Question

Sodium ions bond with chlorine ions to form table salt, NaCl. Why does one sodium ion bond with one chlorine ion?

Answer 1

Why? Because of electrostatic attraction between unlike charges.

Explanation:

And of course, in an ionic solid (say `"sodium chloride"`), an isolated cation, a sodium ion, is BOUND ELECTROSTATICALLY to EVERY OTHER CHLORIDE ANION in the lattice. The attraction/repulsion follows `"Coulomb's Law"`, and is thus strongest between a close-packed ion, and (typically) its six nearest `Cl^-` neighbours. Such ionic bonding is believed to be responsible for the properties of many ionic solids: high melting point; crystallinity, hardness, and brittleness; lack of electrical conductivity in the SOLID state (but not in the liquid state or in solution).

An individual sodium ion is electrostatically repelled by every other positively charged ion in the lattice also, but if you sum the repulsive and attractive forces up across the entire lattice, which can certainly be done quantitatively, a net attractive force can be shown to operate.