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Electronic Structure of Atoms

The periodic table, discussed in Chapter 2, evolved largely from experimental observations. Elements that exhibit similar properties were placed together in the same column of the table. But what are the fundamental reasons for these similarities? Why, for example, are sodium and potassium both soft, reactive metals? Why are helium and neon both unreactive gases? Why do the halogens all react with hydrogen to form compounds that contain one hydrogen atom and one halogen atom?

When atoms react, it is the electrons that interact. Thus, the key to answering questions like those posed above lies in understanding the behavior of electrons in atoms. The arrangement of electrons in an atom is called its electronic structure. The electronic structure of an atom refers not only to the number of electrons that an atom possesses but also to their distribution around the nucleus and to their energies.

As we will see, electrons do not behave like anything we are familiar with in the macroscopic world. Our knowledge of electronic structure is the result of one of the major developments of twentieth-century science, the quantum theory. In this chapter we will describe the development of the quantum theory and how it led to a consistent description of the electronic structures of the elements. We will explore some of the tools used in quantum mechanics, the new physics that had to be developed to describe atoms correctly. In the chapters that follow, we will see how these concepts are used to explain trends in the periodic table and the formation of bonds between atoms.

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