Chemistry Reference
In-Depth Information
Bonding in molecules and solids
2.1 Introduction
Many trends in the properties of solids follow directly from trends in the
properties of the constituent atoms. The semiconductors germanium (Ge),
gallium arsenide (GaAs) and zinc selenide (ZnSe) are all formed from
atoms in the same row of the periodic table: they all have the same crystal
structure and approximately the same lattice constant, but the fundamental
band gap increases on going from the covalently bonded group IV semi-
conductor Ge to the polar III-V compoundGaAs, and again on going to the
evenmore polar II-VI compoundZnSe. Silicon (Si) is fourfold-coordinated,
with four nearest neighbour atoms in almost all of the compounds which
it forms, while nitrogen (N) is generally three-coordinated, as in ammonia
(
NH
3
)
or silicon nitride
(
Si
3
N
4
)
, where each Si has four N and each N three
nearest Si neighbours.
The observation of such properties and their classification through the
Periodic Table of the elements predated the Schrödinger equation by over
fifty years, but it took the development of quantum mechanics to first
explain the structure of the periodic table, and the trends in atomic proper-
ties with increasing atomic number. It took longer still to explain how the
atomic trends give rise to the observed trends in the chemical and physical
properties of matter.
Some of the observed properties, such as high temperature supercon-
ductivity, have still to be fully understood, but there have been many
significant advances in recent years in the development of both approx-
imate and first principles methods to explain and predict a wide range of
material properties, each of which is the subject in its own right of major
text books and review papers.
We are largely concerned in this chapter with understanding the ori-
gins of chemical bonding in molecules and solids: how, as we bring atoms
closer together the atomic energy levels play a significant and predictable
role in determining the electronic energy levels of the resultant molecule or
solid. We illustrate this by first taking the square well as a prototype atom
