Atomic structure wave-particle duality

A basic understanding of the quantum theory is essential in many areas of chemistry, especially in connection with spectroscopy and with theories of atomic and molecular structure. The present book gives an introduction to the theory, and its application to elementary atomic structure, but chemical bonding is not discussed. I have tried to put the essential ideas in their historical context, but without retaining the historical introduction which has been traditional with this topic. With the crucial and difficult concepts of wave-particle duality, it seemed to me more important to give modem illustrations to show that they have current applications in chemistry. 

Quantum chemistry is the appfication of quantum mechanical principles and equations to the study of molecules. In order to nnderstand matter at its most fundamental level, we must use quantum mechanical models and methods. There are two aspects of quantum mechanics that make it different from previous models of matter. The first is the concept of wave-particle duality that is, the notion that we need to think of very small objects (such as electrons) as having characteristics of both particles and waves. Second, quantum mechanical models correctly predict that the energy of atoms and molecules is always quantized, meaning that they may have only specific amounts of energy. Quantum chemical theories allow us to explain the structure of the periodic table, and quantum chemical calculations allow us to accurately predict the structures of molecules and the spectroscopic behavior of atoms and molecules. 

Schrodinger developed the ideas of quantum (wave) mechanics in 1925. It was then applied to determine atomic and molecular structure. The idea of covalent bonding between two atoms based on the sharing of electron pairs was proposed by Lewis in 1916. The Lewis model (of dots and crosses to represent electrons) is still relevant and useful, but the quantum mechanical model (Chapters 2 and 12), incorporating wave-particle duality, Pauli s exclusion principle and Heisenberg s uncertainty principle, gives a deeper understanding of chemical bonds. 

Despite the importance of wave-particle duality for understanding atomic structure, most textbooks ignore important issues based on HPS. Textbook presentations based on HPS can stimulate students to think and go beyond the traditional conceptualization of science as an inductive enterprise (Levere 2006). None of the general chemistry textbooks had a Satisfactory (S) presentation on four of the six HPS related criteria used in this study. On all six criteria the percentage of textbooks that were classified as No mention (N) ranged from 43% (Criterion 1) to 99% (Criterion 3). Textbooks that were classified as Mention (M) ranged from 1% (Criterion 3) to 38% (Criterion 1). 

In 1923, Walter Heitler and Fitz London fully explained electron-pair formation and chemical bonding in terms of quantum mechanics (Heitler and London, 1927). In the same year the French physicist Louise de Broglie proposed that wave-particle duality applied not only to photons, but also to electrons and every other subatomic physical system this work was published in his PhD thesis in 1924. Austrian physicist Wolfgang Pauli (1925) observed that the shell-like structure of the atom could be explained by a set of four parameters that define every quantum energy state, as long as each state was inhabited by no more than a single electron. 

Although Einstein made use of the assumption that light behaves as a particle, there is no denying the validity of the experiments that show that light behaves as a wave. Actually, light has characteristics of both waves and particles, the so-called particle-wave duality. Whether it behaves as a wave or a particle depends on the type of experiment to which it is being subjected. In the study of atomic and molecular structure, it necessary to use both concepts to explain the results of experiments. 

This confirmation of de Broglie s prediction brought duality into the concept of the nature of fundamental particles. A particle was not simply a particle but had a wave aspect to its nature. This idea led very quickly to the development of wave mechanics, or quantum mechanics, by Heisenberg and Schrodinger. All of our modern ideas on atomic and molecular structure are based on wave mechanics. 

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