Page 84 - Electrical Properties of Materials

Bonds

5 Striking the electric chain wherewith we are darkly bound

Byron Childe Harold’s Pilgrimage
Those whom God has joined together,
Let no man put asunder.
Marriage service

5.1 Introduction

As we have seen, an electron and a proton may strike up a companionship,
the result being a hydrogen atom. We have found that the energy of the elec-
tron is a negative number, that is, the electron in the vicinity of the proton has
a lower energy than it would have if it were an infinite distance away, which
corresponds to zero energy. The minimum comes about as some sort of com-
promise between the kinetic and potential energy, but the important thing is
that a minimum exists. The electron comes closer to find lower energy.
Can we say the same thing about two hydrogen atoms? Would they too come
close to each other in order to reduce the total energy? Yes, they come close;
they combine and make up a hydrogen molecule. This combination between
atoms is called a chemical bond, and the discipline that is concerned with these
combinations is chemistry.
You may justifiably ask why we talk of chemistry in a course on electrical
properties of materials. Well, in a sense, chemistry is just a branch of the elec-
trical properties of materials. The only way to explain chemical bonds is to use
electrical and some specific quantum-mechanical properties. And not only is
chemistry relegated to this position: metallurgy, is too. When a large num-
ber of atoms conglomerate and make up a solid, the reason is again to be
sought in the behaviour of electrons. Thus, all the mechanical properties of
solids, including their very solidity, spring from the nature of their electrical
components.
This is true in principle but not quite true in practice. We know the funda-
mental laws, and so we could work out everything (the outcome of all chemical
reactions, the strength of all materials) if only the mathematical problems could
be overcome. Bigger computers and improved techniques of numerical ana-
lysis might one day make such calculations feasible, but for the moment it is
not practicable to go back to first principles. So we are not going to solve the
problems of chemistry and metallurgy here. Nevertheless, we need to under-
stand the nature of the chemical bond to proceed further. The bond between
hydrogen atoms leads to the bond between the atoms of heavier elements. We
shall encounter among others germanium and silicon, their band structure, how
they can be doped, and how they can serve as the basis upon which most of our
electronic devices are built. Is it worth starting with the fundamentals? It is far