Final answer:
An atom must have a charge to be considered an ion, which can be either positive, forming a cation, or negative, forming an anion.
Step-by-step explanation:
In order to be considered an ion, an atom must have a charge. This charge can be either positive or negative. A cation is a positively charged ion that forms when a neutral atom loses one or more electrons from its valence shell. Conversely, an anion is a negatively charged ion that forms when a neutral atom gains one or more electrons in its valence shell. For instance, when a neutral sodium atom loses one electron, it becomes a cation with a 1+ charge, while a neutral oxygen atom becomes an anion with a 2- charge when it gains two electrons.