Silicon dioxide, also known as silica, is an oxide of the element silicon. It has the chemical formula SiO2. Silicon dioxide is most often found in nature as quartz but also occurs as a component of many minerals, rocks and gemstones. Silicon dioxide is one of the most abundant compounds on Earth and is a major component of sand and most types of glass.
Quick Answer: What is Silicon Dioxide Made of?
Silicon dioxide is made of silicon and oxygen atoms. The silicon atom has four valence electrons that it shares with two oxygen atoms. Each oxygen atom also has two lone pairs of electrons, giving the silicon dioxide molecule an overall formula of SiO2.
Chemical Composition
Silicon dioxide has a 1:2 ratio of silicon to oxygen atoms. The silicon atom has four valence electrons in its outermost shell. It shares each of these electrons with an oxygen atom, forming four single covalent bonds. Meanwhile, each oxygen atom has six valence electrons, with two used to bond with the silicon atom and two remaining as lone electron pairs.
The sharing of electrons between the silicon and oxygen atoms forms a tetrahedral molecular geometry. The Si-O-Si angle is around 144 degrees. This tetrahedral structure and wide angle gives crystalline silicon dioxide or silica many unique physical properties, including hardness and low chemical reactivity.
Chemical Formula
The chemical formula for silicon dioxide is SiO2. This indicates that the molecule contains one silicon atom and two oxygen atoms. There is a 1:2 stoichiometric ratio of silicon to oxygen.
The silicon atom has the atomic number 14, indicating 14 protons in its nucleus. It has a ground state electron configuration of [Ne] 3s2 3p2, meaning it has 10 electrons in the 1st and 2nd shells and 4 valence electrons in the 3rd shell.
Oxygen has the atomic number 8, with 8 protons and 8 electrons. Its ground state configuration is 1s2 2s2 2p4. Oxygen has 6 valence electrons in its outer 2p subshell.
So in silicon dioxide, the silicon atom shares its 4 valence electrons with 2 oxygen atoms. Each oxygen atom shares 2 of its 6 valence electrons with silicon. This satisfies the octet rule and creates a stable molecule with strong covalent bonding.
Bonding and Structure
Silicon dioxide has a tetrahedral structure with single covalent bonds between the silicon atom and four oxygen atoms. Although the SiO2 formula suggests double bonds, each Si-O bond is a separate single covalent bond.
Silicon has empty 3d orbitals that allow for sp3 hybridization of its valence orbitals. This lets the silicon atom form four equivalent sigma bonds oriented in a tetrahedral shape. Meanwhile, the oxygen atoms have unhybridized 2p orbitals to bond with the silicon.
The result is a central silicon atom surrounded tetrahedrally by four oxygen atoms. The O-Si-O bond angle is around 109.5 degrees, as is typical for an ideal sp3 hybridized center. However, the Si-O-Si angle between two tetrahedra is wider at 140-150 degrees due to repulsion of the oxygen lone pairs.
Crystalline vs. Amorphous Structure
Silicon dioxide occurs in both crystalline and amorphous forms. In crystalline silicon dioxide, the tetrahedra join together in regular crystalline patterns like those found in quartz. The tetrahedra form a framework by sharing oxygen atoms between them.
Amorphous silicon dioxide has a random arrangement of tetrahedra. Examples are fused silica glass and silica gel. The random network still provides strength but lacks the order and beauty of crystalline forms.
Occurrences
Silicon dioxide occurs naturally in several forms and minerals. Some of the most common natural occurrences include:
- Quartz – Pure crystallized SiO2, may be transparent or colored varieties like amethyst, rose quartz, smoky quartz.
- Sand – Sediment of eroded quartz and silicate minerals, mostly silicon dioxide.
- Agate – Banded form of microcrystalline quartz SiO2.
- Flint – Compact sedimentary form of silica.
- Jasper – Opaque microcrystalline quartz.
- Obsidian – Volcanic glass with 70-80% SiO2.
- Opal – Hydrated amorphous form of silica.
- Chalcedony – Cryptocrystalline fibrous quartz variety.
Silicon dioxide also occurs as a major component of many silicate minerals and rocks including granite, diorite, gneiss, and sandstone. Silica makes up over 10% by mass of the Earth’s crust.
Production
Silicon dioxide is produced both naturally through geological processes and artificially for industrial applications. Some methods of silicon dioxide production include:
- Mining – Quartz mining from open-pit mines and underground tunnels. Provides raw material for many uses.
- Precipitation – Adding dilute acids to sodium silicate water glass solution causes precipitation of amorphous silicon dioxide.
- Flame Hydrolysis – Silicon tetrachloride gas burned with hydrogen and oxygen produces amorphous silica fume.
- Sol-Gel Process – Forming silica gels from aqueous alkoxide precursor solutions.
