Transcript Trends in the Periodic Table

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PERIODIC
TRENDS
General Periodic Trends
• Atomic and ionic size
• Ionization energy
• Electron affinity
Higher effective nuclear charge
Electrons held more tightly
Larger orbitals.
Electrons held less
tightly.
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Effective
Nuclear
Charge
Figure 8.6
Electron cloud
for 1s electrons
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Effective Nuclear Charge
Z*
The 2s electron PENETRATES the region
occupied by the 1s electron.
2s electron experiences a higher positive
charge than expected.
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Effective Nuclear Charge, Z*
• Atom
•
•
•
•
•
•
•
Li
Be
B
C
N
O
F
Z* Experienced by Electrons in
Valence Orbitals
+1.28
------+2.58
Increase in
+3.22
Z* across a
+3.85
period
+4.49
+5.13
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General Periodic Trends
• Atomic and ionic size
• Ionization energy
• Electron affinity
Higher effective nuclear charge
Electrons held more tightly
Larger orbitals.
Electrons held less
tightly.
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Atomic Size
• Size goes UP on going down
a group. See Figure 8.9.
• Because electrons are
added further from the
nucleus, there is less
attraction.
• Size goes DOWN on going
across a period.
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Atomic Radii
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Figure 8.9
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Atomic Size
Size decreases across a period owing
to increase in Z*. Each added electron
feels a greater and greater + charge.
Large
Small
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Trends in Atomic Size
See Figures 8.9 & 8.10
Radius (pm)
250
K
1st transition
series
3rd period
200
Na
2nd period
Li
150
Kr
100
Ar
Ne
50
He
0
0
5
10
15
20
25
Atomic Number
30
35
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Sizes of Transition Elements
See Figure 8.10
• 3d subshell is inside the 4s
subshell.
• 4s electrons feel a more or
less constant Z*.
• Sizes stay about the same
and chemistries are similar!
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Ion Sizes
Li,152 pm
3e and 3p
Does+ the size go
up+ or down
Li , 60 pm
when
an
2e and 3losing
p
electron to form
a cation?
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Ion Sizes
+
Li,152 pm
3e and 3p
Li + , 78 pm
2e and 3 p
Forming
a cation.
• CATIONS are SMALLER than the
atoms from which they come.
• The electron/proton attraction
has gone UP and so size
DECREASES.
Ion Sizes
Does the size go up or
down when gaining an
electron to form an
anion?
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Ion Sizes
F, 71 pm
9e and 9p
F- , 133 pm
10 e and 9 p
Forming
an anion.
• ANIONS are LARGER than the atoms
from which they come.
• The electron/proton attraction has
gone DOWN and so size INCREASES.
• Trends in ion sizes are the same as
atom sizes.
Trends in Ion Sizes
Figure 8.13
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Redox Reactions
Why do metals lose
electrons in their
reactions?
Why does Mg form Mg2+
ions and not Mg3+?
Why do nonmetals take
on electrons?
Ionization Energy
See Screen 8.12
IE = energy required to remove an electron
from an atom in the gas phase.
Mg (g) + 738 kJ ---> Mg+ (g) + e-
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Ionization Energy
See Screen 8.12
IE = energy required to remove an electron
from an atom in the gas phase.
Mg (g) + 738 kJ ---> Mg+ (g) + e-
Mg+ (g) + 1451 kJ ---> Mg2+ (g) +
eMg+ has 12 protons
and only 11
electrons. Therefore, IE for Mg+ > Mg.
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Ionization Energy
See Screen 8.12
Mg (g) + 735 kJ ---> Mg+ (g) + e-
Mg+ (g) + 1451 kJ ---> Mg2+ (g) + e-
Mg2+ (g) + 7733 kJ ---> Mg3+ (g) + eEnergy cost is very high to dip into a
shell of lower n.
This is why ox. no. = Group no.
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General Periodic Trends
• Atomic and ionic size
• Ionization energy
• Electron affinity
Higher Z*.
Electrons held
more tightly.
Larger orbitals.
Electrons held less
tightly.
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Atomic Radii
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Figure 8.9
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Trends in Ionization Energy
1st Ionization energy (kJ/mol)
2500
He
Ne
2000
Ar
1500
Kr
1000
500
0
1
H
3
Li
5
7
9
11
Na
13
15
17
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K
21
23
25
27
29
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Atomic Number
33
35
Trends in Ionization Energy
• IE increases across a
period because Z*
increases.
• Metals lose electrons
more easily than
nonmetals.
• Metals are good reducing
agents.
• Nonmetals lose electrons
with difficulty.
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Trends in Ionization Energy
• IE decreases down a
group
• Because size increases.
• Reducing ability
generally increases down
the periodic table.
• See reactions of Li, Na, K
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Periodic Trend in
the Reactivity of
Alkali Metals
with Water
Lithium
Sodium
Potassium
Ionization Energy
See Screen 8.12
Mg (g) + 735 kJ ---> Mg+ (g) + eMg+ (g) + 1451 kJ ---> Mg2+ (g) + e-
Mg2+ (g) + 7733 kJ ---> Mg3+ (g) + eEnergy cost is very high to dip into a
shell of lower n.
This is why ox. no. = Group no.
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Electron Affinity
A few elements GAIN electrons to
form anions.
Electron affinity is the energy
involved when an atom gains
an electron to form an anion.
A(g) + e- ---> A-(g) E.A. = ∆E
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Electron Affinity of Oxygen
O atom [He] 
 

+ electron
O- ion [He] 
 
EA = - 141 kJ

∆E is EXOthermic
because O has
an affinity for an
e-.
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Electron Affinity of Nitrogen
N atom [He] 
 

+ electron
N- ion
[He] 


EA = 0 kJ

∆E is zero for Ndue to electronelectron
repulsions.
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Trends in Electron Affinity
• See Figure 8.12 and
Appendix F
• Affinity for electron
increases across a
period (EA becomes
more positive).
• Affinity decreases down
a group (EA becomes
less positive).
Atom EA
F
+328 kJ
Cl +349 kJ
Br +325 kJ
I
+295 kJ
Trends in Electron Affinity
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