Also known as negative ion, anion is an atom or molecule that houses more electrons than protons. As electrons are negatively charged particles, this electron-neutron imbalance gives the atom a negative electric charge. Negative atoms are indicated with a minus sign, placed beside the atom’s chemical symbol above the line, or as a superscript. For example, sulfate is an anion whose chemical symbol would be SO42-. Sulfate (S2-), hydride (H-), and oxide (O2-) are a few examples of anions.
Since electrons can move between atoms, thanks to an atom’s desire to have its outermost or valence shell stabilized or filled with sufficient number of electrons, a positive charge atom (cation) can become an anion, or vice-versa. An atom can release an electron to or acquire one from another atom based on whichever is easy. For example, if an electron has five electrons and needs three more in its outer or valence shell to become stable, it will look for additional three electrons from another atom instead of releasing five electrons and getting rid of the shell altogether.