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Incomplete Electron transfer in ionic compounds

Incomplete Electron transfer in ionic compounds. Intermediate Type of Bonding. Nature of pure ionic compounds Ways to get the value of lattice enthalpy Comparison of Theoretical and Experimental value of lattice enthalpy Polarization of ion. Nature of pure ionic compounds.

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Incomplete Electron transfer in ionic compounds

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  1. Incomplete Electron transfer in ionic compounds Intermediate Type of Bonding

  2. Nature of pure ionic compounds • Ways to get the value of lattice enthalpy • Comparison of Theoretical and Experimental value of lattice enthalpy • Polarization of ion

  3. Nature of pure ionic compounds • formed by complete transfer of electrons from metallic atoms to non-metallic atoms. • electrons should be solely confined and not shared with its neighbouring ions In reality, there are some compounds that the transfer of electrons is incomplete.

  4. Lattice Enthalpy of Ionic Compounds • Lattice enthalpy: • Enthalpy change when one mole of ionic crystal is formed from its constituent ions at gaseous state under standard conditions • How can we get the value of lattice enthalpy? • By Energetics (Experimentally derived) • By using the Simple Ionic Model (Theoretical)

  5. By Energetics(Experimentally derived) • We need to construct Born-Haber cycle and apply the Hess’s Law • Let’s use NaCl as an example and calculate the lattice enthalpy

  6. By using the Simple Ionic Model(Theoretical) • Assumptions: • Ions are perfect spheres with uniform charge distribution • The cations and anions are just in contact with each other • Lattice enthalpy is calculated in terms of electrostatic interactionswithin the lattice

  7. Simple Ionic Model • The potential energy (P) required to bring two ions with charge q1and q2 from an infinite distance r is - r + q1 q2

  8. Comparison of the Theoretical and Experimental Values of Lattice Enthalpy • Reveals the nature of the bond in the compound

  9. Table comparing the theoretical and experimentally derived lattice enthaplies of some ionic compounds

  10. Answer for Discussion • For NaCl to KI, the differences between theoretical and experimentally derived lattice enthapies are small • The Simple Ionic Model gives a good representation of the actual lattice structure • Ions are spherical • Charge distribution is uniform • Bond type in these compounds is nearlypurely ionic

  11. Answer for Discussion • For AgCl, AgBr, AgI and ZnS, there is a large difference between the theoretical and experimentally derived lattice enthapies • Assumptions of the Simple Ionic Model are unsatisfactory • Ions are not perfect spheres • Charge distribution is not uniform • Bond type in these compounds is not purely ionic and has some degree of covalent character

  12. Polarization of Ion • Polarization: • The distortion of the electron cloud of an anion under the influence of a nearby cation • the electron cloud is attracted towards the cation • some sharing of electrons between the ions • covalent character in ionic bond

  13. Polarizing Power of Cation • Polarizing Power: • Ability of a cation to distort the electron distributionin a neighbouring atom, molecule or ion • Depends on charge density

  14. The charge density of a cation is high if: • Higher the charge; and/or • Smaller the ionic radius • The higher the charge density of a cation, the higher is its polarizing power

  15. Think about: 1. Among Na+, Mg2+ and Al3+, which one has the highest polarizing power? Why? 2. Among Li+, Na+ and K+, which one has the highest polarizing power? Why?

  16. Answer to Q1 • Al3+has the highest polarizing power • It has the highest charge and smallest size among Na+, Mg2+ and Al3+ • The charge density of Al3+ is the greatest

  17. Answer to Q2 • Li+ has the highest polarizing power • Among Li+, Na+ and K+, Li+ has the smallest size • The charge density of Li+ is the highest • Li+ has the highest polarizing power

  18. Polarizability of Anion • Polarizability: • A measure of the ease of distortion of its electron cloud by neighbouring cations • Higher polarizability of an anion if: • Higher the charge; and/or • The larger the ionic radius of anion

  19. Think about: 1. Among F-, Cl- and Br- and I-, which one has the highest polarizability? Why? 2. Among O2- and F-, which one has a higher polarizability? Why?

  20. Answer to Q1 • F-, Cl-, Br- and I- are all belonged to the same group • I- has the greatest number of electron shell • The size of I- is the largest • I- has the highest polarizability (more easily to be polarized)

  21. Answer to Q2 • O2- and F- have the same number of electrons • The charge of O2- is greater than F- • O2- is easier to be polarized than F-

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