Group iv
Periodicity
1
objectives
×
×
×
Explain the variations in
physical properties of the
elements in terms of structure
and bonding
Describe the bonding of the
tetra chlorides
Explain the reactions of the
tetra chlorides with water
×
Discuss the trends in:
(a) bonding;
(b) acid/base character; and,
×
×
(c) thermal stability of the oxides of
oxidation states II and IV
Discuss the relative stabilities of the
oxides and aqueous cations of the
elements in their higher and lower
oxidation states
Discuss the uses of ceramics based on
silicon (IV) oxide
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3
structure
×
×
×
C, Si and Ge have giant molecular structures.
×
As these bonds become weaker, the melting point from C to Ge also
decreases.
Each atom is covalently bonded to four (4) other atoms.
As the atomic radius increases from C to Ge, the interatomic bonds
become weaker, that is, C – C > Si – Si > Ge – Ge bond.
4
Allotropes of carbon
× Diamond and graphite
× Diamond – 3D, tetrahedral structure
× Graphite – 3D, hexagonal layers (each C bonded to 3 others), layers held
by weak intermolecular forces
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structure
× Sn and Pb are giant metallic structures.
× Due to the big atomic radius of these elements, the metallic bonds in the
structures are as strong as the transition metals.
× Transition elements are able to use d electrons in the sea of delocalized electrons,
but tin and lead use only p electrons from the 5p and 6p sub-shells, respectively. As
a result, these elements have low melting points.
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Atomic radius
× The atomic radius increases from C to Pb.
× This is due to the effect on atomic radius as a result of increased shielding which
outweighs the effect due to increased nuclear charge.
× Shielding increases significantly down a group since successive elements differ
by an extra shell of electrons.
× As the atoms become bigger in size, the outermost electrons become further
away from the nucleus.
× The attraction between the nucleus and the outermost electrons becomes less
hence the 1st ionization energy decreases from C to Pb.
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Electrical conductivity
×
Carbon in the form of graphite, is a good conductor of electricity, due to the
presence of delocalised electrons in the layers of the C- atoms.
×
Si and Ge are semi-conductors and Sn and Pb are good conductors of electricity
since they are metals.
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Tetra chlorides (bonding)
CCl4
×
SiCl4
X(s)
+
GeCl4
SnCl4
PbCl4
2Cl2(g)
→
XCl4(l)
to form simple covalent molecules
Where X = Si, Ge and Sn.
Lead forms the chloride PbCl2, rather than the tetrachloride, because of the thermal
instability of lead (IV) chloride.
×
All are volatile liquids under room conditions.
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Tetrachlorides (shape)
×
×
×
All have a tetrahedral shape (AB4).
Bond angle is 109.5o
This is due to the VSEPR theory pushing the chlorine atoms, (which are all
the same strength), as far from each other as possible.
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Chemical properties of tetra chlorides
×
×
The thermal stability of the chlorides depends on the strength of the X – Cl bond.
As the atomic radius increases from C to Pb, this bond becomes longer and
weaker hence, thermal stability decreases from CCl4 to PbCl4.
Lead(IV) chloride decomposes at room temperature to give the more stable
lead(II) chloride and chlorine gas.
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12
Reaction with water
XCl4(l)
+
4H2O(l)
→
X(OH)4(s)
+
4HCl(g)
×
×
Where X = Si, Ge, Sn and Pb
×
The element must have an available orbital to accept the lone pair. All the
Group IV elements except Carbon (becomes sp3 hybridized to form the 4
bonds with chlorine) have available empty d orbitals to accommodate a lone
pair and start reacting with water.
×
Hence CCl4 does not react with water.
During the reaction with water, a lone pair of electrons from the water
molecules is donated to the central Group IV element.
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Properties of the Group IV oxides in the +4 oxidation state
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Properties of the Group IV oxides in the +2 oxidation state
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Oxidation states
As the group is descended, there
is an increase in the stability of
the +2 oxidation state.
As the group is descended, the
+4 state becomes more
unstable.
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Oxidation states
×
×
The greater the stability of the +2 state as the atomic number
increases is well illustrated by the Eᴓ of the M4+(aq)/M2+(aq)
systems for Ge, Sn and Pb.
As the electrode potentials get more positive from Ge4+ to Pb4+,
the oxidised form is more readily reduced to the +2 state.
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Uses of ceramics
Refer to notes
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End of lesson
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