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SOLID STATE

If you take a paper clip and bend it it would stay bent, it wouldn't, spring back and it wouldn't break. The metal of which the clip is made is ductile. Some other materials are not duc­tile at all. If you tried to bend a glass rod (unless you are holding it in a flame), it would simply break. It is brittle. In this respect as in many others, glass behaves quite different­ly from a metal. The difference must lie either in the parti­cular atoms of which metals and glass are made up or in the way they are put together, probably both. There are of course many other differences between metals and glass.

Metals, for example, conduct electricity and therefore are used for electrical transmission lines, glass hardly conducts electricity at all and can serve as an insulator. Glass being transparent, it can be used in windows whereas a sheet of metal even more than a millionth of an inch thick is quite opaque. It is of course interesting to understand the reasons of these differences in behaviour.

During the past 20 years studies of this kind have been call­ed solid-state physics, or sometimes since the subject Includes a great deal of chemistry, just "solid state". It is a major branch of science that has revealed new and previously unsus­pected properties in metals. Solid-state physics has become one of the most important branches of technology. It has given rise to technological progress. Having studied this branch of techno­logy, engineers could understand much better the phenomenon of quantum mechanics as it is applied to solids. Though solids, of course, were the subject of experimental investigation long be­fore quantum mechanics was invented.

If we consider the faсt known sinсe the earliest studies of electric currents, we should remember that metals conduct electricity well and most materials do not.

It is only the discovery of electron that could help the scientists to understand some of these facts well. With the dis­covery of electron it was assumed that in metals some or all of the atoms had lost an electron and that in insulators such as glass they had not. The electrons in a metal proved thus to move freely, whereas the electrons in insulators do not. Why did this happen in metals? This very question had to await the.discovery of quantum mechanics. The next question was "How are the electrons arranged?"

As far as this question is concerned we can say that solids can be divided into two clasps: crystalline and amorphous. In the crystalline group, which is the largest and includes the metals and most minerals, the atoms are arranged in a regular way.