Which of the following best defines an ion?

An ion is best defined as an electrically charged atom. This definition captures the essence of what distinguishes ions from neutral atoms. Atoms can become charged through the loss or gain of electrons. When an atom loses one or more electrons, it becomes positively charged and is referred to as a cation. Conversely, when an atom gains electrons, it becomes negatively charged and is known as an anion.

Understanding that an ion specifically involves a charge due to an imbalance in the number of protons and electrons clarifies its role in chemical reactions and interactions within various contexts, particularly in solutions and electrolytes, which are highly relevant in water distribution systems. This contrasts with other options that do not accurately reflect the definition of an ion, as an uncharged atom does not possess any charge, a group of molecules refers to multiple entities and lacks the characteristics of an individual charged particle, and a gaseous compound pertains to a state of matter rather than an individual atom’s charge.