Atoms combine by forming chemical bonds, the majority of which fall into three major categories: ionic, covalent and hydrogen bonds. The bonding that occurs is the result of the electrostatic attractive force that exists between opposite charges. The bonds that form between atoms can also be classified as the stronger primary bonds or the weaker secondary bonds.
The stronger chemical bonds are the ionic, covalent bonds and metallic bonds. Because of the strength of attraction between the atoms forming these bonds, they fall into the primary classification. The weaker bonds are formed by hydrogen bonding, dipole-dipole interactions and the London dispersion force.
Quantum theory explains the process by which the bonds between atoms form, but a simpler set of practical rules, such as the octet rule and valance theory, enable chemists to predict the types of bonds that will occur. The number of an atom's valance electrons, which are found in an atom's outermost electron shell, are an important factor regarding the manner in which atoms combine to form molecules and new chemical substances.