Chap. 14 MAIN−GROUP ELEMENTS
Overview
• Recognize hydrogen’s unique properties
• Basic introduction to chemical behavior of
representative element groups (A groups).
• Recognize the influence of electron configuration:
– Reactivity of Groups 1A and 7A
– Lack of reactivity of Group 8A
– Deviations in expected chemical behavior in Groups 3A &
4A
– Trends in metallic behavior in Groups 3A−6A
– Multiple oxidation states in Groups 3A-6A
• Explain why Period 2 elements exhibit anomalous
properties
• Valence electrons are s and p electrons in highest
energy (n) level (§8.3).
• Group number = number of outer/valence electrons.
• Chemistry of transition metals in Chapter 22
Review: Chapter 8
• Understand the nature of allotropes
14.1 Hydrogen, the Simplest* Atom
1A
2A
n = 1
H
H
n = 2
Li
Li Be
Be
n = 3 Na
Na Mg
Mg
3A
4A
5A
6A
7A
8A
He
He
BB
CC
N
N O
O
FF Ne
Ne
KK Ca
Ca
Al
Al Si
Si PP SS Cl
Cl Ar
Ar
Ga
Ga Ge
Ge As
As Se
Se Br
Br Kr
Kr
n = 5 Rb
Rb Sr
Sr
In
In Sn
Sn Sb
Sb Te
Te II Xe
Xe
n = 6 Cs
Cs Ba
Ba
Tl
Tl Pb
Pb Bi
Bi Po
Po At
At Rn
Rn
n = 4
n = 7 Fr
Fr Ra
Ra
• Nonmetal (high IE1) with unique
properties
1A
H
• Produces molecular hyrides
(XHn) in reactions with
nonmetals
H
H
2H2(g) + O2(g) → 2H2O(ℓ)
Sources of H+
• Produces ionic hydrides
reactive metals
(H−)
“7A”
with
H2(g) + 2Li(s) → 2LiH(s)
• Relatively high electronegativity
H
H
2.1
2.1
Li
Li
1.0
1.0
Be
Be
1.5
1.5
B
B
2.0
2.0
Na
Na
0.9
0.9
Mg
Mg
1.2
1.2
Al
Al
1.5
1.5
Si
Si
1.8
1.8
K
K
0.8
0.8
Ca
Ca
1.0
1.0
Ga
Ga
1.6
1.6
Ge
Ge
1.8
1.8
Rb
Rb
0.8
0.8
Sr
Sr
1.0
1.0
In
In
1.7
1.7
Sn
Sn
1.8
1.8
Cs
Cs
0.7
0.7
Ba
Ba
0.9
0.9
C
C
2.5
2.5
*But not in any pejorative sense…
14.2 Group 1A: Alkali Metals
1A
Li
Li
Na
Na
KK
Rb
Rb
Cs
Cs
Fr
Fr
14.3 Group 2A: Alkaline Earth Metals
ns1
2A
• Soft, low-density solids
• Most active metals
Be
Be
• Strongest reducing agents
Mg
Mg
• Ionic compounds
Ca
Ca
• Combine directly with nonmetals:
Sr
Sr
2M(s) + X2(g,ℓ,s) → 2MX(s)
• React vigorously with water:
Ba
Ba
2M(s) + 2H2O (ℓ) → 2MOH(aq) + H2(g)
Ra
Ra
Page 14−1
ns2
• Harder, denser than alkali metals
• React selectively with water:
Ca, Sr —
M(s) + 2H2O (ℓ) → M(OH)2(aq) + H2(g)
• Less active than alkali metals:
Be(s) + H2O (ℓ) → no reaction
Mg(s) + H2O (g) → MgO(s) + H2(g)
• Ionic compounds, except Be
• Combine directly with nonmetals
Period 2 Anomalies
• Period 2 elements (Li, Be,…O, F) are significantly
smaller than other members of their groups ⇒ high
charge density ⇒ stronger Lewis acids/bases
2.0
1.8
rn
• Period 2 elements have no easy access to outerlevel orbitals ⇒ s, p orbitals direct chemistry
——
rn=2
• RESULT: “Anomalous” chemical properties
n = ∞
n = 6
n = 5
n = 4
1.4
COVALENT POLYMER
IONIC
Energy
Be + 2HCl → BeCl2 + H2 ↑
Ca + 2HCl → CaCl2 + H2 ↑
1.6
1.2
Be(OH)2 + 2H3O+ → Be(H2O)42+ IT’S A BASE!
NO, IT’S AN ACID!
