An element is a pure substance made of only one type of atom, with a unique number of protons. The periodic table lists 118 known elements, from common metals like gold to rare elements that exist for only seconds. Atoms are the basic units that make up elements and anything else. They contain protons, neutrons, and electrons. Metals are dense, solid materials that can be shaped, while non-metals are brittle solids, liquids, or gases that conduct heat and electricity poorly. Compounds contain two or more elements bonded together, such as water, which is made of hydrogen and oxygen atoms. Chemical reactions occur when molecules interact to form new substances.