Why is halite crystalline?
Halite is rock salt composed of sodium chloride. The reason it is crystalline in structure is related to the type of bonding that occurs between sodium atoms and chlorine atoms.
The type of bond that forms is called an ionic bond between sodium and chlorine. What occurs is that sodium has 1 valence electron in its third shell and chlorine has 7 valence electrons in its third shell. Sodium transfers its atom to chlorine. This occurs because the two atoms are very unequal in their attraction for valence electrons.
Once this occurs, the sodium will have a net positive charge and is called a cation and the chlorine will have a net negative charge and is called an anion. Because of the difference in charge, these atoms are attracted to one another forming an ionic bond.
Salts like sodium chloride are ionic compounds. They are often found as crystals in nature. Halite forms when seawater evaporates. The atoms in rock salt arrange themselves symmetrically in a cube shape and the opposite charges of sodium and chlorine help to maintain the atoms in their crystalline lattice.
I have attached a link showing the arrangement of sodium and chlorine atoms in rock salt which is organized and stable resulting in its crystalline structure.