Here's why:

* Metals are generally shiny, malleable (can be hammered into sheets), ductile (can be drawn into wires), good conductors of heat and electricity, and tend to lose electrons in chemical reactions.

* Nonmetals are usually dull, brittle, poor conductors of heat and electricity, and tend to gain electrons in chemical reactions.

* Metalloids have properties of both metals and nonmetals.

Sodium exhibits all the classic properties of a metal.