Here's why:

* Crystal Structure: Sodium chloride has a face-centered cubic (FCC) crystal structure, while diamond has a diamond cubic structure. These are distinct and fundamentally different arrangements of atoms in the crystal lattice.

* Chemical Bonding: Sodium chloride is an ionic compound, meaning it forms a crystal lattice due to electrostatic attractions between positively charged sodium ions (Na+) and negatively charged chloride ions (Cl-). Diamond is a covalent network solid, where carbon atoms are linked together by strong covalent bonds in a three-dimensional network.

* Properties: The different bonding and structure lead to vastly different properties. Sodium chloride is easily soluble in water, brittle, and has a relatively low melting point. Diamond is extremely hard, insoluble in water, and has a very high melting point.

Isomorphism refers to two or more compounds having the same crystal structure. While both sodium chloride and diamond are crystalline solids, their structures are fundamentally different, making them non-isomorphic.