Electronegativity and polarity

• In a covalent bond, the nuclei of the bonded atoms attract the shared pair of electrons
• In the molecules of elements like O2, the atoms are the same element and the bonded electron pair is shared evenly
• 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
  • The shared pair of electrons in the covalent bond may now experience more attraction from one of the bonded atoms than the other
• Electronegativity = a measure of the attraction of a bonded atom for the pair of electrons in a covalent bond
• The Pauling scale is used to compare the electronegativity of the atoms of different elements.
• A large Pauling value indicates that the atoms of the element are very electronegative
• Electronegativity increases across and up the periodic table. This results in fluorine being the most electronegative atom and is give a Pauling value of 4.0. The noble gases are not included as they tend not to form compounds
• The non-metals nitrogen, oxygen, fluorine, and chlorine have the most electronegative atoms
• The group 1 metals, including lithium, sodium, and potassium have the least electronegative atoms
• If the electronegativity difference is large, one bonded atom will have a much greater attraction for the shared pair of electrons than the other bonded atom. The more electronegative atom will have gained control of the electrons and the bond will now be ionic rather than covalent
• Electronegativity values can be used to estimate the type of bonding:

  • electronegativity difference of 0 = covalent
  • difference of 0-1.8 = polar covalent
  • difference greater than 1.8 = ionic
• In a non-polar bond, the bonded electron pair is shared equally between the bonded atoms.
• A bond will be non-polar when:

  • the bonded atoms are the same
  • the bonded atoms have the same or similar electronegativity
• When the atoms come from the same element and the electron pair is shared equally, the bond is a pure covalent bond
• Carbon and hydrogen have very similar electronegativities and form non-polar bonds. Hydrocarbon liquids (like hexane) are non-polar solvents and do not mix with water .
• 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, resulting in polar covalent bond
• In Hydrogen chloride, the chlorine atom is more electronegative than than the hydrogen atom. The chlorine atom has a greater attraction for the bonded pair of electrons, resulting in a polar covalent bond. The H-Cl bond is polarised with a small partial positive charge (∂+) on the hydrogen atom and a small partial negative charge (∂-) on the chlorine atom. The delta ∂ sign means small.
• A polar bond has a partial ∂+ charge on one atom and a partial ∂- on the other. This separation of opposite charges is called a dipole.
• Dipole = a separation of chemical charge so that one atom of a polar covalent bond, or one side of a polar molecule, has a small positive charge ∂+ and the other has a small negative charge ∂-
• A dipole in a polar covalent bond does not change and is called a permanent dipole
• A water 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 ∂- charge and the hydrogen end of the molecule has a ∂+ charge
• A carbon dioxide molecule is non-polar:

  • the two C=O bonds each have a permanent dipole
  • the two dipoles act in opposite directions and exactly oppose on another
  • over the whole molecule, the dipoles cancel and the overall dipole is zero
• A sodium chloride lattice being dissolved by water molecules to form aqueous sodium chloride ions:

  • water molecules attract $Na^+$ and $Cl^-$ ions.
  • the ionic lattice breaks down as it dissolves
  • in the resulting solution, water molecules surround the $Na^+$ and $Cl^-$ ions
  • sodium ions are attracted towards the oxygen of water molecules(∂-)
  • chloride ions are attracted towards the hydrogen of water molecules (∂+)