Here's why:

* Composition: Naphthalene is an organic compound composed entirely of carbon and hydrogen atoms. It doesn't contain any metallic elements.

* Properties: Metals generally exhibit properties like good conductivity of heat and electricity, malleability, and ductility. Naphthalene is a solid with poor conductivity and is brittle, characteristics typical of nonmetals.

* Structure: Naphthalene has a planar structure with a fused ring system, which is a common feature of organic molecules, not metals.

In summary, naphthalene's composition, properties, and structure all point to it being a nonmetal.