1. Molecules come in all shapes and sizes
Covalent bonds join atoms together in different ways, forming small and big molecules. While this article is about the big, to appreciate the uniqueness and enormity of giant molecular structure, let’s first recap on the small.
Oxygen gas in the air has a simple molecular structure. Each diatomic molecule is really small, with just two atoms per molecule.
There are bigger simple molecules, like the happiness-inducing sucrose found in all things sweet. Each sucrose molecule, C12H22O11 has 45 atoms.
Even bigger? We have the protein hexokinase, one of the many enzymes in charge of converting your sucrose-rich brown sugar milk tea into energy! Each molecule of the protein has 16000 atoms.
What about the hugest?! Unlike simple molecules of oxygen, sucrose, and hexokinase protein, diamond has a giant molecular structure. A one-carat diamond weighing 0.2 g has 10,000,000,000,000,000,000,000 carbon atoms, all covalently bonded to each other to form one giant molecule that extends vastly.
Giant molecular structures are an extensive network of atoms joined together by strong covalent bonds.
So how do the many carbon atoms join up? Let’s zoom in on a carbon atom of diamond to find out.
2. In diamond, each carbon atom bonds covalently with four other atoms
Carbon is a non-metal from Group IV, with 4 valence electrons. It is short of 4 electrons to complete its octet and achieve noble gas electronic configuration.
Focusing on the central carbon atom, it achieves the noble gas electronic configuration by forming 4 single covalent bonds with 4 surrounding carbon atoms. The surrounding carbon atoms can still form more single covalent bonds with yet more carbon atoms. This will extend the network of covalent bonds between many carbon atoms, hence establishing a giant molecular structure.
As all the electrons of every carbon atom are involved in covalent bonding, there is no free-moving electron to conduct electricity.
With neither free-moving electrons or ions, diamond does not conduct electricity.
3. A giant molecule of extremes: high melting point, insane hardness, and impossible to dissolve
Unlike simple molecules, diamond has a very high melting point. To melt diamond is to break the covalent bonds between carbon atoms. They are very strong, hence requiring a large amount of energy to break.
Diamond is hard, in both senses of the word. It is hard to own with its sky-high price. It is also the hardest material, owing to the strong covalent bonds between carbon atoms that take extraordinary force to break. This physical property makes diamond an excellent material for cutting tools.
In other news, it is insoluble in both water and organic solvents. It’s weird to see your precious diamond dissolve huh.
Diamond has a high melting point, is hard, and cannot dissolve in water. This is because of the strong covalent bonds between its carbon atoms that take a large amount of energy to break.
4. A cousin of diamond: graphite as another allotrope of pure carbon
Besides diamond, carbon atoms can be arranged differently to give another allotrope: graphite. The different structures give rise to vastly different physical properties.
However, graphite and diamond have the same chemical composition and properties. They comprise carbon atoms that can oxidise at high temperature to form carbon dioxide gas.
Allotropes are different structural forms of an element.
5. Delocalised, wild and free: graphite conducts electricity
In graphite, each carbon atom uses just 3 of its valence electrons to form 3 single covalent bonds with 3 other carbon atoms. The remaining electron is delocalised and can move about freely.
Due to the presence of delocalised, free-moving electrons, graphite can conduct electricity.
6. Hippity hoppity, graphite is so slippery
While the covalent bonds within a layer are strong, we can easily overcome the weak intermolecular forces of attraction between layers by applying a small force. Therefore, graphite is slippery.
This is why we make pencil “lead” with graphite. When we write, the layers of graphite come off easily, leaving a thin layer of black graphite we call pencil mark. Furthermore, we make lubricants with graphite, like the one to oil bicycle chain.
7. Copycat properties: high melting point and impossible to dissolve
Within each extensive layer of carbon atoms in graphite, the strong covalent bonds require a large amount of energy to break, hence conferring graphite a high melting point.
Also, like diamond, graphite is insoluble in water and organic solvents. This is during paper chromatography, we mark the starting line with pencil. The graphite marking will not dissolve in the solvent to affect the results.