Key concepts
An anion is an atom or group with more electrons than protons. Fluorine and chlorine usually gain one electron; oxygen and sulfur commonly gain two; nitrogen can gain three in ionic nitrides.
The nucleus does not change during ordinary ion formation. Added electron–electron repulsion and reduced effective attraction per electron usually expand the electron cloud.
Applications & misconceptions
Applications: salts, batteries, electrolysis, biological electrolytes, corrosion, and ionic crystal formation.
Misconception: An anion is not formed by adding neutrons or removing protons. It forms by gaining electrons.