What is an ion?

An ion is defined as an atom or a group of atoms that has gained or lost one or more electrons, leading to an overall electric charge. This occurs because the number of protons in the nucleus, which are positively charged, no longer equals the number of electrons, which are negatively charged. As a result, ions can be either positively charged (cations) when electrons are lost, or negatively charged (anions) when electrons are gained. This concept is fundamental in understanding chemical reactions, especially in the context of hair coloring, where chemical reactions between different ions and molecules play a critical role in the development and permanence of hair color.

The other choices do not accurately define an ion. An atom without charge refers to a neutral atom that has equal numbers of protons and electrons. A molecule composed of one element may describe homonuclear diatomic molecules, but it does not capture the essence of an ion. A vacuum of atomic particles describes a space devoid of matter, which is unrelated to the concept of ions as charged entities.