Elements and Compounds

Pure substances come in two kinds:

• An element is made of only one type of atom and cannot be broken into simpler substances by chemical means. Copper ($\text{Cu}$), oxygen ($\text{O}_2$) and carbon ($\text{C}$) are elements. The periodic table is the list of all known elements.
• A compound is two or more elements chemically bonded in a fixed ratio. Water ($\text{H}_2\text{O}$) is always two hydrogen atoms to one oxygen. Table salt is sodium chloride ($\text{NaCl}$).

A compound's properties are usually nothing like the elements that formed it. Sodium is a metal that reacts violently with water; chlorine is a poisonous green gas — yet bonded together they make the salt on your chips.