What is an anion?

An anion is defined as a negatively charged ion, which means it has gained one or more electrons, resulting in an overall negative charge. In the context of the provided options, a negatively charged chloride atom (Cl-) is the correct representation of an anion. Chlorine, when it gains an electron, becomes Cl-, thereby acquiring a negative electrical charge. This characteristic of gaining electrons distinguishes anions from cations, which are positively charged ions formed by the loss of electrons.

The other options do not describe an anion accurately. A positively charged sodium atom (Na+) represents a cation, not an anion. A substance that does not carry a charge is referred to as a neutral substance and does not fit the definition of an ion at all. Lastly, a compound formed by acids and bases refers to the result of a neutralization reaction but does not specifically define what an anion is. The clarity of the definition and its implications in chemistry direct us firmly to the negatively charged species as the correct answer.