The Electronic Configuration of Atoms

Shells, Subshells, and Orbitals Orbital Shape Buildup Principle Electronic Structure and the Periodic Table Solved Problems 

The first plausible theory of the electronic structure of the atom was proposed in 1914 by Niels Bohr, a Danish physicist. To explain the hydrogen spectrum, he suggested that in each hydrogen atom the electron revolves about the nucleus in one of several possible circular orbits, each having a definite radius corre- 

Copyright 2003 by The McGraw-Hill Companies, Inc. Click Here for Terms of Use. 

The wavelength of a photon is inversely proportional to the energy of the light, and when the light is observed through a spectroscope, lines of different colors, corresponding to different wavelengths, are seen. 

The electrons in atoms other than hydrogen also occupy various energy levels. The maximum number of electrons that can occupy a given shell depends on the shell number. For example, in any atom, the first shell can hold a maximum of only two electrons, the second shell can hold a maximum of eight electrons, the third shell can hold a maximum of 18 

In Chap. 3 the elementary structure of the atom was introduced. The facts that protons, neutrons, and electrons are present in the atom and that electrons are arranged in shells allow us to explain isotopes (Chap. 3), chemical bonding (Chap. 5), and much more. However, with this simple theory, we still have not been able to deduce why the transition metal groups and inner transition metal groups arise, and many other important generalities. In this chapter we introduce a more detailed description of the electronic structure of the atom which begins to answer some of these more difficult questions. 

The modern theory of the electronic structure of the atom is based on experimental observations of the interaction of electricity with matter, studies of electron beams (cathode rays), studies of radioactivity, studies of the distribution of the energy emitted by hot solids, and smdies of the wavelengths of light emitted by incandescent gases. A complete discussion of the experimental evidence for the modern theory of atomic structure is beyond the scope of this book. In this chapter only the results of the theoretical treatment will be described. These results will have to be memorized as rules of the game, but they will be used so extensively throughout the general chemistry course that the notation used will soon become familiar. In the rest of this course, the elementary theory presented in Chap. 3 will suffice. You should study only those parts of this chapter that are covered in your course. 

---

There are two immediate questions we ask about the transition elements once we know where they are in the periodic table (1) Why do we consider these elements together (2) What is special about their properties These questions are closely related because they both depend upon the electron configurations of the atoms. What, then, is the electron configuration we might expect for these elements ... 

The valence electron configuration of the atoms of the Group 2 elements is ns1. The second ionization energy is low enough to be recovered from the lattice enthalpy (Fig. 14.18). Flence, the Group 2 elements occur with an oxidation number of +2, as the cation M2+, in all their compounds. Apart from a tendency toward nonmetallic character in beryllium, the elements have all the chemical characteristics of metals, such as forming basic oxides and hydroxides. 

As seen in Section 10.4, the quantum numbers determine the electronic configuration of the atom and also the shape of the atomic orbitals (Fig. 

The illustrations that depict the electron configurations of the atoms of each element are based on the Bohr model of quantum energy shells. 

Indicate the position of oxygen in Mendeleev s periodic table of the elements, the size of its atom, the electron configurations of the atom and molecule, and its oxidation states. How can the paramagnetic properties of liquid oxygen be explained ... 

Write the electron configurations of the atoms of the zinc subgroup elements. What oxidation states do these elements exhibit in their compounds Give examples. What position do these elements occupy in the electrochemical series ... 

In keeping with the 3f/ 4v1 electron configuration of the atom, chromium forms compounds in which it is in the stable 6+ state. The compounds of 2+ and 3+ chromium arc also quite numerous. The compounds with chromium oxidation numbers of 4+ and 5+ are unstable except under extremely alkaline conditions. The one known chromium I f complex is also unstable. 

PERIODIC LAW. Originally stated in recognition of an empirical periodic variation of physical and chemical properties of the elements with atomic weight, this law is now understood to he based fundamentally on atomic number and atomic structure. A modern statement is the electronic configurations of the atoms of the elements vary periodically with their atomic number. Consequently, all properties of the elements that depend on their atomic structure (electronic configuration) tend also to change with increasing atomic number in a periodic manner. 

A set of pairs of quantum numbers n,7, with the indicated number of electrons having these quantum numbers, is called an electronic configuration of the atom (ion). Thus, we have already discussed the cases of two non-equivalent electrons and a shell of equivalent electrons. If there is more than one electron with the same nf, then the configuration may look like this ... 

---