Principles of General Chemistry. 2 nd ed.

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Opposites attract
attract
repel
In a many-electron atom, each electron feels both
the attraction to the protons in the nucleus and
the repulsion from other electrons
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
Effective nuclear charge
In a many-electron atom, each electron feels both
the attraction to the protons in the nucleus and
the repulsion from other electrons
• Force of attraction increases as nuclear
charge (# of protons) increases
• Force of attraction decreases as the
electron goes farther from the nucleus
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
In effect, the charge felt by an electron is
less than the full nuclear charge
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
In effect, the charge felt by an electron is
less than the full nuclear charge
• The effective nuclear charge (Zeff) is the
nuclear charge that is actually felt by an
electron
• An electron is shielded from the full charge
of the proton
– greatly by the inner electrons
– only slightly by the other electrons in the
same principal quantum number (n)
– not at all by outer electrons
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
The effective nuclear charge and electron
configuration are key in understanding the
periodic trends
• Atomic radius
Physical properties
• Ionic radius
– neutral vs. charged
– isoelectronic series
• Ionization energy
Chemical properties
– one atom vs. another
– same atom
• Electron affinity
Atomic radius
Atomic radius is ½ the distance between the
two nuclei in two adjacent atoms
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
*Radii in pm
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
Left to right: decreasing atomic radius
Decreasing atomic radius
• Zeff dominates
• number of protons increases
• electrons are added to the same n, so
shielding by inner electrons does not
change while shielding by electrons
belonging to the same n is poor
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
Top to bottom: increasing atomic radius
• n dominates
• going down the group, each
member has one more level of
inner electrons that shield the outer
electrons very effectively
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
Ionic radius:
Neutral vs. charged
If the atom forms a cation, its radius decreases
• same number of
protons
• less electrons
• electron-electron
repulsion is reduced
• the electron cloud
becomes smaller
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
If the atom forms an anion, its radius increases
• same number of
protons
• more electrons
• electron-electron
repulsion is
enhanced
• the electron cloud
becomes bigger
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
Ionic radius:
Isoelectronic series
Ions having the same electron configuration
and same number of electrons are said to
be isoelectronic
Ion
F-
Electron
Configuration
[Ne]
Number of
Electrons
10
O2-
[Ne]
10
N3-
[Ne]
10
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
Left to right: decreasing ionic radius
• same number of electrons
• increasing number of
protons
• stronger attraction
between the protons and
the electrons
Ionization energy:
One atom vs. another
Ionization energy (IE) is the minimum energy
required to remove an electron from a
gaseous atom
energy + X(g)  X+(g) + e• The higher the IE, the more difficult it is to
remove the electron
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
Brown, , E. LeMay, and B. Bursten. 2000. Chemistry: The Central Science. 8th ed. Phils: Pearson Education Asia Pte. Ltd.
Left to right: increasing ionization energy
Increasing ionization energy
•
•
•
•
Zeff dominates
number of protons increases
same n
stronger attraction between the protons and
the electrons
• harder to remove an electron
Brown, , E. LeMay, and B. Bursten. 2000. Chemistry: The Central Science. Phils: Pearson Education Asia Pte. Ltd.
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
Decreasing ionization energy
Top to bottom: decreasing ionization energy
• n dominates
• there are more electrons in
between the protons and the
outer electrons
• weaker attraction
• easier to remove an electron
Brown, , E. LeMay, and B. Bursten. 2000. Chemistry: The Central Science. Phils: Pearson Education Asia Pte. Ltd.
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
Ionization energy:
Same atom
If more than one electron could be removed from
the same atom, there will be different IE values
X(g)  X+(g) + e-
IE1
X+(g)  X2+(g) + e-
IE2
X2+(g)  X3+(g) + e-
IE3
.
.
.
.
.
.
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
For the same atom, IE1 < IE2 <IE3
• same number of protons, less electrons
• same nuclear charge, less electronelectron repulsion
• greater attraction between the proton and
the remaining electrons
• harder to remove another electron
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
There is a dramatic increase in IE when an
electron is removed from an atom/ion with a
noble gas configuration
• the noble gas configuration is stable
• removing another electron from it will
result in instability
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
Electron affinity
Electron affinity (EA) is the energy change
accompanying the addition of electrons to
atoms or ions
X(g) + e-  X-(g)
energy gained (+) or released (-)
*usually released
• The more negative the electron affinity, the
greater the tendency to accept an electron
• Note: different books may use different
sign conventions
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
*Energies in kJ/mol
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
Left to right: increasing(?) electron affinity
Increasing(?) electron affinity
• Zeff dominates
• size decreases
• stronger attraction between the protons and
the added electron
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
Decreasing electron affinity
Top to bottom: no general trend except for
Group 1A
• For Group 1A,
– the nucleus is farther away
from an electron being added
– weaker attraction between the
protons and the added
electron
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
Factors other than Zeff and atomic size affect
electron affinities, so trends are not as
regular as those for the other properties
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