What is an ion?

An ion is defined as an atom or molecule that has a net electric charge because it has lost or gained one or more electrons. This process alters the balance between protons, which are positively charged, and electrons, which are negatively charged. When an atom loses electrons, it becomes positively charged and is referred to as a cation. Conversely, if an atom gains electrons, it becomes negatively charged and is known as an anion. This charge imbalance is what characterizes ions, distinguishing them from neutrally charged atoms, which have equal numbers of protons and electrons.

The other options present definitions that do not accurately encapsulate what an ion is. A fixed mass refers to particles like atoms in a specific isotopic form, while an atom with no electrons would not represent a typical ion, as it would lack the electron aspect entirely. Lastly, a neutral atom indicates a state without charge; thus, it is not classified as an ion.