Here's why:

* Non-metal: Phosphorus is a non-metal, and non-metals are generally poor conductors of heat.

* Structure: Phosphorus exists in different forms (allotropes), but in its common forms (white, red, black phosphorus), it has a molecular structure. This means the atoms are held together by relatively weak bonds. Heat energy is transferred through vibrations of atoms within a material. The weak bonds in phosphorus prevent efficient transfer of this vibrational energy.

* Comparison: Metals are known for their excellent heat conductivity due to their strong metallic bonds and the presence of free electrons.

Therefore, phosphorus is considered a poor conductor of heat compared to metals like copper or aluminum.