In the bonding of elements to form compounds, intermolecular forces must be in place. These forces are due to the difference in electronegativity. The force is directed towards the more electronegative element because it attracts more electrons towards itself. In this case, Chlorine is more electronegative than Carbon.
You have to make the Lewis structure as shown in the figure. Since there are 4 valence electrons for Carbon and 7 for Cl, the total electrons would be 4 +3(7) = 25. A polar molecule is when there is an imbalance of forces creating a dipole moment. In this case, the opposite Cl branches cancel out leaving one for for the upper Cl. There is an imbalance, therefore CCl3⁻ is a polar molecule. The basis is: the molecule is nonpolar if the net molecular dipole moment is zero.
