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Chapters 18/19 – "Representative Elements"
18.1) Survey of the Representative Elements
18.2) Group 1A Elements
18.3) The Chemistry of Hydrogen
18.4) Group 2A Elements
18.5) Group 3A Elements
18.6) Group 4A Elements
19.1) Group 5A Elements
19.2) The Chemistry of Nitrogen
19.3) The Chemisty of Phosphorus
19.4) Group 6A Elements
19.5) The Chemistry of Oxygen
19.6) The Chemistry of Sulfur
19.7) Group 7A Elements
19.8) Group 8A Elements
2
s-block
elements
p-block
elements
d-block
elements
The Chemistry of Representative Elements
3
alkali
metals
Group 1A Elements
(ns
1
valence electron configuration)
nonmetal
4
Common Theme: The first member is
usually the “black sheep” of the family
reactivity and nature
of compounds formed
are noticeably different
H
Be B
7
N
2 (g)
+ 3 H
2 (g)
2 NH
3 (g)
heat
catalyst
The Chemistry of Hydrogen
industrial
fertilizer
required for
plants to
synthesize
proteins
Nature uses a Metalloenzyme called Nitrogenase
to Synthesize NH 3 under mild conditions
Found: anaerobic bacteria
blue-green algae
symbiotic plant bacteria (root nodules)
Function: N 2 + 8e
NH 3 + H 2
12 ATP 12 ADP + Pi
N 2 ase
biological enzyme
catalytic site:
9
H
(aq) + 2e-^
H
(hydride)
H
(aq) +^
H
H 2 (g)
LiH, CaH 2 (hydride salts)
The "Hydrogen Economy"
2H
2 (g)
+ O
2 (g)
2 H
O
(g)
+ Energy!
How do we store and release hydrogen safely?
Among the methods receiving consideration is storage as
covalent boro-hydrides ("boranes").
Borane chemistry is very rich. The boranes are typically
very reactive because they are extremely electron deficient.
Current research is targeting development of reversible
storage chemistries involving boranes.
NaBH
4 (s)
+ 2H
O
(l)
4H
2 (g)
+ NaBO
2 (s)
Figure 18.
Figure 18.
Bonding in Boranes
Valence configuration of B = 2s
2
2p
1
Electron deficient- "bridge bonds" require unusual 3-
center, 2-electron bonding!
(similar bonding observed in Be hydrides and halides, section 18.4)
Another source of H 2 : metalloenzymes
A billion years ago, enzymes developed that could convert protons from
water into H 2 , and then burn the H 2 as a source of energy. These
enzymes are called Hydrogenases.
16
alkali
metals
Group 1A Elements
(ns
1
valence electron configuration)
nonmetal
20
M
0 (alkali metal) + H 2 O (^) MOH + H 2
exothermic
increasing
violence
of reaction
X= Li, Na, K, Rb, Cs Y= H
21
oxidation
state
O
1–
O
2–
O 2
2–
O 2
all M
1+
*Basic oxides
M 2 O (^) + H 2 O 2 M
OH
(aq)
*Basic oxides:
22
Basic Oxides
(react with H
Acidic Oxides
(react with OH
Amphoteric Oxides
(react with H
or OH
Be B
25
Alkaline Earth Metals combine with Oxidants
to Form Ionic Compounds
Redox
all M
2+
26
M
0 (alkaline earth) + 2H 2 O (^) M(OH) 2 + H 2
less
exothermic
increasing
reactivity
X= Ca, Sr, Ba Y= H
27
Unlike the other alkaline earth metals, Be forms
covalent compounds with unusual bonding properties
28
dative bond
Be= electron acceptor
Cl= electron donor
BeCl 2 = electron deficient
polymeric structure
