In CH2Cl2 all bond moments reinforce each other (C-Cl at the same side, draw a tetrahedral molecular structure), while in CHCl3 the bond moment of the Cl's opposes the net moment of the other two. This effect is more apparent when the tetrahedral nature of the molecule is considered.
2. Oxididation number is a value assigned to an atom based upon the relative electronegativities. The bonding e- 's are given to the more electronegative element of the bond, and all remaining e- 's, including the unshared ones are counted. the oxidation number equals the periodic group number minus the counted e's. The oxidation number is not a real charge, but nevertheless is useful for revealing whether reactions of covalent species are redox reactions.
3. The sume of the oxidation numbers of all the atoms in a species is equal to its charge, which is zero in the case of a molecule.