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CHEMISTRY FORM 2


1. STRUCTURE OF THE ATOM, AND THE PERIODIC TABLE
2. CHEMICAL FAMILIES AND PATTERNS IN PROPERTIES
3. CHEMICAL BONDING AND STRUCTURE
4. SALTS
5. EFFECT OF AN ELECTRIC CURRENT ON SUBSTANCES
6. CARBON AND SOME OF ITS COMPOUNDS
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Chemical Bonding and Structure: Giant ionic structure

3.0 Chemical Bonding and Structure


3.3 Giant ionic structure


We have seen sodium chloride (NaCl) and copper (II) oxide as examples of ionic compounds.
Observe the demonstration involving strong heating of sodium chloride crystals, copper (II) oxide, and zinc oxide separately.


(courtesy Youtube-Strongly heating sodium chloride crystals by Joseph Rabari)

(courtesy Youtube-Strongly heating copper oxide by Joseph Rabari)

(courtesy Youtube-Strongly heating zinc oxide by Joseph Rabari)

Questions 3.3(a)

  1. Is there any noticeable effect of heat on sodium chloride crystals, copper (II) oxide or zinc oxide?
  2. What can you say about the melting points of sodium chloride and copper (II) oxide?
  3. What can you say about the amount of heat energy required to melt sodium chloride or copper (II) oxide?

Answers to Questions 3.3(a)


Melting occurs when all the bonds are completely broken. It has been found that the amount of energy required to melt a sodium chloride crystal is over a billion trillion (1021) times larger than the energy required to break the bond between two ions (sodium ion and chloride ion).


Questions 3.3(b)

From the information given, which of the following diagrams best represents the structure of sodium chloride crystal? Explain your choice.


test structures, high school chemistry

Figure 3.3b:Test structures


Answers to Questions 3.3(b)


If sodium and chloride ions occurred in small separate units such as in (a), (b) and (d), the structure would have a low melting point because nothing ties such units together. They can be easily separated from one another. High melting point therefore indicates that trillions of the ions are bonded continuously (in a repeated pattern), one to the next. The resulting structure is therefore called a giant ionic structure (Figure 3.3(c)).


Giant ionic strucures of sodium chloride high school chemistry

Figure 3.3(c): Giant ionic structure of sodium chloride


Note that, in this structure, each positive ion (Na+) is surrounded by negative ions (Cl-) and vice versa. Only then can the structure hold together. Why? This applies to all other ionic compounds. We have used balls (◯) to represent ions and sticks (__) to represent bonds. This is called a ball-and-stick model.


Questions 3.3(c)

In the same manner as Figure 3.3(c), represent the giant ionic structures of copper (II) oxide (CuO) and lithium fluoride (LiF).


Answers to Questions 3.3(c)


It should be noted that in ionic bonding, the number of negative ions surrounding a positive ion and vice versa, does not depend on valency but size of ions. This is because an electric charge can attract any number of opposite charges around it.


Properties of ionic compounds

  1. They have giant ionic structures
  2. They have high melting and boiling points.
  3. They do not conduct electricity as solids, but conduct electricity in molten state or solution. This is the property used to identify ionic compounds.

In solids, the ions are locked together in ionic bonds so they are not free to move about. In molten state and solutions, the ions are free to move about and conduct electricity. This explains Property 3 above. Observe the demonstration with molten lead (II) bromide.


(courtesy Youtube-The electrolysis of lead bromide by David Read)

What range of temperatures is considered as low and high melting points?


Generally, temperatures a few hundred degrees above and below room temperature (25oC) are considered as low. High temperatures are around 1000 oC and higher.