fields. I hope you enjoyed the chemistry of silica!
