A Level Biology Practice Exam

What defines an ion?

1. An atom with an equal number of protons and electrons

2. An atom or molecule with an overall electric charge

3. A charged particle made solely of neutrons

4. A molecule that cannot bond with other atoms

The correct answer is: An atom or molecule with an overall electric charge

An ion is defined as an atom or molecule that has an overall electric charge due to the loss or gain of one or more electrons. When an atom loses electrons, it becomes positively charged, known as a cation, while gaining electrons results in a negatively charged ion, or anion. This charge differentiation is what distinguishes an ion from a neutral atom, where the number of protons equals the number of electrons. In contrast, an atom that has an equal number of protons and electrons is neutral, while a charged particle made solely of neutrons is not physically possible, as neutrons are electrically neutral and do not create an overall charge. Lastly, while all molecules have some capacity to bond, it is inaccurate to define an ion as a molecule that cannot bond with other atoms, as many ions can readily participate in bonding interactions.