The properties of a solid mainly reflect the properties of the electronic structure of its atoms. The existence of a solid state as the stable low-temperature configuration of the atoms proves that there are attractive forces between the atoms. If two atoms are far apart, this attractive force is small and caused by the dipole moment of their electron structure. A dipole moment results from the fact that the centers of gravity of the positive charge (nucleus) and the negative charge (electrons) are never identical, because of fluctuations of the electron density distribution. A locally separated positive and negative charge forms an electrical dipole. The interaction among dipoles always is attractive and, therefore, causes atoms to approach each other (Fig. 10.1). The attractive force increases rapidly with decreasing distance between dipoles. If the atoms reach a distance where their outermost electrons interact, various outcomes can occur depending on the different types of chemical bonding as covered in Chapter 2. A further decrease of the atomic spacing causes the electron orbitals to overlap. According to the Pauli principle, two electrons cannot share the same energy state. Correspondingly, an overlap forces some electrons to change to “free” states, which are of higher energy. The corresponding rapid increase of electron energy results in a strong repulsive interatomic force. The sum of the attractive and repulsive forces constitutes the total interaction force between the atoms (Fig. 10.2). At the equilibrium spacing, repulsive and attractive forces balance, i.e., the sum of forces is zero. This simple concept of the formation of a two-atomic molecule can be generalized to a solid consisting of many atoms, where the same interactions determine the next neighbor arrangement, qualitatively.
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- Physical Properties
Professor Dr. Günter Gottstein
- Springer Berlin Heidelberg
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