While metals steal the spotlight with their shine, nonmetals quietly run the show. Oxygen, carbon, nitrogen, hydrogen, sulfur, phosphorus — this small group of elements forms the basis of life, the atmosphere, and the chemistry of everyday existence. Only about 18 elements are classified as nonmetals, yet they are astonishingly abundant: nitrogen and oxygen make up 99% of the air you breathe, hydrogen and oxygen form every drop of water, and carbon is the backbone of every organic molecule in your body. If metals built civilization's infrastructure, nonmetals built life itself.
Nonmetals are in many ways the mirror image of metals. Where metals are shiny, nonmetals are often dull. Where metals conduct electricity, nonmetals are typically insulators (with carbon in its graphite form being a notable exception). Where metals are malleable, solid nonmetals tend to be brittle — try bending a chunk of sulfur and it snaps. Most importantly, while metals lose electrons, nonmetals gain or share them. This electron-hungry nature gives nonmetals high electronegativities, and it is why they form the negative ions in ionic compounds and the shared-electron bonds in covalent molecules.
Nonmetals show remarkable diversity in their physical forms. At room temperature, some are gases (oxygen, nitrogen, fluorine, chlorine, the noble gases), one is a liquid (bromine), and a few are solids (carbon, sulfur, phosphorus, selenium, iodine). Carbon alone can exist as diamond, graphite, fullerenes, and graphene — all with wildly different properties. This versatility is why nonmetals are irreplaceable: chlorine purifies drinking water, silicon (a metalloid neighbor) powers computers, phosphorus is essential for DNA and ATP, and fluorine strengthens your tooth enamel in toothpaste.