Periodic Properties and Electronic Structure

The sizes of atoms and ions influence how they interact in chemical compounds. Although atomic radius is not a precisely defined concept, these sizes can be estimated in several ways. If the electron density is known from theory or experiment, a contour surface of fixed electron density can be drawn, as demonstrated in Section 5.1 for one-electron atoms. Alternatively, if the atoms or ions in a crystal are assumed to be in contact with one another, a size can be defined from the measured distances between their centers (this approach is explored in greater detail in Chapter 21). These and other measures of size are reasonably consistent with each other and allow for the tabulation of sets of atomic and ionic radii, many of which are listed in Appendix F. 

FIGURE 5.22 Ionic and atomic radii plotted versus atomic number. Each line connects a set of atoms or Ions that have the same charge all species have noble-gas configurations. 

FIGURE 5.23 The moiar voiumes (measured in cm moi of atoms) of some eiements in their soiid states. Note the iarge vai-ues for the aikaii metais. 

The ionization energy tends to decrease down a group in the periodic table (for example, from lithium to sodium to potassium). As the principal quantum number increases, so does the distance of the outer electrons from the nucleus. There are some exceptions to this trend, however, especially for the heavier 

FIGURE 5.24 First and second ionization energies of atoms of the first three periods. 

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