Transcript Chapters 19 & 20

Chapter 19
The Representative Elements:
Groups 1A Through 4A
19.1 Survey of the Representative
Elements
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Elements in group 1A through 8A are called
representative elements because they display a wide
range of physical and chemical properties.
Representative elements display the range of
possible valence electrons from one in group 1A to
eight in group 8A.
The valence electrons of representative elements
are in s or p orbitals.
Metals tend to lose their valence electrons to form
cations with a configuration of the noble gas from
the preceding period
Nonmetals tend to gain electrons to form anins with
a configuration of the noble gas in the same period
Metalloids or semi metals
Atomic size and Group anomalies
1. H, Be, B, C, N, O & F have
properties that distinguish
them from their groups due
to their relatively small sizes
2. H is nonmetal and forms
covalent bonds; Li is metal
and forms ionic bonds
3. All oxides of G 2A are ionic
except
that of Be (BeO) it is
2.
covalent also amphoteric.
4. Same is applicable for G 3A
5. G 5A: carbon forms readily C-C
but Si forms readily Si-O
6. Si-Si exists but less stable
the C-C
7. O=C=O exists but O=Si=O is
not stable; Si-O-Si is stable.
8.Si does not form  bonds. Si 3p
valence orbitals do not overlap
with the O- 2p orbitals
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 bonding is important for relatively small elements
of the 2nd period.
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N exists as N≡N due to tendency to form  bonds
P exists as P4; P large atoms are like Si do not form
strong  bonds. They prefer to achieve noble gas
configuration but forming single bonds.
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O (G 6A) exists as O=O; tendency to form 
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bonding
S does not form  bonding thus it exists as
S8.
F has smaller electron affinity than Cl (not expected)
the small size of F in F-F with 6 lone pairs of
electrons leads to much greater repulsion compared
to Cl
Abundance and Preparation (P. 917): Self study
Distribution (Mass Percent) of the 18 Most Abundant
Elements in the Earth's Crust, Oceans, and Atmosphere
Abundance of Elements in the Human Body
19.2 Group 1A Elements
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ns1 valence electron configuration.
Comprise H and alkali metals
Alkali Metals
Sources and Methods of Preparation of the
Pure Alkali Metals
Selected Physical Properties of the Alkali Metals
Group 1: M(s) + H2O(l)  2M+(aq) + 2 (OH)-(aq) + H2(g)
• Li has the highest ionization potential and the highest oxidation
potential. Due to its small size and large energy of hydration, thus Li+
attracts water effectively and large energy is released when the ion is
formed. Thus formation of Li+ is favored and Li behaves as a strong
•Reducing agent.
Types of Compounds Formed by the Alkali Metals
with Oxygen
Selected Reactions of the Alkali Metals
19.3 Hydrogen
Also prepared from the electrolysis of water
Covalent hydrides
Hydrides
Metallic/interstitial hydrides
19.4 Group 2A Elements
Group 2A Elements
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Ns2 metals. They are called “Alkaline Earth metals”
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Their oxides are basic
MO(s) + H2O
M(OH)2
The differences in reactivity among them
are shown by their reaction with water:
M(s) + 2H2O
M(OH)2 + H2(g)
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Ca, Ba, Sr react easily with cold water
 Mg reacts with hot water
 Be does not react with water
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Selected Physical Properties, Sources, and Methods of
Preparation for the Group 2A Elements
BeCl2 and BeF2
• BeCl2 and BeF2 melts are poor conductors:
– Therefore they are covalent rather than ionic solids.
Selected Reactions of the Group 2A Elements
Ions in Natural Waters: Hard Water
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Rainwater is not chemically pure water.
 Contains dissolved atmospheric gases.
 Once on the ground it may pick up a few to about
1000 ppm of dissolved substances.
 If the water contains Ca2+ and or Mg 2+ ions we say
that the water is hard.
Hardness may be permanent or temporary.
Temporary Hard Water
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Contains HCO3- ion.
 When heated gives CO32-,
CO2 and H2O.
 The CO32- reacts with
multivalent ions to form
precipitates.
(for example CaCO3, MgCO3)
Water softening on a large scale
is carried out by precipitating
the multivalent ions using
slaked lime Ca(OH)2. CaCO3
would be precipitated
Permanent Hard Water
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Contains significant concentrations of anions
other than carbonate.
 For example SO42-, HSO4-.
 Usually soften by precipitating the Ca2+ and
Mg2+ using sodium carbonate leaving sodium
salts in solution.
Water Softening by ion-exchange
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Ion exchange.
Undesirable
cations, Mg2+ Ca2+
and Fe3+ are
changed for ions
that are not as
undesirable, ex.
Na+.
 Resins or zeolites.
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A typical cation-exchange resin.
when hard water is passed over the cation-exchange
resin, the ca2+ and mg2+ bind to the resin
19.5 Group 3A Elements
 ns2np1 elements
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Boron is a nonmetal
that forms covalent
bonds; other elements
are metals
Al forms significant
covalent bonding with
nonmetals; that is why
Al2O3 has amphoteric
nature.
Selected Physical Properties, Sources, and Methods of
Preparation for the Group 3A Elements
Selected Reactions of the Group 3A Elements
19.6 Group 4A Elements
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They have the
valence electron
configuration:
ns2np2
Contain the two
most important
elements on the
earth, C and Si
which form the
basic of geologic
world.
