Matter is anything that has mass and takes up space. A substance made of just one type of atom is an element.
Scientists organize the elements into a table called the periodic table.
Compounds are formed when atoms share electrons or when they exchange electrons. A molecule is a particle that forms when atoms of some elements share electrons. Water, H 2 O, is a compound composed of molecules. The formula shows that water is made up of hydrogen and oxygen. The symbol for the element chlorine is Cl.