What condition must be met for an atom to become an ion?

An atom becomes an ion when it gains or loses electrons, which is the basis for ion formation. This process alters the overall charge of the atom. When an atom gains one or more electrons, it takes on a negative charge and is referred to as an anion. Conversely, when it loses electrons, it results in a positive charge, creating a cation.

The other choices do not lead to the ionization of an atom. For instance, gaining or losing protons would fundamentally change the identity of the element itself, as protons determine the atomic number. Gaining or losing neutrons affects the atom's isotopic form but does not alter its charge or lead to the formation of an ion. Sharing electrons is a characteristic of covalent bonding, which involves atoms forming molecules rather than becoming ions. Thus, the correct understanding lies in the role of electrons in defining the charge and ionization of an atom.