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C06_021A
Trends in First Ionization Energies
Generally increases
Generally decreases
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Trends within periods
As shown in
Figure 17
and by the values in
Table 5
,
first ionization energies generally increase as you
move from left to right across a period. The increased
nuclear charge of each successive element produces
an increased hold on the valence electrons.
Trends within groups
First ionization energies generally decrease as you
move down a group. This decrease in energy occurs
because atomic size increases as you move down the group. Less energy is required to
remove the valence electrons farther from the nucleus.
Figure 18
summarizes the group
and period trends in first ionization energies.
Octet rule
When a sodium atom loses its single valence electron to form a 1
+
sodium ion, its
electron configuration changes as shown below.
Sodium atom 1s
2
2s
2
2p
6
3s
1
Sodium ion 1s
2
2s
2
2p
6
Note that the sodium ion has the same electron configuration as neon (1s
2
2s
2
2p
6
), a
noble gas. This observation leads to one of the most important principles in chemistry,
the
octet rule.
The octet rule states that atoms tend to gain, lose, or share electrons in
order to acquire a full set of eight valence electrons. This reinforces what you learned
earlier, that the electron configuration of filled s and p orbitals of the same energy
level (consisting of eight valence electrons) is unusually stable.
Note that the first-period elements are an exception to the
rule, as they are complete with only two valence electrons.
Electronegativity
The
electronegativity
of an element indicates the relative
ability of its atoms to attract electrons in a chemical bond.
As shown in
Figure 19
, on the next page, electronegativity
generally decreases as you move down a group.
Figure 19
also indicates that electronegativity generally increases as
you move from left to right across a period. Fluorine is the
most electronegative element, with a value of 3.98, meaning
it attracts electrons more strongly than any other element
in a chemical bond. Cesium and francium are the least
electronegative elements, with values of 0.79 and 0.70,
repectively. In a chemical bond, the atom with the greater
electronegativity more strongly attracts the bond’s elec-
trons. Note that because the noble gases form very few
compounds, they do not have electronegativity values.
Figure 18
Ionization energies
generally increase from left to right in
a period and generally decrease as
you move down a group.
Real-World Chemistry
Ionization Energy
SCUBA DIVING The increased pressure
that scuba divers experience far below
the water’s surface can cause too much
oxygen to enter their blood, which would
result in confusion and nausea. To avoid
this, divers sometimes use a gas mixture
called heliox—oxygen diluted with helium.
Helium’s high ionization energy ensures
that it will not react chemically in the
bloodstream.
Lesson 3 • Periodic Trends
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