Entropy, Randomness, and Disorder
A very useful, though somewhat rough, description of the entropyA thermodynamic state function, symbol S, that equals the reversible heat energy transfer divided by temperature; higher entropy corresponds to greater dispersal of energy on the molecular scale. See also standard entropy. of a substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. is as a measure of the randomness or disorder of the atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. and molecules which constitute that substance. In these terms the second law of thermodynamicsA formal statement that any spontaneous (product-favored) process is accompanied by an increase in the entropy (dispersal of energy) of the universe. is seen as a tendency for the disorder of the universe to increase. This way of looking at entropy is entirely compatible with the approach presented above. A situation which we intuitively recognize as being orderly is also one which can only be achieved in a limited number of ways. By contrast, situations which we recognize as disordered, random, or chaotic, can he achieved in a whole variety of ways. In other words, W, and hence S, is small for an ordered situation but large for a disordered situation.
There are limits to the lengths one can take this order-disorder approach to entropy, though. It does not lend itself to a quantitative treatment, and it is also difficult to explain some things like the effect of massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object. in these terms. There is nothing in our intuition about order, for example, which suggests that 1 mol Xe gasA state of matter in which a substance occupies the full volume of its container and changes shape to match the shape of the container. In a gas the distance between particles is much greater than the diameters of the particles themselves; hence the distances between particles can change as necessary so that the matter uniformly occupies its container. is more disordered than 1 mol He gas, even though its entropy is in fact larger.