Both NH3 and NF3 are pyramidal in shape with one lone pair on N. However, as F has higher electronegativity than H, the electron pair is attracted more towards F in NF3, i.e., the bond pairs of electrons are away from N or in other words, distance between bond pairs is more. Hence the repulsions between bond pairs in NF3 is less than in NH3. Thus the lone pair repels the bond pairs of NF3 more than it does in NH3. As a result, the bond angles decreases to 102.4º only.
