Explain the reason NaCl is an ionic compound whereas CuCl is a covalent compound.

The reason sodium chloride (NaCl) is an ionic compound while copper(I) chloride (CuCl) is primarily covalent lies in the nature of the bonding between the atoms.

NaCl (Sodium Chloride):

Sodium chloride is formed by the reaction between a metal (sodium) and a non-metal (chlorine).
Sodium (Na) is a metal with one valence electron, while chlorine (Cl) is a non-metal with seven valence electrons.
Sodium easily loses its one valence electron to achieve a stable electronic configuration similar to that of a noble gas (like neon), forming a sodium cation (Na⁺).
Chlorine readily gains one electron to complete its valence shell, forming a chloride anion (Cl⁻).
The electrostatic attraction between the oppositely charged ions (Na⁺ and Cl⁻) results in the formation of an ionic bond.
In the solid state, NaCl exists as a three-dimensional array of ions held together by strong electrostatic forces, forming a crystalline lattice.
CuCl (Copper(I) Chloride):

Copper(I) chloride is formed by the reaction between a transition metal (copper) and a non-metal (chlorine).
Copper (Cu) is a transition metal with variable oxidation states. In CuCl, copper is in the +1 oxidation state.
The electronegativity difference between copper and chlorine is not as high as that between sodium and chlorine.
In copper(I) chloride, the bond between copper and chlorine tends to be more covalent than ionic.
While there is some ionic character due to the electronegativity difference, the sharing of electrons between copper and chlorine is more significant in forming the bond.
As a result, CuCl tends to exist as discrete molecules in which copper and chlorine atoms share electrons in covalent bonds rather than as a crystal lattice of ions.
In summary, the difference in the nature of the bonding and the electronegativity between the elements involved accounts for why NaCl is an ionic compound while CuCl is primarily covalent.