They all form
covalent bonds
with nonmetals:
CH4, SiF4, GeBr4,
SnCl4, PbCl4 (ِ All
sp3 hybridization)
Selected Physical Properties, Sources, and Methods of
Preparation for the Group 4A Elements
Selected Reactions of the Group 4A Elements
Chapter 20
The Representative Elements:
Groups 5A Through 8A
20.1 Group 5A Elements
The Nitrogen Family
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ns2np3 valence
electrons
configuration
Nitrogen can
exist in many
oxidation states.
N and P are
nonmetallic.
As and Sb are
metalloid.
Bi is metallic.
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Bi and Sb tend to be metallic
But no ionic compounds containing Bi5+ and Sb5+
are known
BiF5, SbF5 and SbCl5 are molecular rather than ionic
G 5A elements can form molecules or ions that
involve 3, 5 or 6 covalent bonds to the G 5A atom
NH3, PH3, NF3, and AsCl3. They all behave as Lewis
base.
All G 5A elements except N can form molecules
(MX5) with 5 covalent bonds.
The ability of G 5A elements to form  bonds
decreases dramatically after N.
This is why N exists as N2 molecules; while other
elements in the group exist as larger aggregates
containing single bonds: P4, As4, Sb4
The Molecules of
the Types MX3,
MX5, and MX6
Formed by
Group 5A
Elements
The Structures of the
Tetrahedral MX4+ and
Octahedral MX6- Ions
20.2 The Chemistry of Nitrogen
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Since N2 molecule contains a triple bond, most
binary compounds (except NH3) containing N
decompose exothermically to the elements
In the preparation of NH3 from N2 and H2, too much
energy is needed to disrupt the N≡N bond.
Thus, though K (106) is high the reaction is very slow
at room temperature.
Haber process is used to prepare NH3 (high
pressure, high temperature and a catalyst are
needed)
Nitrogen fixation: The process of transforming N2 to
other nitrogen containing compounds
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Nitrogen fixation can be carried out by:
 Haber process (ammonia can be applied to the
soil as a fertilizer)
 High temperature combustion process in
automobile engines. NO produced is converted
into NO2 that with moisture is concerted into NO3that reaches soil.
 Natural. Lightning produces the energy that
disrupt N2 and O2 molecules producing reactive N
and O atoms that attack other molecules to form
nitrogen oxides that convert eventually to NO3 Nitrogen-fixing bacteria that reside on the root of
nodules of plants such as beans and peas. This
converts N2 to ammonia and other nitrogen
containing compounds.
Denitrification: return of N element to the
atmosphere as N2 gas. Bacteria changes NO3- to N2
The Nitrogen Cycle
Some Common Nitrogen Compounds
20.3 The Chemistry of Phosphorus
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Chemical properties of P are significantly different
from N for the following reasons:
 Nitrogen’s stability to form much stronger 
bonds
 Grater electronegativity of N
 Larger size of P atom
 Availability of empty valence d orbitals on P
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White phosphorus exists as P4: very reactive
and bursts into flames on contact with air.
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It is commonly stored under water
Black P and Red P are network solids
(a) The P4 Molecule Found in White Phosphorus
(b) The Crystalline Network Structure of Black Phosphorous
(c) The Chain Structure of Red Phosphorus
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P is essential for plant growth
Soluble phosphate fertilizer is made by treating
phosphate rock with sulfuric acid to make
superphosphate of lime, that is a mixture of :
CaSO4. 2H2O and Ca(H2PO4)2. H2O
A reaction of NH3 and P produces NH4H2PO4
a very efficient fertilizer
20.4 The Group 6A Elements
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The valence electron
configuration is ns2np4
Non of these elements
behaves as a metal
They achive the noble gas
configuration by adding 2
electrons to become 2anion
G 6A elements can form
covalent bonds with other
nonmetals
Due to the presence of
empty d orbitals (except
O), they form molecules in
which central atom is
surrounded by more than
8 electrons: SF4 and SF6
Group 7A
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ns2 np5 valence electron configuration.
All nonmetals
Reactive. Not free in nature. Found as halide (X-) ions.
Astatine radioactive with t1/2 = 8.3 hrs for its longest living
isotope
Very high electronegativities (4, 3, 2.8, 2.5 and 2).
Ionic bonds with metals and covalent bonds with
nonmetals in low oxidation states & polar covalent in
metals in high oxidation states.
Hydrogen Halides
HCl is the most important acid.
HF is used for etching glass.
SiO2(s) + 4HF(aq)  SiF4(g) + 2H2O(l)
X
H–X
Bond energy
(kJ/mol
F
Cl
Br
I
565
427
363
295
2O
X  ( g ) H
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 X  (aq )
Ho
Ho
(kJ/mol)
(J/mol.K)
-510
-159
-366
-96
-334
-81
-291
-64
Go
(kJ/mol)
+ 102.4
90
53
23
Group 8A
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ns2p6 configuration; Un-reactive.
He. Component of the sun.
Present in natural gas (from decay of
radioactive elements).
 Used as Coolant and a rocket pressurizing gas.
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Ne. Used in Luminescent lighting.
Ar. Used as a non-corrosive atmosphere in
light bulbs.
Xe & Kr form compounds with O and F.