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Chemistry - Simple Molecules and Covalent Bonds

Grade 9IGCSE

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

🔑Concepts

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A covalent bond is formed when two non-metal atoms share a pair of electrons to achieve a stable outer shell (noble gas configuration).

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In a single covalent bond, each atom contributes one electron to the shared pair, denoted as A−BA-B.

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Double bonds involve sharing two pairs of electrons (e.g., O=OO=O), and triple bonds involve sharing three pairs (e.g., N≡NN \equiv N).

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Simple molecules consist of a small number of atoms joined by strong covalent bonds, but they have weak intermolecular forces (Van der Waals forces) between the molecules.

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Physical properties: Simple molecular substances have low melting and boiling points because only the weak intermolecular forces need to be overcome, not the strong covalent bonds.

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Electrical conductivity: Simple molecules do not conduct electricity in any state because they do not have free-moving ions or delocalized electrons.

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Dot-and-cross diagrams are used to represent the arrangement of electrons in the outer shells of the atoms involved in bonding.

📐Formulae

H2 (Hydrogen molecule)H_{2} \text{ (Hydrogen molecule)}

Cl2 (Chlorine molecule)Cl_{2} \text{ (Chlorine molecule)}

H2O (Water molecule)H_{2}O \text{ (Water molecule)}

CH4 (Methane molecule)CH_{4} \text{ (Methane molecule)}

NH3 (Ammonia molecule)NH_{3} \text{ (Ammonia molecule)}

CO2 (Carbon Dioxide molecule)CO_{2} \text{ (Carbon Dioxide molecule)}

💡Examples

Problem 1:

Explain the bonding in a molecule of Chlorine (Cl2Cl_{2}) using the electronic configuration of Chlorine (2,8,72, 8, 7).

Solution:

Each Chlorine atom has 77 electrons in its outer shell and requires 11 more to be stable. Two Chlorine atoms share one pair of electrons: Cl⋅+⋅Cl→Cl:ClCl \cdot + \cdot Cl \rightarrow Cl : Cl.

Explanation:

By sharing one electron each, both atoms achieve a full outer shell of 88 electrons (2,8,82, 8, 8). This forms a single covalent bond.

Problem 2:

Why does Oxygen (O2O_{2}) have a much lower boiling point than Diamond, even though both contain covalent bonds?

Solution:

O2O_{2} is a simple molecular substance, whereas Diamond is a giant covalent structure.

Explanation:

To boil Oxygen, you only need to overcome the weak intermolecular forces between O2O_{2} molecules. In Diamond, you must break many strong covalent bonds throughout the entire lattice, which requires significantly more energy.

Problem 3:

Draw the dot-and-cross diagram for Water (H2OH_{2}O) and state the total number of shared electrons.

Solution:

The central Oxygen atom shares one electron with each of the two Hydrogen atoms. Oxygen (2,62, 6) ends up with 88 electrons, and each Hydrogen (11) ends up with 22 electrons.

Explanation:

There are 22 single covalent bonds in H2OH_{2}O. Since each bond consists of 22 electrons, the total number of shared electrons is 2×2=42 \times 2 = 4.