Be(OH)2 + 2OH− → Be(OH)42−
2
3
4
n = 3
5
n
6
n = 2
Results:
• LESS METALLIC CHARACTER
0.9
IE1(n)
————
IE1(n=2)
– Less likely to form M+
– More likely to form covalent bonds
Easier
to form
M+
• MORE π-BONDING POTENTIAL
0.8
C
C
O
0.7
O
O
C
poor
overlap
2
3
4
5
Si
6
O
X
• linear molecule
• molecular solid
O
O•
Si
O
•O
Si
O•
O•
n
covalent
network
solid
Results, cont.:
Groups 5A, 6A, 7A:
• STRONGER ELECTRON-PAIR REPULSION
– Smaller atomic radii ⇒ shorter bond lengths, BUT:
– Less diffuse lone pairs
– Increased atom-atom repulsion ⇒ weaker bonds
1.50
BEX—X(n)
—————
BEX—X(n=2)
1.25
R
R
1.00
O
0.75
DX—X, pm:
BE, kJ/mol:
2
3
n
4
5
Page 14−2
R
S
O
148
204
R
S
204
266
Groups 3 & 4: Influence of Transition Elements
1A
n = 1
LiLi
Be
Be
m/z:
BB
CC
152
152
112
112
85
85
77
77
186
186
160
160
143
143
118
118
Na
Na
Mg
Mg
Al
Al
Si
Si
EN:
2+
LiLi++ Be
Be2+
BB3+3+
CC4+4+
+
2+
3+
Na
Na+ Mg
Mg2+ Al
Al3+
4+
Si
Si4+
6.9
6.9
n = 4
4.5
4.5
23.0
23.0 12.1
12.1
3.3
3.3
9.0
9.0
LiLi
n = 5
Be
Be
BB
CC
3.0
3.0
1.0
1.0
1.5
1.5
2.0
2.0
2.5
2.5
7.0
7.0
0.9
0.9
1.2
1.2
1.5
1.5
1.8
1.8
Na
Na
Mg
Mg
Al
Al
n = 6
ns2np1
Al
Al
Ga
Ga
• Al → Tl
In
In
– Metallic bonding (element)
– Covalent, ionic bonding (compounds)
Be
Be
BB
CC
1.5
1.5
2.0
2.0
2.5
2.5
0.9
0.9
1.2
1.2
1.5
1.5
1.8
1.8
0.8
0.8
1.0
1.0
1.6
1.6
1.8
1.8
0.8
0.8
Cs
Cs
1.0
1.0 •
Ba
Ba
1.7
1.7
1.8
1.8
KK
Rb
Rb
0.7
0.7
Al
Al
Ca
Ca • d, f electrons:
Sr
Sr
0.9
0.9
less effective
shielding
Zeff larger than
expected
Ga
Ga
In
In
Tl
Tl
1.8
1.8
Si
Si
Ge
Ge
Sn
Sn
Pb
Pb
1.9
1.9
Tl
Tl
4A
• Carbon
– First nonmetal (after H)
– Covalent network bonding (element)
BB
BF3(g) + :NH3(g) → F3BNH3(g)
LiLi
1.0
1.0
Na
Na Mg
Mg
14.5 Group 4A: Carbon et al.
3A
– First metalloid
– Covalent bonding only
– Bridge bonding
H
BH2
B2H6: H2B
H
– Forms Lewis acids
4A
Si
Si
14.4 Group 3A: Boron et al.
• Boron
2.1
2.1
Metallic Character
r, pm:
n = 3
3A
Metallic Character
n = 2
• Outcome of relative atomic sizes, mass-to-charge
ratios, electronegativities:
2A
HH
Metallic Character
• Certain Period 2 elements have chemical properties
very similar to those of Period 3 elements in the
Group to the right.
Metallic
Character
Diagonal Relationships: Periods 2 & 3
• Si, Ge
– Covalent network bonding (element)
• Sn, Pb
– Metallic bonding (element)
– +2 oxidation state common
• C, Sn exist as ALLOTROPES:
different crystalline or molecular
forms of an element
ns2np2
CC
Si
Si
Ge
Ge
Sn
Sn
Pb
Pb
C(graphite) → C(diamond) ∆H° = 1.9 kJ
∆H° = 3 kJ
Sn(white) → Sn(gray)
14.6 Group 5A: Nitrogen et al.
C–C bonds only
DIAMOND
(network)
– Covalent molecules (elements)
N2, P4
– Covalent compounds
– Multiple oxidation states:
−3 → +5
– Oxoanions
NOx−, POx3−
multiple bonds
GRAPHITE
(sheets)
FULLERENES
(spheres, etc)
5A
• N, P
• Phosphorus exists as allotropes:
P(white) → P(red)
nP4 → P4n
Page 14−3
∆H° = −17.6 kJ
N
N
PP
As
As
Sb
Sb
Bi
Bi
ns2np3
14.7 Group 6A: Oxygen et al.
• As, Sb
– Covalent network bonding
• Oxygen: strong oxidizing agent
– +3, +5 oxidation states common
• O, S
• Bi
– Covalent molecules (elements)
O2, S8
– Metallic bonding (element)
– +3 oxidation state most common
– Anions:
X + 2e− → X2−
• O, S bond covalently with other
nonmetals
• Se, Te bond covalently
∆H° = 143 kJ
S(rhomb) → S(mono)
∆H° = 0.3 kJ
• Most nonmetallic of the
elements
• Strongest oxidizing agents
• Sulfur chemistry more complex than that of
oxygen (d orbitals):
• Readily form anions:
S8(s) + 8O2(g) → 8SO2(g)
X2 +
2SO2(g) + 2O2(g) → 2SO3(g)
2e−
→
2X−
• Combine directly with metals,
hydrogen:
X2(g,ℓ,s) + H2(g) → 2HX(g)
X2(g,ℓ,s) + 2M(s) → 2MX(s)
Group 8A: Noble Gases
• Monatomic gases
• Filled s, p subshells
• Exceptionally unreactive
(formerly: “inert gases”)
• Kr, Xe react with fluorine
Xe(g) + 2F2(g) → XeF4(s)
Kr(g) + F2(g) + hν → KrF4(s)
ns2np4
O
O
SS
Se
Se
Te
Te
Po
Po
14.8 Group 7A: Halogens
• O, S exist as allotropes:
1½O2(g) → O3(g)
6A
8A
ns2np6
He
He
Ne
Ne
Ar
Ar
Kr
Kr
Xe
Xe
Rn
Rn
Page 14−4
7A
FF
Cl
Cl
Br
Br
II
At
At
ns2np5