6.2 Electronegativity and Polarity

6.2 Electronegativity and Polarity

1. Electronegativity

   • Electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond
   • The bonded electron pair is shared evenly between to atoms unless...
     • ... the nuclear charges are different
     • ... the atoms may be different size
     • ... the shared pair of electrons may be closer to one nucleus than the other
     • If these conditions are met, the shared pair of electrons in the covalent bond may now experience more attraction from one of the bonded atoms than the other
   • Ionic or Covalent?
     • If the electronegativity difference is large, one bonded atom will have a much greater attraction for the shared pair than the other bonded atom.
       • The more electronegative the atom will have gained control of the electrons and the bond will now be ionic rather than covalent

2. Bond Polarity

   • Non-polar bonds
     • Bonded electron pair shared equally
     • Bonded atoms will be the same or the bonded atoms have the same/similar electronegativity
       • Pure Covalent Bond
   • Polar Bonds
     • Bonded pair shared unequally
     • A bond will be polar when the bonded atoms are different, or have a large difference in electronegativity
       • Polar Covalent Bond
   • A H-Cl bond is polarised as Cl atom is more electronegative than H
     • Due to it being polar, H has a slightly positive charge, where as Cl has a slightly negative charge (as it is the more electronegative one)
       • This separation of opposite charges is 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
           • induced dipoles will be on my 6.3 Intermolecular Forces mind map

3. Polar Molecules

   • 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, H2O molecule is polar
       • The two O-H bonds each have a permanent dipole
       • The two dipoles act in different directions but do not exactly oppose one another
       • Overall the oxygen end of the molecule has a slightly negative charge and the hydrogen end of the molecule has a slightly positive charge
     • A carbon dioxide, CO2 molecule is non-polar
       • The two C=O bonds each have a permanent dipole
       • The two dipoles act in opposite directions and exactly oppose each other
         • Dipoles cancel out and the overall dipole is zero