What type of ion is known as an anion?

An anion is defined as a negatively charged ion. This occurs when an atom gains one or more electrons, which increases the overall negative charge of the ion. Atoms typically strive for a more stable electron configuration, often resembling the nearest noble gas. By gaining electrons, they can achieve a full outer electron shell, leading to the formation of anions.

In contrast, the other options do not accurately describe anions. Positively charged ions, known as cations, result from an atom losing electrons. A neutral ion has no charge, meaning it has an equal number of protons and electrons, and therefore does not fit the definition of an anion. Lastly, the description of an ion that has lost electrons specifically pertains to cations, not anions. Therefore, the correct definition of an anion is indeed as a negatively charged ion.