A Level Chemistry OCR Practice Exam

The correct answer is indeed the description of an ion, which is a positively or negatively charged atom or group of atoms. Ions form when an atom either loses or gains electrons, resulting in a net charge. When electrons are lost, the atom becomes a positively charged cation, while gaining electrons results in a negatively charged anion. This behavioral characteristic is crucial to many chemical reactions and the formation of compounds.

In contrast, the other descriptions do not correctly define an ion. The first choice refers to an atom's inability to gain electrons, which does not account for the possibility of losing electrons to form a positive charge. The second choice describes a neutral atom with a full outer shell, which means it is stable and not participating in ionic formation. The last choice pertains to molecules, which refers to covalent compounds and does not apply to the definition of an ion, which is fundamentally about charge and electron transfer rather than the molecular independence.