Download The periodical trends of elements in chemical properties

Additional Chapter: The periodical trends of elements in chemical properties
Mendeleev’s periodic table
In 1869 Dmitri Mendeleev (1834-1907) succeeded in
organizing the 62 elements known at that time into a
system of rows and columns on the basis of increasing
mass and similar chemical and physical properties.
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Uniqueness principle
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π bonding
π-bonds are more efficient between 2p orbitals
Chemical properties of second period elements are
quite different from their heavier congeners
small size
π bonding
no d orbitals
Size effect
increased covalence for small cations due to greater
polarizing power or charge density
•LiCl dissolves in alcohols; NaCl does not
•BeCl2 dissolves in diethyl ether; MgCl2 does not
•LiR dissolve in hexane; NaR are insoluble in
hydrocarbons (R usually denotes an organo
group)
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Effects: silenes (R2Si=SiR2) and silynes (RSi≡SiR) are rare and unknown
respectively, while alkenes and alkynes are common.
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1
Allotropes of carbon
So CO2 is molecular (O=C=O, has double bonds)
But SiO2 (quartz, sand, glass…) is a 3-dimensional solid network:
Cristobalite
Fullerene C60
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Structure of a-Quartz:
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O2 is molecular (O=O, has a double bond)
Two optically isomeric forms
But S forms rings (e.g., S8)
In each form, two helices
composed of -O-Si-O-Si-O- chains.
Being able to generate
piezoelectricity, electricity
resulting for the application of
mechanical pressure on a crystal.
Such crystals can convert
vibrations into electrical
signals, also used in crystal
microphone. The inverse
effect, electrical signals to
specific vibrations, find use
in clock, watch, …
Nitric Oxide (NO)
Nitrogen Dioxide (NO2)
Nitrous Oxide (N2O)
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Inert pair effect
lack of d orbitals
Some heavier p-block elements display a maximum oxidation
state 2 units less than their lighter congeners:
ns2np1
ns2np2
ns2p3
B
C
N
Al
Si
P
Ga
Ge
As
In
Sn
Sb
Tl
Pb
Bi
maximum coordination number is 4; octet rule applies
hypervalent compounds can occur if d orbitals available
Al+ is extremely rare
Tl+ is very common
Si4+ is well known
Pb4+ is uncommon
Si2+ is rare
Pb2+ is the usual ox. state
Highest oxidation states harder to achieve for heavier elements:
Tl electron configuration:
[Xe] 4f14 5d10 6s2 6p1 acts as if the 6s electrons are inert
Remark: although d orbital participation is commonly invoked to
explain hypervalence, this phenomenon can be explained
satisfactorily without using d orbitals by a more sophisticated M.O.
treatment.
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Al3+ is well known
Tl3+ is uncommon
Sb3+ and Bi3+ are also very common
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Tendency for catenation
Catenation: formation of chain molecules
Carbon, very common (polymers)
Silicon, less common
Sn and Pb, very rare
General trend: C > Si > Ge > Sn > Pb
Why: sigma bonding strength
Trend for other groups:
N < P > As > sb > Bi
O < S > Se > Te > Po
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