Both natural and synthetic amorphous silicon dioxide are purified and condensed to produce silica gel, colloidal silica, and fumed silica nanoparticles. Quartz can be crystallized into high-purity crystalline silicon dioxide for optics and electronics.
Uses
The unique properties of silicon dioxide make it suitable for uses in many industries. Some major uses include:
- Glassmaking – Silica is the primary ingredient in most glass. It provides hardness, chemical stability, and high melting point.
- Semiconductors – Silicon wafers are made from high-purity monocrystalline SiO2 and serve as substrates.
- Refractory Products – The high melting point of silica makes it useful for bricks and ceramics in kilns and furnaces.
- Abrasives – Silicon carbide and silicon nitride abrasive powders rely on SiO2 for hardness.
- Hydraulic Fracturing – Silica sand (frac sand) props open fractures in shale gas drilling.
- Filtration – Silica sand and silica gels are used extensively for filtration.
Silicon dioxide also sees many other applications in adhesives, ceramics, cement, concrete, cosmetics, incendiary devices, insulation, molecular sieves, paints, toothpaste, and more.
Chemical Properties
Some key chemical properties of silicon dioxide include:
- Exhibits high strength covalent Si-O bonding but limited reactivity due to saturation of valence shell electrons.
- Has a high melting point of 1713°C and boiling point of 2230°C.
- Insoluble in water and organic solvents. Soluble in hot aqueous alkali and hydrogen fluoride.
- Amphoteric – Can react as weak acid or weak base in chemical reactions.
- Acts as a network former in glasses with Si-O-Si linkages between tetrahedra.
- Hydroxyl groups allow for hydrogen bonding between silanol groups.
- Forms precipitates and colloids in solution.
- Exhibits surface silanol groups that give adsorption and catalytic activity.
The chemical inertness and thermal stability of silicon dioxide make it useful for many applications. It can withstand high temperatures, pressures, and pH extremes.
Physical Properties
Silicon dioxide exhibits the following physical properties:
- Appearance – Colorless crystalline solid or white powder in amorphous forms.
- Odor – Odorless.
- Density – 2.648 g/cm3 (crystalline), 2.196 g/cm3 (amorphous).
- Melting point – 1713°C.
- Boiling point – 2230°C.
- Refractive index – 1.46.
- Solubility – Insoluble in water and organic solvents.
- Hardness – 7 on the Mohs hardness scale.
The high density, hardness, melting point, and thermal shock resistance of silicon dioxide make it valuable for many applications.
Quartz Varieties
As a crystalline form of silicon dioxide, quartz is found in many different colored varieties based on included impurities:
| Quartz Variety | Color | Impurities |
|---|---|---|
| Rock crystal | Colorless | Pure SiO2 |
| Amethyst | Purple | Iron |
| Rose quartz | Pink | Titanium, iron |
| Smoky quartz | Gray to brown | Aluminum |
| Milky quartz | White | Included air bubbles |
| Citrine | Yellow to orange | Iron |
In nature, quartz crystals often grow in massive formations in veins and geodes. The variety depends on the impurities present during formation.
Silica Gel
Silica gel is a form of amorphous silicon dioxide synthesized from sodium silicate. It exhibits high porosity and adsorption capability. Key properties of silica gel include:
- Amorphous, porous, granular morphology
- Large specific surface area of 800 m2/g
- Pore sizes of 2 to 3 nm
- High adsorption capacity
- Often used as desiccant and adsorbent
- Indicating types change color when saturated
- Can be regenerated by heating to 120°C
The most common use of silica gel is as a desiccant to control humidity. It is also used commercially for chromatography column stationary phases and catalyst support.
Health Effects
Silicon dioxide has low toxicity but lung exposure to fine silica dust can cause silicosis. Possible health effects include:
- Chronic silicosis – Lung inflammation and scarring from silica dust.
- Acute silicosis – Severe rapid onset after heavy exposure.
- Silica toxicity – Cell damage from free radical oxidation.
- Increased risk of tuberculosis, lung cancer, kidney disease.
- Cutting, sawing, drilling silica releases more breathable fine particles.
Quartz and cristobalite dust are classified as carcinogenic for lungs. Safety procedures are followed in mining, fabrication, construction to limit silica exposure.
Conclusion
Silicon dioxide is a compound of silicon and oxygen with the chemical formula SiO2. It occurs abundantly in nature as quartz and is a major component of sand and glass. Silicon dioxide has a tetrahedral structure with silicon at the center bonded covalently to four oxygen atoms. It exhibits high strength, hardness, and chemical resistance. Silicon dioxide has many uses including in glass, cement, abrasives, filtration, and electronics. It also poses health hazards when inhaled as fine crystalline dust.
