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Chapter 19 The structure of materials
The structure and bonding of materials
 
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1.

Studying structures

  • The structures of substances can be studied using X-rays.
  • A beam of X-rays is directed at a well-formed crystal of the substance.
    Some of the X-rays are deflected and reinforced (diffracted) by the particles in the crystal onto X-ray sensitive film.
    From the diffraction pattern on the X-ray film, it is possible to work out the arrangement of particles in the crystal.
  • The regular arrangement of particles in a crystal is called a lattice.
2.

The structure of substances

The structure and bonding of a substance determines its properties and, in turn, the properties of a substance determine its uses.

  • There are only three different types of particle in all substances – atoms, molecules and ions.
    The three different types of particle lead to four different types of solids.
    Giant metallic composed of metal atoms.
    Giant covalent (giant molecular) composed of non-metal atoms joined together in large molecules.
    Giant ionic composed of ions.
    Simple molecular composed of non-metal atoms in small molecules.
    The table below summarises the particles, bonding and properties of these four different solid structures.
Type of
structure
Particles in the structure Type of
substance
Bonding Properties Structure

3.

Chemical bonding

  • When elements react, they try to gain, lose or share electrons in order to get a more stable electron structure. The more stable electron structure is often like that of a noble gas.
  • Ionic bonding involves transfer of electrons from metal atoms to non-metal atoms forming positive and negative ions. The attractions between the oppositely-charged ions produce strong ionic bonds.


  • Covalent bonding involves the sharing of electrons by two atoms. Each atom contributes one electron to the shared pair in a covalent bond between two atoms.

4.

Energy changes in chemical reactions

  • The energy change in a chemical reaction can be calculated from the strengths (bond energies) of the bonds broken and the new bonds formed in the reaction.
  • Bond breaking requires energy and is an endothermic process. Bond making helps to make the atoms more stable and this is an exothermic reaction.
    The energy change when methane reacts with chlorine to form chloromethane and hydrogen chloride is calculated below.
    The equation for this reaction is:


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