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Atoms, Elements and Compounds - Macromolecules and metallic bonding

Grade 12IGCSEChemistry

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

🔑Concepts

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Macromolecules (Giant Covalent Structures) consist of a vast network of atoms held together by strong covalent bonds. Examples include Diamond, Graphite, and Silicon(IV) oxide (SiO2SiO_2).

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In Diamond, each carbon atom is covalently bonded to four other carbon atoms in a rigid tetrahedral lattice. Because there are no free electrons, Diamond is an electrical insulator and is extremely hard.

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In Graphite, each carbon atom is bonded to three others in planar hexagonal layers. The fourth valence electron is delocalized between layers, allowing Graphite to conduct electricity. Weak Van der Waals forces between layers allow them to slide, making it a good lubricant.

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Silicon(IV) oxide (SiO2SiO_2) has a structure similar to diamond. Each Silicon (SiSi) atom is bonded to four Oxygen (OO) atoms, and each Oxygen atom is bonded to two Silicon atoms, resulting in a high melting point and hardness.

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Metallic bonding is the electrostatic attraction between a regular lattice of positive metal ions (Mn+M^{n+}) and a 'sea' of delocalized valence electrons.

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The malleability and ductility of metals arise because the layers of positive ions can slide over each other without breaking the metallic bond, as the delocalized electrons move to maintain the attraction.

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Metals conduct electricity and heat due to the mobility of delocalized electrons, which can carry charge or thermal energy through the lattice.

📐Formulae

SiO2SiO_2

C(diamond)→C(graphite)C_{(diamond)} \rightarrow C_{(graphite)}

F=kq1q2r2F = k \frac{q_1 q_2}{r^2}

M→Mn++ne−M \rightarrow M^{n+} + ne^-

💡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 3 of its 4 valence electrons for covalent bonding. The 4th4^{th} electron is delocalized and free to move throughout the structure. In Diamond, all 4 valence electrons are involved in fixed covalent bonds.

Explanation:

Electrical conductivity requires mobile charge carriers. The delocalized electrons in the sp2sp^2 hybridized layers of Graphite fulfill this, whereas the sp3sp^3 structure of Diamond lacks free charge carriers.

Problem 2:

Describe the structure and bonding in Silicon(IV) oxide (SiO2SiO_2) and state one similarity to Diamond.

Solution:

Structure: Each SiSi atom is tetrahedrally bonded to 4 OO atoms, and each OO atom is bonded to 2 SiSi atoms. Similarity: Both have a giant covalent tetrahedral lattice and very high melting points.

Explanation:

The macroscopic properties of SiO2SiO_2 are derived from the strength of the Si−OSi-O covalent bonds and the energy required to break the giant 3D lattice.

Problem 3:

Why does Magnesium (Mg2+Mg^{2+}) have a higher melting point than Sodium (Na+Na^+)?

Solution:

Magnesium ions have a higher charge (2+2+ vs 1+1+) and contribute more delocalized electrons (2e−2e^- vs 1e−1e^-) to the sea. This results in a stronger electrostatic attraction (metallic bond).

Explanation:

The strength of the metallic bond depends on the charge density of the cations and the number of delocalized electrons per atom.

Macromolecules and metallic bonding Revision - Grade 12 Chemistry IGCSE