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Chemistry: Atomic Structure, Bonding, and Periodicity - Giant Covalent Structures (Diamond, Graphite, and Silicon Dioxide)

Grade 8IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Giant Covalent Structures (also known as macromolecules) consist of a massive network of atoms held together by strong covalent bonds extending throughout the entire lattice.

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Diamond: A form of carbon where each CC atom is covalently bonded to 44 other carbon atoms in a rigid tetrahedral arrangement. Because of the strength of the C−CC-C bonds and the lack of free electrons, it is extremely hard and does not conduct electricity.

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Graphite: A form of carbon where each CC atom is bonded to 33 other carbon atoms, forming hexagonal layers. One electron per carbon atom is delocalized, allowing graphite to conduct electricity. The layers are held by weak intermolecular forces, allowing them to slide over each other.

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Silicon Dioxide (SiO2SiO_{2}): Also known as silica or quartz. Its structure is similar to diamond; each silicon (SiSi) atom is bonded to 44 oxygen (OO) atoms, and each oxygen atom is bonded to 22 silicon atoms.

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Physical Properties: These structures have very high melting and boiling points because a large amount of energy is required to break the numerous strong covalent bonds.

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Conductivity: Most giant covalent structures are insulators (like diamond and SiO2SiO_{2}), but graphite is a conductor due to its delocalized electrons (e−e^{-}).

📐Formulae

SiO2SiO_{2}

Ratio in Silica→Si:O=1:2\text{Ratio in Silica} \rightarrow Si:O = 1:2

Melting Point of Diamond≈3550∘C\text{Melting Point of Diamond} \approx 3550^{\circ}C

Melting Point of SiO2≈1710∘C\text{Melting Point of } SiO_{2} \approx 1710^{\circ}C

💡Examples

Problem 1:

Explain why Graphite can conduct electricity while Diamond cannot, even though both are made of Carbon (CC).

Solution:

In Graphite, each carbon atom uses only 33 of its 44 valence electrons for bonding, leaving one delocalized electron (e−e^{-}) per atom. In Diamond, all 44 valence electrons are involved in covalent bonds.

Explanation:

Electricity requires the movement of charged particles. The delocalized electrons in graphite are free to move through the structure, whereas in diamond, the electrons are localized within the covalent bonds, preventing the flow of current.

Problem 2:

Why does Silicon Dioxide (SiO2SiO_{2}) have a high melting point compared to simple molecular substances like Carbon Dioxide (CO2CO_{2})?

Solution:

To melt SiO2SiO_{2}, strong covalent bonds in a giant lattice must be broken, whereas to melt CO2CO_{2}, only weak intermolecular forces must be overcome.

Explanation:

SiO2SiO_{2} exists as a giant covalent lattice where every atom is bonded to others. CO2CO_{2} exists as small, discrete molecules. Even though the C=OC=O bonds are strong, the forces between the CO2CO_{2} molecules are very weak, requiring much less energy to separate.

Problem 3:

Determine the number of covalent bonds formed by a single Silicon atom in a crystal of Quartz.

Solution:

4 covalent bonds4 \text{ covalent bonds}

Explanation:

In the giant covalent structure of SiO2SiO_{2}, the central SiSi atom follows the group 1414 bonding pattern, sharing its 44 valence electrons with 44 different Oxygen atoms to achieve a stable octet.