What is an ion?

An ion is defined as an atom or molecule that has gained or lost electrons, resulting in a net electrical charge. When an atom loses one or more electrons, it becomes positively charged and is referred to as a cation; conversely, when it gains electrons, it becomes negatively charged and is known as an anion. This process of ion formation is crucial in many chemical reactions, as it affects properties such as conductivity and reactivity.

The other definitions do not accurately capture what an ion is. A neutral atom, for example, has an equal number of protons and electrons, resulting in no overall charge. Atoms with varying numbers of neutrons are known as isotopes, which also do not involve the gain or loss of charge. The formation of ions is specifically related to the transfer of electrons, not protons, making the correct answer focus on the role of electrons in determining charge.