other electrostatically if the distance between them is small enough. A typical example is the water molecule in its liquid and solid states. This interaction force is called dipole-dipole interaction. It is a weak intermolecular force, i.e. a strictly physical bond operating between molecules. Physical bonding differs from chemical bonding in that physically bonded molecules remain unchanged when the interaction between them is terminated. When chemical bonds are broken, new molecules are formed.

Hydrogen and carbon have a similar electronegativity range, and must share binding electrons almost equally. Therefore molecules comprising these two elements are balanced in the distribution of their binding electrons and form non-polar molecules. Built exclusively from hydrogen and carbon, the so-called hydrocarbons are held together only by weak physical interaction forces. Hydrocarbon molecules can be easily separated from each other because they are only weakly held together. The little energy needed for their separation means that they have low melting and boiling points with respect to their molecular mass (molecular weight).

Water is an interesting example of a highly polar molecule. It consists of two hydrogen atoms linked to one oxygen atom. Oxygen exhibits a considerably higher eletronegativity than the hydrogen atoms attached to it. The difference in electronegativity between oxygen (3.4) and hydrogen (2.2) amounts to 1.2 units (Table 1.4). Oxygen has four electrons, i.e. two lone pairs, in its outermost shell that are not involved in the bonding.

The -O-H bond must be regarded as a covalent bond of considerable polarity. The electronegative oxygen pulls the binding electrons towards itself, leaving the partially positively charged nuclei of the two attached hydrogen atoms nearly unshielded by the joint molecular orbital. As the electrons carry the negative charge, water molecules become electrically polarised due to this shifting of electrons. The oxygen becomes partially negatively charged leaving the hydrogen positively charged. However, the molecule as a whole is electrically balanced and therefore is electrically neutral. Strong dipole-dipole interaction forces are effective between molecules where an unshielded positive hydrogen nucleus is strongly attracted to an atom such as oxygen that is rich in electrons. Water is an important example of such a molecule. Two factors account for the strength of these interactions: the high polarity of the -O-H bond and the close proximity of the molecules resulting from the small size of the hydrogen atom. These particular physical attraction forces are of exceptional strength. Therefore they are given a special term that includes the word bond,