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Atoms, Elements and Compounds - Covalent bonding

Grade 11IGCSEChemistry

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

🔑Concepts

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Covalent bonding occurs when two non-metal atoms share one or more pairs of electrons to achieve a stable outer shell (noble gas configuration).

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A single covalent bond consists of one shared pair of electrons (11 pair = 22 e−e^-), represented as a single line, e.g., H−HH-H.

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Multiple bonds occur when atoms share more than one pair: double bonds share two pairs (O=OO=O) and triple bonds share three pairs (N≡NN \equiv N).

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Simple molecular structures (e.g., Cl2Cl_2, H2OH_2O, CH4CH_4) have strong covalent bonds within molecules but weak intermolecular forces (Van der Waals forces) between molecules, leading to low melting and boiling points.

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Giant covalent structures (macromolecules) such as Diamond (CC), Graphite (CC), and Silicon(IV) oxide (SiO2SiO_2) have extremely high melting points because many strong covalent bonds must be broken to melt the substance.

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In Graphite, each carbon atom is bonded to three others in hexagonal layers, leaving one delocalised electron per atom, which allows it to conduct electricity. In Diamond, each carbon is tetrahedrally bonded to four others, making it very hard and an insulator.

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Dot-and-cross diagrams are used to represent the arrangement of electrons in the outer shells of atoms within a covalent molecule.

📐Formulae

H−H (Single bond in hydrogen molecule)H-H \text{ (Single bond in hydrogen molecule)}

O=C=O (Double bonds in carbon dioxide)O=C=O \text{ (Double bonds in carbon dioxide)}

N≡N (Triple bond in nitrogen molecule)N \equiv N \text{ (Triple bond in nitrogen molecule)}

CH4 (Methane)CH_4 \text{ (Methane)}

NH3 (Ammonia)NH_3 \text{ (Ammonia)}

SiO2 (Silicon(IV) oxide)SiO_2 \text{ (Silicon(IV) oxide)}

💡Examples

Problem 1:

Draw the dot-and-cross diagram for a molecule of Ammonia (NH3NH_3) showing only the outer shell electrons.

Solution:

Nitrogen (2,52,5) is the central atom, sharing three of its electrons with three Hydrogen (11) atoms.

Explanation:

Nitrogen needs 33 electrons to complete its octet. It forms three single covalent bonds with three HH atoms. This leaves one lone pair of electrons on the Nitrogen atom: 1×lone pair+3×bonded pairs=81 \times \text{lone pair} + 3 \times \text{bonded pairs} = 8 electrons in the valence shell of NN.

Problem 2:

Explain why Methane (CH4CH_4) is a gas at room temperature while Diamond (also containing Carbon) is a solid with a very high melting point.

Solution:

CH4CH_4 is a simple molecular substance, while Diamond is a giant covalent structure.

Explanation:

In CH4CH_4, the intermolecular forces are weak and require very little thermal energy to overcome. In Diamond, every CC atom is bonded to four others by strong covalent bonds in a giant 3D3D lattice; breaking these bonds requires massive amounts of energy (>3500∘C> 3500^\circ C).

Problem 3:

Why can Graphite conduct electricity but Diamond cannot?

Solution:

Presence of delocalised electrons in Graphite versus localized electrons in Diamond.

Explanation:

In Graphite, each carbon atom forms only 33 covalent bonds. The 4th4^{th} valence electron is delocalised and free to move throughout the layers, acting as a charge carrier. In Diamond, all 44 valence electrons are involved in rigid covalent bonds, so there are no free electrons to carry current.

Covalent bonding - Revision Notes & Key Formulas | IGCSE Grade 11 Chemistry