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120 CHEMISTRY FOR THE UTTERLY CONFUSED
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The goal of this chapter is to help you relate the properties of elements to their
position on the periodic table. These properties include ionization energies and
electron affinities. You may want to review the basic structure of the periodic
table in Chapter 2 and electron configurations in Chapter 7. And Don’t forget!—
Practice, Practice, Practice.
8-1 Periodic Table Revisited
In Chapter 2, we showed you the arrangement of the periodic table. The
columns are groups or families. The group members have similar chemical
properties and somewhat similar physical properties. The rows are periods. The
row members have predictably different properties. In Chapter 7, we showed
you the electron configurations of the elements. You learned how similar electron
configurations lead to similar chemical properties. There are, however,
other periodic properties. These properties relate to the magnitude of the
attractive force that the protons in the nucleus have for electrons. These include
atomic radii, ionization energies, and electron affinities. The inner electrons
(core electrons) somewhat screen the valence electrons from the attraction of
the nucleus. The effective nuclear charge is the nuclear charge minus the
screening effect of the core electrons. This screening is essentially constant for
all of the elements in any period on the periodic table. However, the screening
increases towards the bottom of any family.
The effective nuclear charge is useful in explaining most of the observed trends.
For example, there is a trend of decreasing atomic radii when moving from left to
right across any period on the periodic table. The atomic radius is the distance the
valence electrons are from the nucleus. In going from top to bottom on the periodic
table, each row adds an additional energy level. Thus, the valence electrons
are farther from the nucleus and the atoms increase in size. (There are some
minor exceptions, especially in the transition metals.) The atomic radius decreases
when moving from left to right within a period. The nuclear charge is increasing
across the period. The number of core electrons is constant, so the screening is
constant. The effective nuclear charge is increasing due to the increase in nuclear
charge minus the constant screening. This increase exerts a greater attractive
force on the valence electrons. The increased attraction pulls the outer electrons
closer to the nucleus, resulting in a smaller atom.