Hexagon is a noun. Hexagonal is an adjective. A noun is usually preceded by an article: "a" or "the". Example: A hexagon has six sides. An adjective normally precedes a noun that it describes. Example: A hexagonal table has six sides.

Hexagon is a noun and hexagonal is an adjective. Examples of use: As a noun: He drew a hexagon on the paper. In America, the road sign that means "stop" is shaped like a hexagon. As an adjective: He drew a hexagonal box on the paper. In American, a stop sign is hexagonal.

Hexagon is a noun. Hexagonal is an adjective. The building is hexagonal. The building is built in the shape of a hexagon.