How does NaCl maintain its crystalline structure?

The formation of an ionic crystal such as sodium chloride is a delicate balance between electrostatic attraction and Pauli repulsion. Electrostatic attraction says that between two different charges, q+ and q−, there is a Coulomb force given by,
F=kq1q2/r^2
where r is the distance between the two charges. If one plays with the numbers then it's easy to see that at short distances the force is strongest, but there is a limit to how close they may come together. Eventually a repulsive force due to a quantum mechanical principle called the Pauli Exclusion Principle overpowers the attraction.