MOLECULES AND SOLIDS

Almost all atoms will attract one another when they come within a couple of diameters of one another. However, the Pauli Principle demands that the probability distributions of the atoms cannot significantly overlap, so in effect there is an infinite repulsive barrier encountered when the overlap begins. When two or more atoms can form a stable bound state we refer to “atomic bonding.” Among the types of bonds encountered are the ionic bond, the covalent bond, the hydrogen bond, the Van der Waals bond, and, in solids, the metallic bond.

There are two types of internal excitations possible for most molecules. “Rotational” excitations have energies like Eℓ = [ℏ2/(2I)][ℓ(ℓ + 1)], where ℓ = 0, 1, etc. “Vibrational” excitations go like En = ℏω(n + 1/2), where n = 0, 1, etc. The spacing between rotational levels is typically about 10−3 eV, while the spacing between vibrational levels is more like 0.1 eV. The transition selection rules are Δℓ = ±1, and Δn = ± 1.

Molecular spectra are characterized by distinctive “bands,” very different from the line spectra of individual atoms.

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