Polarity

__Bond Polarity__ - Measure of how equally the electrons in a bond are shared to form the bonds between atoms in molecules. Bond Polarity is largely determined by the difference in electronegativity. As the difference in electronegativity increases, the bond polarity also increases.

__Molecular Polarity__ - Depends on difference of electronegativity and asymmetry. One atom will pull the electrons closer to them, giving it a more negative charge. This also means that part of the molecule will end up with a slightly positive charge Note: For a molecule to be polar, polar bonds must exist! If polar bonds exist, but they are evenly spread out (symmetry exists), then they cancel out and there is a net charge of 0, so the molecule is non polar.

Since the electronegativity decreased from Fluorine to Iodine, the difference in electronegativity gets smaller. Consequently polarity decreases.



__Examples__ 1. HF - Polar because Difference in electronegativity is high. (F slightly negative) 2. NH3 - Polar (Polar bonds in a pyramidal shape) 3. CH4 - Non Polar (Polar bonds symmetrical in a tetrahedron) 4. BF3 - Non Polar (Polar bonds symmetrical in a trigonal planar formation)

Sources: Chem Book [|Wikipedia]