Here's why:

* Metals are generally shiny, malleable, ductile, good conductors of heat and electricity, and tend to lose electrons to form positive ions (cations).

* Nonmetals are generally dull, brittle, poor conductors of heat and electricity, and tend to gain electrons to form negative ions (anions).

* Metalloids have properties of both metals and nonmetals and can act as semiconductors.

Noble gases have very weak interatomic forces, making them gases at room temperature. They are very unreactive, as their outermost electron shell is full (octet rule). This makes them poor conductors of heat and electricity, and they don't easily form ions.

So, based on their properties, noble gases are clearly classified as nonmetals.