International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Which of the following compounds has a covalent bond?

1. NaCl

2. MgO

3. H2O

4. KCl

The correct answer is: H2O

The compound H2O, or water, is indeed an example of a molecule that contains covalent bonds. In H2O, each hydrogen atom shares electrons with the oxygen atom, creating covalent bonds. This sharing allows both the hydrogen atoms and the oxygen atom to achieve greater stability, as they attain full outer electron shells according to the octet rule for the oxygen atom and the duet rule for the hydrogen atoms. H2O's structure also illustrates the nature of covalent bonding, where the shared electrons lead to the formation of a distinctive molecular geometry, which is bent due to the presence of lone pairs of electrons on the oxygen atom. This molecular shape is key for many of water’s unique properties, including its polarity, which affects how it interacts with other substances. In contrast, compounds like NaCl, MgO, and KCl are characterized by ionic bonds, where electrons are transferred from one atom to another, resulting in the formation of charged ions. These compounds typically consist of metals and nonmetals, where the metal donates electrons and the nonmetal accepts them, leading to strong electrostatic attractions between the positive and negative ions that form. This differing nature of bonding is one of the core distinctions between ionic and covalent compounds in