Understanding chemical bonds and how they affect the properties of materials is an important part of material science and heat treatment. For example, why do some atoms join together to form molecules, but others do not? How can a sheet of gold (Au) be so malleable as to be pounded thin enough for light to pass through, whereas osmium (Os) is so hard (18 times harder than gold) and extremely brittle? Why is copper 25 times more thermally conductive and 43 times more electrically conductive than stainless steel? These questions can be answered by considering the most fundamental topic in chemistry: that of the chemical bond.

Chemical compounds are formed by the joining of two or more atoms together. A stable compound occurs when the total energy of the combination is lower than the energy of the separate atoms. Thus, a chemical bond is simply an attractive force between atoms that holds them together. It is the bonds between atoms that give molecules different properties than the atoms they are composed of.

Types of Chemical Bonds

Van der Waals Bonds

Liquid water molecules are an example of chemical bonds called van der Waals bonds. The forces that attract each atom to the other are electrostatic in nature and are called van der Waals forces. Even though the water molecule as a whole is electrically neutral, the distribution of charge in the molecule is not symmetrical, resulting in what is called a dipole moment (a microscopic separation of the positive and negative charges). This leads to a net attraction, which results in a cohesive force between water molecules and contributes to properties such as viscosity and surface tension. This is true of all such polar molecules.

Covalent Bonds

A covalent bond is a bond that is formed when two atoms share electrons (in contrast to the transfer of electrons, which takes place in ionic bonds). Such bonds try to create a noble-gas configuration for each atom and lead to stable molecules if they share electrons in this way.

These bonds are oriented in definite directions in space, giving rise to the complex geometry of organic molecules. A single covalent bond is formed when one pair of electrons is shared; a double bond is formed when two pairs of electrons are shared; and a triple bond is formed when three pairs of electrons are shared.

Ionic Bonds

An ionic bond results from the attraction of oppositely charged ions. In cases where one or more atoms lose electrons and other atoms gain them (in order to produce a noble-gas electron configuration), the bond is considered ionic. Ionic bonds often produce extremely stable compounds. Chlorine (Cl) and sodium (Na), for example, are particularly unstable molecules, but become very stable when joined together in an ionic bond. Ionic bonding involves a very strong type of interaction between atoms, and strongly bonded ionic compounds have high melting points.

Comparison of Covalent and Ionic Bonds

The nature of ionic and covalent bonds produces materials that have significantly different macroscopic properties. The atoms of covalent materials are bound tightly to each other in stable molecules, but those molecules are generally not very strongly attracted to other molecules in the material. On the other hand, the atoms (ions) in ionic materials show strong attractions to other ions in their vicinity. This generally leads to low melting points for covalent solids and high melting points for ionic solids.

This table compares the properties of ionic and covalent compounds.

*For example, the molecule carbon tetrachloride (CCl₄) is a nonpolar covalent molecule with a melting point of -23°C. By contrast, the ionic solid NaCl has a melting point of 800°C.