Electronegativity and Polarity

• In a covalent bond, the nuclei of the bonded atoms attract the shared pair of electrons. In molecules of elements, e.g. H2, O2, N2, CL2, the atoms are the same element and the bonded elctron apir is shared evenly.
• When does electronegativity of a molecule change?

  When the bonded atoms are different elements:

  the nuclear charges are different

  the atoms may be different sizes

  the shared pair of electrons may be closer to one nucleus than the other

  Therefore the shared pair of electrons in the covalent bond may experience more attraction from one of the bonded atoms than the other
• What is electronegativity?

  The attraction of a bonded atom for the pair of electrons in a covalent bond
• How is electronegativity measured?

  Pauling scale
• Across the periodic table?

  Nuclear charge increases

  Atomic radius decreases
• What does a large Pauling value indicate?

  atoms of the element are very electronegative

  Electronegativity increases across and up the periodic table - F is the most electronegative (4.0)
• The non-metals nitrogen, oxygen, fluorine and chlorine have the most electronegative atoms. Whereas the group 1 metals, e.g. lithium, sodium and potassium have the least electronegative atoms.
• Ionic or covalent?

  If the electronegativity difference is large, one bonded atom will have a much greater attraction for the shared pair of electrons than the other. The more electronegative atom will have gained control of the electrons - bond will be ionic rather than covalent.
• Difference in electronegativity = 0?


  covalent
• Difference in electronegativity = 0 - 1.8?

  Polar covalent
• Difference in electronegativity = greater than 1.8?

  Ionic
• Non-polar bonds?

  The bonded electron pair is shared equally between bonded atoms because the bonded atoms are the same, or have the same or similar electronegativity - if atoms the same = pure covalent bond
• In a polar bond, the bonded electron pair is shared unequally between the bonded atoms. A bond will be polar when the bonded atoms are different and have different electronegativity values - polar covalent bond
• For example H-Cl

  Bond is polarised, small partial positive charge on H (smaller electronegativity) and small partial negative charge on Cl.
• What is the separation of opposite charges called?

  A dipole
• A dipole in a polar covalent bond does not change and is called a permanent dipole, to distinguish it from an induced dipole
• Polar molecules?

  e.g. H-Cl - has one permanent dipole acting in the direction of the H-Cl bond. For molecules with more than 2 atoms, there may be 2 or more polar bonds. Depending on the shape of the molecule, the dipoles may reinforce one another to produce a larger dipole over the whole molecule or cancel out if the dipoles act in opposite directions
• A water molecule is polar

  The 2 O-H bonds each have a permanent dipole

  The 2 dipoles act in different directions but do not exactly oppose each other

  Overall the oxygen end of the molecule has a delta - charge and the hydrogen end of the molecule has a delta + charge
• A carbon dioxide molecule is non-polar

  The 2 C=O bonds each have a permanent dipole

  The two dipoles act in opposite directions and exactly oppose one another

  over the whole molecule, the dipoles cancel and the overall dipole is zero
• Polar Solvents and Solubility:

  e.g. sodium chloride lattice being dissolved by water

  1. water molecules attract Na+ and Cl- ions
  2. the ionic lattice breaks down as it dissolves
  3. in the resulting solution, water molecules surround the Na+ and Cl- ions
  4. Na+ ions are attracted to the oxygen of water molecules (delta -)
  5. Cl- ions are attracted towards the hydrogen of water molecules (delta +)