Concepts of Modern Physics by Arthur Beiser

Author:Arthur Beiser
Language: eng
Format: epub, pdf
Tags: Physics; Special Relativity; Photoelectric; Compton; DeBroglie; Bohr; Hydrogen; Quantum; Solid State; Atom; Nucleus; Elementary Particle

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244 PROPERTIES OF MATTER

FIGURE 8 2 Scale model of NaCI crystal.

In Hj the bond is purely covalent and in NaCI it is purely ionic, hut in many other molecules an intermediate type of bond occurs in which the atoms share electrons to an unequal extent. An example is the MCI molecule, where the CI atom attracts the shared electrons more strongly than the H atom. A strong argument can be made for thinking of the ionic Iwnd as no more than an extreme case of the covalent bond.

8.2 ELECTRON SHARING

The simplest possible molecular system is l\ 2 ' , the hydrogen molecular ion, in which a single electron bonds two protons. Before we consider the bond in H.,~ in detail, let us look in a general way into how it is possible for two protons to share an electron and why such sharing should lead to a lower total energy and hence to a stable system.

In Chap. 5 we discussed the phenomenon of quantum-mechanical barrier penetration: a particle can "leak" out of a box even though it does not have enough energy to break through the wall because the particle's wave function extends l>eyond it. Only if the wall is infinitely strong is the wave function wholly inside the box. The electric held around a proton is in effect a Ijox for an electron, and two nearby protons correspond to a pair of boxes with a wall between them (Fig. 8-3). There is no mechanism in classical physics by which the electron in a hydrogen atom can transfer spontaneously to a neighboring proton more distant than its parent proton. In quantum physics, however, such a mechanism does exist. There is a certain proI>ability that an electron trapped in one box will tunnel through the wall and get into the other box, and once there it hie* the same probability for tunneling back. This situation can be described bv saving that the electron is shared by the protons.

To lie sure, the likelihood that an electron will pass through the region of high potential energy—the "wall"—between two protons depends strongly upon how far apart the protons are. If the pro ton- proton distance is 1 A, the electron

THE PHYSICS OF MOLECULES

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