Wwwengbookspdfcom periodic trends in atomic

PERIODIC TRENDS IN ATOMIC PROPERTIES 313
electron is also 2p and the nuclear charge has
increased.
• Nitrogen has a still higher ionization energy
because of the further increase in nuclear charge.
• There is now a kink in the curve because the
ionization energy of oxygen is lower than would
be expected by simple extrapolation.
• At oxygen, a 2p orbital must become doubly
occupied, and the electron–electron repulsions are
increased above what would be expected by simple
extrapolation along the row. (The kink is less pro-
nounced in the next row, between phosphorus and
sulfur, because their orbitals are more diffuse.)
• The values for oxygen, fluorine, and neon fall
roughly on the same line, the increase of their ion-
ization energies reflecting the increasing attrac-
tion of the nucleus for the outermost electrons.
• The outermost electron in sodium is 3s. It is far
from the nucleus, and the latter’s charge is
shielded by the compact, complete neon-like core.
As a result, the ionization energy of sodium is sub-
stantially lower than that of neon.
• The periodic cycle starts again along this row,
and the variation of the ionization energy can be
traced to similar reasons.
The electron a G nity , E
ea
, is the difference in energy
between a neutral atom and its anion. It is the energy
released in the process
X(g) + e
−
(g) → X
−
(g)
E
ea
= E (X) − E (X
−
)
(13.10a)
The electron a A nity is positive if the anion has a
lower energy than the neutral atom. Care should be
taken to distinguish the electron a A nity from the
electron-gain enthalpy (Section 3.2): they have very
similar numerical values but differ in sign:
X(g) + e − (g) → X − (g)
Δ
ea
H = H
m
(X − ) − H
m
(X)
(13.10b)
Electron a A nities (Table 13.3) vary much less
systematically through the periodic table than
ionization energies. However, the following general
observations are important:
• Broadly speaking the highest electron a A nities
are found close to fluorine. In the halogens, the
incoming electron enters the valence shell and
experiences a strong attraction from the nucleus.
• The electron a A nities of the noble gases are
negative—which means that the anion has a
higher energy than the neutral atom—because
the incoming electron occupies an orbital outside
the closed valence shell. It is then far from the
nucleus and repelled by the electrons of the
closed shells.
• The first electron a A nity of oxygen is positive for
the same reason as for the halogens. However, the
second electron a A nity (for the formation of O 2 −
from O − ) is strongly negative because although
the incoming electron enters the valence shell, it
experiences a strong repulsion from the net nega-
tive charge of the O
−
ion.
Table 13.3
Electron affinities of main-group elements, E
ea
/eV*
H He
+ 0.75 < 0
†
Li Be B C N O F Ne
+ 0.62 − 0.19 + 0.28 + 1.26 − 0.07 + 1.46 + 3.40 − 0.30 †
Na Mg Al Si P S Cl Ar
+ 0.55 − 0.22 + 0.46 + 1.38 + 0.46 + 2.08 + 3.62 − 0.36
†
K Ca Ga Ge As Se Br Kr
+ 0.50 − 1.99 + 0.3 + 1.20 + 0.81 + 2.02 + 3.37 − 0.40
†
Rb Sr In Sn Sb Te I Xe
+ 0.49 + 1.51 + 0.3 + 1.20 + 1.05 + 1.97 + 3.06 − 0.42 †
Cs Ba Tl Pb Bi Po At Rn
+ 0.47 − 0.48 + 0.2 + 0.36 + 0.95 + 1.90 + 2.80 − 0.42 †
* 1 eV = 96.485 kJ mol − 1 . See also Table 3.3.