Electronegativity is the ability of an atom to attract electrons towards itself. When two atoms with a large difference in electronegativity bond, the more electronegative atom pulls the electrons away from the less electronegative atom, creating an ionic bond.

For example, in sodium chloride (NaCl), the electronegativity of sodium is 0.9, while the electronegativity of chlorine is 3.0. This large difference in electronegativity means that the electrons are pulled away from the sodium atom and towards the chlorine atom, creating an ionic bond.