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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: Molecular structures

3.0 Chemical Bonding and Structure


3.6 Molecular structures

Let us revisit some of the substances we have dealt with so far under covalent and coordinate bonds (Figure 3.6).


Molecular structures, high school chemistry

Figure 3.6: Molecular structures


In each unit in Figure 3.6, energy levels of all the atoms are filled. So they are stable like noble gases and exist independently. They are molecules; their structure is molecular. Unlike ions, they are neutral and do not attract or combine with neighboring particles. Also, they are non-metals and compounds of non-metals only, and occur mainly as gases and liquids.


Questions 3.6

  1. Define a molecular structure.
  2. Which bond type leads to molecular structures?
  3. Which class of elements forms bonds that lead to molecular structures?
  4. Distinguish between a bond and structure.
  5. Comment about the texture (hardness and softness), and melting and boiling points of molecular structures.
  6. Do substances with molecular structures conduct electricity? Explain your answer.

Answers to Questions 3.6


Both co-ordinate and covalent bonds lead to molecular structures.


Van der Waal's forces of attraction

Because water exists as molecules, we would expect it to be a gas; that is, a state in which molecules are far apart because there is no bond holding them together. But water is a liquid, which can be readily condensed to ice. That means there are forces of attraction holding its molecules together.


The forces that hold molecules together are called Van der Waal's forces of attraction. Van der Waal's forces are not chemical bonds, but weak attractions between molecules. They occur in all molecular structures. Ice, liquid water and other substances with molecular structures have low melting and boiling points because of the weak Van der Waal's forces of attraction in them.


Properties of substances with molecular structures

  1. They have low melting and boiling points, because molecules exist independently of one another, only linked by weak Van der Waal's forces of attraction.
  2. They occur as gases, liquids and soft solids.
  3. They do not conduct electricity in solid, molten state or solution because molecules are electrically neutral. This property is used to identify molecular structures.