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    The question is:
    Name and compare briefly the mechanisms of inter-atomic bonding in a crystal of copper and in a crystal of polyethylene.

    The sample answer contains the sentence: 'Van der Waals bonding is much weaker than metallic bonding, hence the crystal of polyethylene perpendicular to molecules is much weaker than Cu.'

    I don't understand the use of crystal (doesn't the crystal contain all of the molecules) in this sentence or the perpendicular to molecules bit.

    Please can someone help. Thanks.
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    (Original post by runny4)
    The question is:
    Name and compare briefly the mechanisms of inter-atomic bonding in a crystal of copper and in a crystal of polyethylene.

    The sample answer contains the sentence: 'Van der Waals bonding is much weaker than metallic bonding, hence the crystal of polyethylene perpendicular to molecules is much weaker than Cu.'

    I don't understand the use of crystal (doesn't the crystal contain all of the molecules) in this sentence or the perpendicular to molecules bit.

    Please can someone help. Thanks.
    A crystal simply means when the compound/element is in its solid form.

    So solid copper is a metallically bonded giant lattice.

    A polyethylene molecule is a very long chain of repeating units. When lots of these chains come together to form a solid (crystal) they line up and are bound by Van der Waals forces, which are weak.

    What the answer is getting at is the fact that, its easy to separate the chains by 'pulling' perpendicular to their direction.

    ie

    <------IIIIIIIIIII ---->



    IIIIII......IIIIIIII

    is easy compared to separating the copper

    If you pull in the other direction, then to break apart the crystal of polyethylene you have to break the chains, and that means breaking covalent bonds which is much more difficult.
 
 
 
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