Atoms form bonds with other atoms because of the electrostatic attraction between positively-charged protons and negatively-charged electrons. When this force of attraction brings atoms together to form substances containing two or more atoms, the bond is called a chemical bond. There are many types of chemical bonds, but the three major, or primary, types are ionic, covalent and metallic.
The three major chemical bond types represent the three different ways that metals and nonmetals combine. The attraction between the atoms is the result of the varying characteristics of the outermost electrons, which are called valence electrons. These are the electrons that are involved in the creation of the chemical bond.
An ionic bond occurs between a metal and a nonmetal. An example is the chemical bond that forms between the metal sodium and the nonmetal chlorine to create the compound sodium chloride commonly known as table salt (NaCl). In an ionic chemical bond, electrons from the metallic substance are transferred to the atoms of the nonmetallic substance.
Covalent bonds occur between nonmetallic substances. An example would be the bond formed between two hydrogen atoms and one oxygen atom to form the compound water (H2O). In a covalent bond, the electrons are shared between the atoms. The third major type of chemical bond, the metallic bond, is formed between metallic atoms. Valance electrons are easily set free from metallic substances and can move freely, which is a characteristic that gives metals the properties of thermal and electrical conductivity.