What defines a monatomic ion?

A monatomic ion is defined as an ion that consists of a single atom that has gained or lost one or more electrons, resulting in a net positive or negative charge. This definition clearly indicates that the ion originates from a single atom rather than being composed of multiple atoms or molecules. For example, when a sodium atom loses one electron, it becomes a sodium ion (Na⁺), while a chlorine atom that gains an electron becomes a chloride ion (Cl⁻). Both of these are examples of monatomic ions, demonstrating how a single atomic source can be transformed into an ion by altering its electron count.

Other options suggest incorrect characteristics of monatomic ions. One points to the presence of multiple atoms, which would define a polyatomic ion, not a monatomic one. Another option implies that all monatomic ions must be negatively charged, which is not accurate as monatomic ions can be either positively or negatively charged depending on electron transfer. The last option restricts monatomic ions to noble gases, which is misleading because while noble gases can form monatomic ions under certain conditions, many other elements outside of this group can also form monatomic ions. Therefore, the defining feature remains that a monatomic ion is simply a single atom with a charge.