Basically, the elements that lack the properties of metals are said to have nonmetallic properties and are therefore called nonmetals.
They are encountered most often as compounds or mixtures of compounds, though some occur in their elemental forms and are very important.
Nitrogen (N2) and oxygen (O2) make up most of the air we breathe.
They cannot be seen, tasted, or smelled, and therefore difficult to experience their existence.
The most observed nonmetallic element is carbon, which is found in graphite, coal, and diamonds--all of which are pure carbon atoms.
The nonmetals have properties almost completely opposite of metals.
They are poor conductors of heat and electricity (except for graphite--which is attributed to molecular struture).
Many of the nonmetals are solids at STP, while many others are gases.
All of the group 0 (18) elements (the noble gases--mostly inert) are gases consisting of single atoms.
The other gaseous elements, hydrogen, oxygen, nitrogen, fluorine, and chlorine, are diatomic molecules--H2, O2, N2, F2, and Cl2.
Bromine and iodine are also diatomic, but bromine is a liquid and iodine is a solid at room temperature.
The nonmetals also lack the malleability and ductility of the metals.
A lump of sulfur crumbles when hit or pulled on.
Diamond cutters rely on the brittle nature of carbon when they split the stone by carefully striking a quick blow with a sharp blade.
Like metals, nonmetals also produce a broad range of chemical reactivity.
Flourine is extremely reactive, reacting readily with almost all other elements.
Helium is inert and does not react with anything.