By Evelyn Trimborn, updated March 24, 2022
Atoms consist of protons, neutrons, and electrons. Protons carry a positive charge, neutrons are neutral, and electrons carry a negative charge. Electrons orbit the nucleus in shells, and the arrangement of electrons determines whether an atom can form positive or negative ions.
Ionization energy is the energy required to remove an electron from an atom or molecule. Elements that already possess a full outer shell of eight electrons (the noble gases) are typically stable, while those with fewer or more than eight electrons have weaker or stronger bonds that can be influenced by ionization energy.
A positive ion (cation) forms when an atom loses one or more electrons. For example, sodium (Na) has 11 protons and 11 electrons, with a single valence electron in its outer shell. This valence electron is weakly bound and can be removed by ionization energy, leaving the atom with a net positive charge: Na⁺.
A negative ion (anion) forms when an atom gains an electron. Fluorine (F), with seven valence electrons, is highly electronegative. When it accepts an extra electron, it completes its octet and carries a net negative charge: F⁻.
In both cases, the ionization process alters the electron count, changing the atom’s charge while preserving its overall identity.
