6 Electronegativity And Electron Affinity


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6 Electronegativity And Electron Affinity

  1. 1. 6 Electronegativity and Electron Affinity
  2. 2. Electronegativity (p 344) <ul><li>Electronegativity is the ability of an atom in a covalent bond to attract electrons to itself. </li></ul><ul><li>Fluorine is the most electronegative element - caesium the least </li></ul>
  3. 3. Trends
  4. 5. <ul><li>The bigger the difference between the electronegativities of the elements in an ionic compound the ionic the compound </li></ul><ul><li>The compound with the highest ionic character would be ceasium fluoride </li></ul><ul><li>Electronegativity F 4.0 </li></ul><ul><li>Cs 0.7 </li></ul><ul><li>difference 3.3 </li></ul><ul><li>Compare this to methane (a covalent compound) </li></ul><ul><li>C 2.5 </li></ul><ul><li>H 2.1 </li></ul><ul><li>difference 0.4 </li></ul>
  5. 6. <ul><li>So we can find out the % ionic character of a compound by finding the electronegativity difference between the elements of the compound </li></ul>Non-polar 0 Covalent <0.4 Polar covalent 0.4-1.7 Ionic >1.7 Ionic character  EN
  6. 8. Electron affinity (p 310) <ul><li>Electron affinity is the energy change accompanying the addition of 1 mol of electrons to 1 mol of gaseous ions or atoms. </li></ul><ul><li>Atom (g) + e -  ion - (g) </li></ul><ul><li>In most cases energy is released when an electron is added because it is attracted to the positive charge in the nucleus. </li></ul>
  7. 10. Influence of electronegativity on properties As the electronegativity decreases from F to C the polarity of the hydrides will decrease HF H 2 O NH 3 CH 4  decreasing polarity  decreasing hydrogen bond strength
  8. 11. <ul><li>We can also compare boiling points of hydrides down groups. For example for group 6 (O to Te). Only oxygen at the top of the group is electronegative enough to form a polar molecule  only water has hydrogen bonds and the boiling point is much higher than expected compared to other hydrides down the group. </li></ul><ul><li>The other hydrides do not form hydrogen bonds so their bp is influenced only by their size.  the trend for the rest of the group is increasing boiling points due to increasing van der waals forces </li></ul>
  9. 13. <ul><li>For group 4 (C to Pb ) CH 4 does not form hydrogen bonds  the trend depends solely on Van der Waals forces and is shows an increase in bp’s down the group. </li></ul><ul><li>You may have noticed that, although HF forms stronger hydrogen bonds than water the bp of water is higher than the bp of HF. This is due to the fact that water forms more hydrogen bonds than HF due to the larger no of atoms in the molecule. </li></ul>