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1.47 - 1.48 Simple molecular substances

Ice melts at 0 degrees C, Diamond remains solid until its temperature is above 3500 degrees

Look at the images of ice and diamond.  In some senses you might say the two substances are similar.  Both substances are solid,  transparent and crystalline. In terms of bonding, they have similarities too :  

  • In ice, atoms of oxygen are held to hydrogen atoms by covalent bonds. 
  • In diamond atoms of carbon are held to other atoms of carbon by covalent bonds. 
However ice melts at 0oC whilst diamond remains solid until its temperature is above 3500oC. This is because:
  • the molecules in water are held together by relatively weak intermolecular forces 
  • the atoms in diamond interconnect with one another to make a giant covalent lattice whose atoms are held together strongly

1.47 Covalent substances : Simple molecules

Compared with the covalent bonds inside the molecules, the forces between molecules are relatively weak

 Students should:

  • 1.47 explain why substances with simple molecular structures are gases or liquids, or solids with low melting and boiling points
  • 1.48 explain why the melting and boiling points of substances with simple molecular structures increase, in general, with increasing relative molecular mass

We have seen how covalent bonds form when electrons are shared between atoms of non-metals. When molecules form in this way the forces within the molecules (intramolecular forces) are relatively strong. In contrast - the forces between molecules (intermolecular forces) are relatively weak.

 1.47 Activity. A look at the data...

Use the infogram to examine the melting point data of a number of covalent substances.

  1. Select the simple covalent molecules tab
  2. Roll your cursor over each of the blocks in turn to reveal the melting point and make a note of it.
  3. Calculate an average value for the melting points for this group of compounds
  4. The melting point  values are given in Kelvin. To convert Kelvin to Celsius subtract 273. 
  5. Convert the average melting point temperature value you calculated in 3 to Celsius.

Weak intermolecular forces = easy melting 

In this animation - three types of intermolecular forces are shown. All are weak relative to the intramolecular forces known as covalent bonds. Watch the video and list the three types of intermolecular forces in order of  decreasing strength.

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