positive pole on one side and a negative pole on the other. Such a particle is called dipolar or simply polar. The molecule as a whole remains electrically neutral, but the shift of electrons creates positive and negative, i.e. polarised, regions. Polar molecules will stick together by aligning their oppositely charged poles if they come closely enough together. To distinguish them from chemical bonding, the electrical attraction forces that exist between polar molecules are called physical interaction forces or molecular attraction forces. Molecules that are fixed together by physical interaction can be separated without interfering in their chemical integrity. Physical interaction forces are of overall importance with respect to the formation of molecular aggregates, that is the formation of molecular solids, liquids and, in the case where the electrical interaction forces are negligible, gases.

In the gaseous state, molecules are in constant chaotic and rapid motion (Fig. 1.2). Their velocity prevents interaction, because the force of attraction is too weak and the speed of the particles is too high. So each of them behaves as a non-socialised individual by acting mostly independently of the rest (Munowitz 2000). A gas has no defined volume or shape but assumes the shape and the volume of its container. Things change when the molecules are forced closer together, e.g. by lowering the temperature, which makes them move much slower, or by increasing the pressure, causing the available space to become smaller. Then molecules are unable to escape the attraction of those in their immediate vicinity. They must organise themselves in a way that they ‘communicate’, i.e. physically interact, with their neighbours through the increasing influence of electrostatic attraction forces (Fig. 1.3). In the liquid state, molecules are still in significant motion but they may cluster together according to the intensity of the cohesive force that is effective between them. However, this order is not permanent, it is broken up and rearranged in extremely short time periods. The clusters are said to flicker (Animation 2.1).

A liquid has a definite volume but because of its impermanent internal structure it does not have a definite shape. It flows and moulds itself to the shape of the vessel in which it is held. A molecular substance assumes its solid state when the mobility of its molecules is further reduced. This is achieved by a reduction in temperature. In the case of water, solidification occurs when the freezing point (0°C) is reached. At the point of solidification, the electrostatic attraction forces between the molecules are stronger than the competing forces of random motion that are dominant in the liquid and gaseous states. Interaction between neighbouring molecules thus intensifies, which enforces strict rules of order. In molecular solids, molecules are forced by physical interaction forces into fixed positions.