When atoms are joined to each other by covalent bonds, the result is a molecule. A molecule is a small, separate group of atoms with a fixed number of atoms in it. The formula of a molecule shows how many atoms of each element it contains.
Some molecules contain atoms of one element. Hydrogen, H2, is made of molecules with two hydrogen atoms, and oxygen, O2, is made of molecules with two oxygen atoms. Other molecules contain different elements. A water molecule, H2O, has two hydrogen atoms and one oxygen atom. A methane molecule, CH4, has one carbon atom and four hydrogen atoms. A carbon dioxide molecule, CO2, has one carbon atom and two oxygen atoms. In every case the atoms are held together by covalent bonds.
