What are simple molecules in the context of Cambridge IGCSE Chemistry?

In Cambridge IGCSE Chemistry, simple molecules refer to small groups of atoms bonded together by covalent bonds. These molecules typically consist of a few atoms, such as H2O (water), CO2 (carbon dioxide), and O2 (oxygen). They are characterized by strong covalent bonds within the molecule but weak intermolecular forces between molecules, leading to low melting and boiling points.

How do covalent bonds form between atoms?

Covalent bonds form when two atoms share one or more pairs of electrons to achieve a full outer shell, resembling the electron configuration of noble gases. This sharing of electrons allows each atom to attain stability. For example, in a water molecule (H2O), each hydrogen atom shares an electron with the oxygen atom, resulting in a stable covalent bond.

What is the difference between covalent bonds and ionic bonds?

Covalent bonds involve the sharing of electron pairs between atoms, typically between non-metal elements, resulting in the formation of simple molecules. In contrast, ionic bonds occur when electrons are transferred from one atom to another, usually between metal and non-metal elements, creating ions that attract each other due to opposite charges. This difference in bonding leads to distinct properties in the resulting compounds.

Why do simple molecules have low melting and boiling points?

Simple molecules have low melting and boiling points because they are held together by weak intermolecular forces, such as van der Waals forces or hydrogen bonds. These forces are much weaker than the covalent bonds within the molecules, requiring less energy to overcome. As a result, simple molecular substances are often gases or liquids at room temperature.

Can you explain the concept of a double covalent bond with an example?

A double covalent bond occurs when two pairs of electrons are shared between two atoms. This type of bond is stronger and shorter than a single covalent bond. An example is the oxygen molecule (O2), where each oxygen atom shares two electrons with the other, forming a double bond. This sharing allows both oxygen atoms to achieve a stable electron configuration.

Atoms, Elements and CompoundsCambridge IGCSE Chemistry (0620)

Simple Molecules and Covalent Bonds

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Study Notes

Simple molecules are formed when non-metal atoms share electrons, creating covalent bonds that lead to noble gas electronic configurations. These bonds can be single, double, or triple, depending on the number of shared electron pairs.

Covalent Bond — a bond formed by the sharing of a pair of electrons between two atoms. Example: HCl is a simple molecule with a single covalent bond.
Simple Molecular Compounds — compounds with low melting and boiling points due to weak intermolecular forces. Example: Water (H2O) is a simple molecular compound.
Poor Electrical Conductivity — characteristic of covalent compounds because they lack free charged particles. Example: CH4 (methane) does not conduct electricity.

Exam Tips

Key Definitions to Remember

Covalent bond: a bond formed by the sharing of electrons between atoms.
Simple molecular compounds: compounds with low melting and boiling points.

Common Confusions

Confusing covalent bonds with ionic bonds.
Misunderstanding why covalent compounds do not conduct electricity.

Typical Exam Questions

What is a covalent bond? A covalent bond is formed by the sharing of a pair of electrons between two atoms.
Why do simple molecular compounds have low melting points? They have low melting points due to weak intermolecular forces.
Why do covalent compounds not conduct electricity? They do not conduct electricity because they lack free charged particles.

What Examiners Usually Test

Ability to draw dot-and-cross diagrams for simple molecules.
Understanding of the properties of covalent compounds.

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