STRUCTURE AND BONDING

• Metals


  Elements that react to form positive ions
• Non-metals


  Elements that do not form positive ions
• Students should be able to

  • Explain the differences between metals and non-metals on the basis of their characteristic physical and chemical properties
  • Explain how the atomic structure of metals and non-metals relates to their position in the periodic table
  • Explain how the reactions of elements are related to the arrangement of electrons in their atoms and hence to their atomic number
• Chemical bonds


  Ionic, covalent and metallic
• Ionic bonding


  Occurs in compounds formed from metals combined with non-metals
• Covalent bonding


  Occurs in most non-metallic elements and in compounds of non-metals
• Metallic bonding


  Occurs in metallic elements and alloys
• Chemical bonding can be explained in terms of electrostatic forces and the transfer or sharing of electrons
• The charge on the ions produced by metals in Groups 1 and 2 and by non-metals in Groups 6 and 7 relates to the group number of the element in the periodic table
• Ionic compound


  A giant structure of ions held together by strong electrostatic forces of attraction between oppositely charged ions
• The structure of sodium chloride can be represented in different forms

  
• Students should be familiar with the structure of sodium chloride but do not need to know the structures of other ionic compounds
• Covalent bonding


  Occurs when atoms share pairs of electrons
• Covalently bonded substances may consist of small molecules or have very large molecules, such as polymers
• Some covalently bonded substances have giant covalent structures, such as diamond and silicon dioxide
• Metallic bonding


  Occurs in metals, where the electrons in the outer shell of metal atoms are delocalised and free to move through the whole structure
• The sharing of delocalised electrons gives rise to strong metallic bonds
• The three states of matter are solid, liquid and gas. Melting and freezing take place at the melting point, boiling and condensing take place at the boiling point
• The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles of the substance
• The nature of the particles involved depends on the type of bonding and the structure of the substance
• The stronger the forces between the particles the higher the melting point and boiling point of the substance
• Atoms themselves do not have the bulk properties of materials
• The particle theory has limitations in relation to changes of state when particles are represented by solid inelastic spheres which have no forces between them
• In chemical equations, the three states of matter are shown as (s), (l) and (g), with (aq) for aqueous solutions
• Ionic compounds


  Have regular structures (giant ionic lattices) in which there are strong electrostatic forces of attraction in all directions between oppositely charged ions
• Ionic compounds have high melting points and high boiling points because of the large amounts of energy needed to break the many strong bonds
• When melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move and so charge can flow
• Knowledge of the structures of specific ionic compounds other than sodium chloride is not required

  
• Substances that consist of small molecules

  Are usually gases or liquids that have relatively low melting points and boiling points
• These substances have only weak forces between the molecules (intermolecular forces)
• The intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points
• These substances do not conduct electricity because the molecules do not have an overall electric charge
• Polymers


  Have very large molecules where the atoms are linked by strong covalent bonds
• The intermolecular forces between polymer molecules are relatively strong and so these substances are solids at room temperature
• Substances that consist of giant covalent structures

  Are solids with very high melting points where all of the atoms are linked by strong covalent bonds
• Giant covalent structures


  • Diamond, graphite (forms of carbon), silicon dioxide (silica)
• Metals


  Have giant structures of atoms with strong metallic bonding
• Most metals have high melting and boiling points
• In pure metals, atoms are arranged in layers, which allows metals to be bent and shaped
• Pure metals are too soft for many uses and so are mixed with other metals to make alloys