What is an anion?

An anion is indeed defined as a negatively charged ion that is formed when an atom gains electrons. This process occurs because atoms seek to achieve a full outer electron shell, similar to that of the nearest noble gas configuration, which is generally associated with stability. When an atom gains one or more electrons, it acquires a negative charge due to the imbalance in the number of protons (which are positively charged) and electrons (which are negatively charged).

Understanding the nature of anions is fundamental in chemistry, as they play crucial roles in various chemical reactions, particularly in ionic bonding where they pair with positively charged ions, known as cations. This interaction is essential in the formation of salts and other ionic compounds.

Given this context, the other choices do not correctly define an anion. A positively charged ion describes a cation, while an ion with no charge refers to a neutral atom, which does not categorize as either anion or cation. Lastly, although nonmetals commonly form anions, the notion that anions can only form from nonmetals is not accurate, as certain metal ions can also exist with an anionic character under specific conditions.