Question

HF is polar while F2 is nonpolar.Explain

Answer 1

Well, Expand each of those mocecules out. 

H2 is really H-H. Both atoms in the molecule have the same electronegativity (because they are the same atom!) and so they share teh electrons between them equally. 

F2 is F-F. For the same reason, they share their bonding pair of electrons equally as well. Neither is eletromagnetically strong enough to pull the electron pair away from the other atom. 

HF is H-F. The two atoms have differing electronegativities, and so they do not share the bonding pair of electrons equally. F is stronger than H, and so F has the electrons closer to it than H. This causes what is caller a "dipole" to appear, where a slightly positive charge is attributed to H and slightly negative to F, due to the proximity of the bonding electrons. This is what is meant by "polar", that one side of the bond is partially +ve, and the other side is partially -ve (Similar to North and South on a compass, hence, Polar!)