Here's how it works:

* Electrostatic Attraction: Sodium (Na) is a metal and readily loses an electron to become a positively charged ion (Na+). Chlorine (Cl) is a nonmetal and readily gains an electron to become a negatively charged ion (Cl-). These opposite charges attract each other strongly, forming an ionic bond.

* Crystal Lattice: The attraction between ions in a salt extends beyond just the nearest neighbors. Many Na+ and Cl- ions arrange themselves in a regular, repeating three-dimensional structure called a crystal lattice. This arrangement maximizes the electrostatic attraction and stabilizes the structure.

Therefore, the force holding Na+ and Cl- together in NaCl is the strong electrostatic attraction between oppositely charged ions, which is a defining characteristic of ionic bonding.