Sodium and chlorine form an ionic bond because they have a large difference in their electronegativities. Electronegativity is a measure of an atom's ability to attract electrons. Sodium has a low electronegativity, meaning that it easily gives up electrons. Chlorine has a high electronegativity, meaning that it strongly attracts electrons. When these two elements come into contact, the sodium atoms lose electrons to the chlorine atoms, resulting in the formation of positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). These ions are then attracted to each other to form an ionic bond.