1. The moment of any two B-F dipoles of symmetrical trigonalmolecule has equal magnitude and opposite direction. Themoment of BF3 molecule is zero, like CO2. 2. H is less electromagnetic than N in NH3. Hence, N pulls theelectrons form H towards itself so, the direction of moment and the direction of lone pair of electron of nitrogen is same.
3. The size of I-atom is more than the Cl-atom as the electronegativity I-atom is less than the Cl-atom. So, HI has lower dipole moment than HCl .