A Level Chemistry OCR Practice Exam

A molecule is defined as a small group of atoms that are held together by covalent bonds. This definition highlights the significance of covalent bonding, where atoms share pairs of electrons. Molecules can consist of two or more atoms of the same or different elements. For instance, a water molecule (H2O) comprises two hydrogen atoms and one oxygen atom bonded covalently, forming a stable unit with distinct chemical properties.

In contrast, the other terms listed have different meanings. An ion refers to an atom or molecule that has gained or lost one or more electrons, resulting in a net electric charge. A compound consists of two or more different elements that are chemically bonded, which may include ionic or covalent bonds but is broader than just the molecule concept. Lastly, a mixture involves two or more substances combined physically but not chemically bonded, which means the individual substances retain their own properties. Thus, the term molecule specifically captures the essence of a group of atoms connected by covalent bonds, making it the correct choice.