Here's why:

* Structure: Phosphine consists of discrete PH₃ molecules held together by weak intermolecular forces (London dispersion forces). These forces arise from temporary fluctuations in electron distribution around the molecule.

* Bonding: The P-H bonds within the molecule are covalent, meaning they involve sharing of electrons between the phosphorus and hydrogen atoms.

* Properties: Molecular solids generally have lower melting and boiling points compared to ionic or metallic solids. They are also typically soft and easily broken. Phosphine is a colorless gas at room temperature, further supporting its molecular nature.

Let me know if you have any other questions!