Giant Covalent Structures

What are giant covalent structures?

You have seen how non-metals form covalent bonds to form molecules. Sometimes these molecules are simple and small, like the Chlorine molecule. Chlorine molecule is diatomic, meaning it is made up of 2 atoms. Sometimes, atoms of certain elements come together to form a giant molecule which is made up of millions of atoms. These molecules are called giant covalent structures.

Examples of giant covalent structures are Diamond, Graphite, and Silicon Dioxide. All atoms in a giant covalent structure are bonded by strong covalent bonds, and are arranged in crystal lattices. Their entire structure is repetitions of uniform arrangement of atoms.

Giant Covalent Structures: Properties

Giant covalent structures are different from simple molecules. Although atoms in both types of molecules are bonded with covalent bonds, in simple molecules those bonds only exist between a few atoms. In giant covalent structures, all the atoms in the entire molecule are bonded with strong covalent bonds.

All giant covalent structures have these general properties -

High Melting and Boiling Points

Covalent bonds are very strong. Therefore, to break covalent bonds between any atoms, a lot of energy is required. To melt or boil a substance of simple molecules (like Chlorine), we don't need to break the covalent bonds between the Chlorine atoms, but we only need to break the intermolecular forces between the molecules. However, to melt a giant covalent molecule, we need to break all the covalent bonds between all the atoms. This takes a lot of energy and that is why giant covalent molecules have high melting and boiling points.

Low Electrical Conductivity

Giant covalent structures have low electrical conductivity. They have no free charge carriers and no net charge on the molecule, and so cannot conduct electricity.

Insoluble in Water

Giant covalent molecules are insoluble in water. For a substance to be soluble in water, they have to make strong enough bonds with water molecules that can break their molecular bonds. But the giant covalent molecules are generally inert because of the stable structure and the presence of strong covalent bonds. Due to this, they do not react with water and do not dissolve.

Giant Covalent Structures: Examples

Diamond

We started this article with the mention of Diamond. Everyone knows what diamond is, but do you know how it's made, and what its chemical composition is? Diamond is an allotrope of Carbon. This means that Diamond is completely made up of Carbon atoms! It is a naturally occurring mineral which is formed under the high temperature and pressure conditions found between the Earth's crust and the upper mantle. The giant structure of diamond has Carbon atoms arranged in tetrahedrons. Each Carbon atom forms single covalent bonds with 4 other Carbon atoms. You can see the figure to visualise the structure.

Tetrahedral Arrangement of Carbon Atoms in Diamond | Tutormyself

Properties of Diamond

• The structure of diamond is extremely stable. Combined with the strong covalent bonds between the Carbon atoms, Diamond is the hardest material in the world!
• Diamond is an excellent conductor of heat.
• Diamond is a poor conductor of electricity. This is because there are no free charges in the structure of diamond.
• Diamond has a broad optical transparency. The crystal structure of Diamond allows light ranging from ultraviolet to infrared to pass through.
• low chemical reactivity - The structure of Diamond is very stable and the atoms are tightly packed with strong covalent bonds between them. Thus, they do not react with any substance, and are inert.
• High melting point - 4,027°C.

Diamonds are used in many applications like jewellery, cutting and grinding tools (because of hardness), engraving, polishing, heat sinks, and many other applications. If you're interested in knowing more about Diamond, you can find an article dedicated to it here.

Graphite

Graphite is another allotrope of Carbon, which means it is completely made up of Carbon atoms. The structure of Graphite is very interesting. Each Carbon atom forms only 3 single covalent bonds with 3 other Carbon atoms. The atoms form hexagonal rings, which are joined together and forma layer. The structure of Graphite is formed when many such layers are stacked together. See the figure for better visualisation -

Structure of Graphite | Flash Education

Since each Carbon atom only makes 3 covalent bonds, there is a free unpaired electron left with each atom. This electron is delocalised in the structure of Graphite. This means that those electrons are not associated with any atom in particular and are free to move about. These electrons are essentially charge carriers, and thus facilitate the conductance of electric current. Therefore, Graphite is a good conductor of electricity.

Let's see some other properties of Graphite.

Properties of Graphite

• High melting and boiling point - can withstand 3000^oC without changing chemically in the absence of Oxygen.
• High conductivity of heat and electricity
• Low chemical reactivity
• Soft and Smooth. The layers have weak intermolecular forces between them and can slide over each other.

Graphite is used in pencil leads, as lubricators and electrical contacts in electric motors, for the anode terminal in batteries, and many other industrial applications. You can read in more detail about Graphite here.

Silicon Dioxide

The chemical formula of Silicon Dioxide is SiO₂. It is the main component of sand, and also glass. Silicon Dioxide is also called Silica. Like Diamond, the structure of Silica is tetrahedral.

Tetrahedral arrangement of atoms in Silicon Dioxide | Mathsmadeeasy

Silicon and Oxygen atoms are present in the ratio of 1:2. Each vertex of a tetrahedron has a Silicon atom, and there is an Oxygen atom between any two Silicon atoms.

Properties of Silicon Dioxide

Since its structure is similar to that of Diamond, its properties are also similar to Diamond:

• High hardness.
• High melting point of 1713^oC and high boiling point of 2950^oC.
• Insoluble in water.
• Very low chemical reactivity.

Silicon Dioxide has a very important role in the making of semiconductors. All electronic microprocessor chips have Silicon Dioxide as their component material. They are also used in the manufacturing of adhesives, concrete, and glass. It's also used in the manufacturing of some sedatives.