General Chemistry Principles, Patterns, and Applications, 2011

could therefore be described quantitatively by the average of the values of its first ionization energy and
the absolute value of its electron affinity. Using our definition of electron affinity, we can write Mulliken’s
original expression for electronegativity as follows: [1]
Equation 7.15
X =I+|EA|/2
Elements with a large first ionization energy and a very negative electron affinity have a large positive
value in the numerator of Equation 7.15, so their electronegativity is high. Elements with a small first
ionization energy and a small electron affinity have a small positive value for the numerator in Equation
7.15, so they have a low electronegativity. [2]
As noted previously, all electronegativity scales give essentially the same results for one element relative
to another. Even though the Mulliken scale is based on the properties of individual atoms and the Pauling
scale is based on the properties of atoms inmolecules, they both apparently measure the same basic
property of an element. In the following discussion, we will focus on the relationship between
electronegativity and the tendency of atoms to form positive or negative ions. We will therefore be
implicitly using the Mulliken definition of electronegativity. Because of the parallels between the Mulliken
and Pauling definitions, however, the conclusions are likely to apply to atoms in molecules as well.
Electronegativity Differences between Metals and Nonmetals
An element’s electronegativity provides us with a single value that we can use to characterize the
chemistry of an element. Elements with a high electronegativity (χ ≥ 2.2 in Figure 7.15 "Pauling
Electronegativity Values of the ") have very negative affinities and large ionization potentials, so they are
generally nonmetals and electrical insulators that tend to gain electrons in chemical reactions (i.e., they
are oxidants). In contrast, elements with a low electronegativity (χ ≤ 1.8) have electron affinities that have
either positive or small negative values and small ionization potentials, so they are generally metals and
good electrical conductors that tend to lose their valence electrons in chemical reactions (i.e., they
are reductants). In between the metals and nonmetals, along the heavy diagonal line running from B to At
in Figure 1.24 "The Periodic Table Showing the Elements in Order of Increasing ", is a group of elements
with intermediate electronegativities (χ ~ 2.0). These are the semimetals, elements that have some of
the chemical properties of both nonmetals and metals. The distinction between metals and nonmetals is
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