|
Structure
and bonding
Francine
Taylor-Campbell, Contributor
 |
|
Members of Glenmuir High's Schools' Challenge quiz team pose at the end of a match.
- Ricardo
Makyn/Staff Photographer | Syllabus
requirements Students
should be able to: - Define
and give examples of ionic, simple molecular and giant molecular crystals.
- Explain
the term allotropy.
- Relate
structure of sodium chloride, diamond and graphite to their properties.
- Distinguish
between ionic and molecular solids.
Attempt
the following questions. 1.
The table gives information about the ability of four substances to conduct electricity.
| Substance | Property |
| W
| Does
not conduct electricity under any conditions | | X
| Conducts
only in aqueous solution | | Y
| Conducts
in both molten and solid states
| | Z
| Conducts
in both molten and aqueous states | What
could these four substances be?
| | W | X | Y | Z |
| A. | HCl | S | NaCl | Pb |
| B. | Pb | HCl | NaCl | S |
| C. | S | HCl | NaCl | Pb |
| D. | S | HCl | Pb | NaCl |
| E.
| S | NaCl | HCl | Pb |
2. Which
of the following substances is an element and forms crystals containing small
molecules? A.
copper
B. graphite
C. ice
D. iodine
E. sugar 3.
Which statement about graphite is correct? A.
It conducts electricity because it has ions which are free to move.
B.
It has an ionic lattice.
C.
It has the same structure as diamond.
D.
Its structure contains rings of carbon atoms. 4.
Which substance has the highest melting point? A.
Diamond
B. lead (II) bromide
C.
methane
D. water 5.
Which statement explains why magnesium oxide has a very high melting point? A.
Magnesium atoms and oxygen atoms are joined by strong double bonds.
B.
The crystal lattice of magnesium oxide resembles that of diamond.
C.
The magnesium ions are strongly attracted to the oxide ions.
D.
The reaction between magnesium and oxygen is strongly exothermic. ANSWERS
1.
D. Sulphur is a non-metal, which does not conduct electricity. All other substances
can conduct electricity. The ions in HCl are mobile in solution, Pb being a metal
conducts when solid and molten while the ions in NaCl are able to conduct only
when molten or in solution. 2.
D. Iodine, copper and graphite (C) are all elements, but only iodine forms small
molecules. Sugar and ice are compounds. 3.
D. The carbon atoms of graphite are arranged in a layer of hexagonal rings, which
are able to slide over each other. Note that graphite conducts electricity because
free electrons not ions are available to carry the current. 4.
A. Diamond has the highest melting point because of its macromolecular structure
and the strong covalent bonds that require a lot of energy to break. Lead bromide
is ionic and also has high melting points, but not as high as diamond. Methane
and water are simple covalent compounds which exhibit very low melting points.
5. C.
Magnesium oxide consists of magnesium and oxide ions which are strongly attracted
to each other. To break this attractive force requires lots of energy resulting
in a very high melting point. For this reason, magnesium oxide is used as lining
for furnaces since it can withstand high temperature without melting or breaking
down. Contributed
by Francine Taylor-Campbell. | 
