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Using your periodic table as a source of information.

Using your periodic table as a source of information. 1.The compound NaF is used in tooth paste. Give the electron configurations of the Ions involved. 2.What type of bonding is present in LiCl and MgO ? Explain why MgO has a much higher melting point. Answers. 1. Sodium Fluoride

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Using your periodic table as a source of information.

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  1. Using your periodic table as a source of information. • 1.The compound NaF is used in tooth paste. Give the electron configurations of the Ions involved. • 2.What type of bonding is present in LiCl and MgO? Explain why MgO has a much higher melting point.

  2. Answers 1. Sodium Fluoride Na atom 1s22s22p63s1(GCSE 2.8.1) has become Na+ ion 1s22s22p6(GCSE 2.8) F atom 1s22s22p5(GCSE 2.7) has become F-ion 1s22s22p6(GCSE 2.8) 2. Both Ionic. MgO higher because of stronger charge attraction between Mg2+ and O2- ions. More energy needed to break this bond (strictly speaking to break down the many bonds in the lattice of which the energy is an average).

  3. Types of ionic packing in lattice This depends on the sizes of the ions involved. With NaCl the ions are not too dissimilar in size – as a result we have a coordination number of 6 or 6:6 (around each +ion are 6 –ions & around each –ion are 6 +ions) So in the lattice there are alternate ions in the same horizontal or vertical plane. + - + - + - + - + - - + - + - + - + - +

  4. CsCl Cs+ is a much larger ion than the Cl-. As a result 8 Cl- ions can pack around it. (this also means 8 Cl- ions surround each Cs ion) The co-ordination number is 8 or 8:8 There is only one type of ion in a given horizontal or vertical plane. + + + + - - - - - - - - - -

  5. Electron density maps • A way of imaging electron density of ions and showing it graphically. The lines join up areas of the same electron density. (eg in an element the energy levels can be clearly seen) • Useful when looking at ionic bonds as shows us that not all ionic bonds are the same....

  6. Degree of ionic (and covalent) bonding • In GCSE we had either ionic or covalent bonding. • At A level we can have a degree of ionic or covalent character within a bond. • The ionic bonds that have covalent tendencies show a degree of polarisation of the bond (distortion of electron density)

  7. What is polarisations and what factors influence it? • Distortion of electron density from spherical patterns -See also diagrams on board (it is the anion that is distorted). ZnOZnSZnSe Fajan’s rules: Basically it is size and charge: • The smaller the positive cation the more polarising it is. • The higher the charge on positive cation the more polarising it is. • The higher the charge on the negative anion the more it is polarised. • The larger the negative anion the more it is polarised. Increased polarisation

  8. Lattice Energy • Energy released when one mole of a lattice crystal is formed from the gaseous ions under standard conditions. • This is loosely similar to bond energy EXCEPT that as it is the formation of the lattice so it must be exothermic and ... • .....really we know that a in a crystal lattice each ion is held by attractive forces to a number of other ions, which is why ionic bonds are hard to break (egNaCl not 1:1 as formula might imply)

  9. Lattice Energy Values • We can calculate Lattice energies indirectly from the values that we can measure. • We can apply Hess’s law to do this. • The energy diagram in this case is known as a Boen-Haber cycle. • (see worked example to follow)

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