CHEMISTRY 2.4
An assessment for AS90308
Describe the nature of structure and bonding in different substances.
Credits: Four
INSTRUCTIONS
Answer ALL questions.
© New Zealand Institute of Chemistry 2005
2
You should spend 40 minutes answering the questions in this booklet
QUESTION ONE
1
(a)
Lewis diagrams
Complete the table below by:
• drawing the Lewis structures
• naming the shape
• stating whether the compound is polar or non-polar
Molecule
hydrogen
chloride
HCl
(b)
beryllium
chloride
BeCl2
(c)
sulfur
dichloride
SCl2
(d)
Carbon
disulfide
CS2
(e)
phosphorus
trichloride
PCl3
(f)
methane
CH4
Lewis structure
Name of shape
Polar or non-polar
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3
QUESTION TWO:
1
Shapes of molecules
Using HCl as an example, explain what is meant by the term 'polar bond'.
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2
Explain why the shape of the ammonia molecule, NH3, is trigonal pyrimidal rather than
triangular.
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3
Explain why water, H2O, and carbon dioxide, CO2 have different shapes, even though both
molecules contain three atoms.
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4
Discuss the effect the shape of the molecules PCl3 and CCl4 have on their polarity and relate
this effect to the solubility of each molecule in water.
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4
QUESTION THREE:
1
Structures of solids
The table below supplies information about the oxides of some of the third-row elements.
Complete the following table by stating
 the type of solid
 the type of particle(s) in the solid crystal
 the name of the bond or force broken when the solid melts
Name and
formula of solid
Melting
point
°C
2
sodium oxide,
Na2O
1275
silicon dioxide
SiO2
1710
sulfur trioxide
SO3
27
chlorine dioxide
ClO2
–75
Type of solid
(covalent network,
ionic, metallic, or
molecular)
Type of
particle(s) in
crystal
(atoms, ions or
molecules)
Name of bond broken
when the solid melts
(covalent, ionic,
metallic or weak
molecular)
The table in Question 1 shows both Na2O and SiO2 have high melting points while SO3 and
ClO2 have low melting points. For each of the following, state whether their melting points
will be high or low and explain your choice.
Oxide
Relative Melting Point
(high/low)
Explanation
MgO
P4O6
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5
QUESTION 4:
1
Properties of solids
Explain, in terms of structure and bonding within the solid, why each of the following has the
property described.
Property
Explanation
Gold is very malleable and
can be shaped into many
items of jewellery.
Solid sodium chloride is a
poor conductor of
electricity. However, when
molten, sodium chloride is a
good electrical conductor.
Iodine crystals are easily
changed into a gas when
warmed gently.
2
Diamond and graphite are allotropes of carbon, yet graphite is soft enough to be used as the
'lead' in pencils, while diamond is so hard that it can cut glass.
Discuss these properties in terms of the structure and bonding in these solids.
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End of Assessment
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