What is a negative ion characterized by?

A negative ion, also known as an anion, is characterized by having extra electrons. This means that the number of electrons in the atom exceeds the number of protons, resulting in a net negative charge. The addition of these extra electrons allows the atom to achieve a more stable electron configuration, often resembling that of noble gases.

In contrast, the other options do not accurately define a negative ion. A deficit of electrons would result in a positive ion (cation), as there would be more protons than electrons. Extra neutrons do not influence the charge of an atom as they carry no charge; they contribute to the atomic mass. Similarly, the loss of protons would decrease the positive charge of the nucleus, also leading to a positive ion if it were to occur. Thus, the defining characteristic of a negative ion is indeed its extra electrons.