Why is arsenic trichloride more likely to be polar than aluminum trichloride? According to my chemistry teacher, $\ce{AsCl3}$ is more likely to be polar than $\ce{AlCl3}$. Why is this? As I understand it, the difference in electronegativity is what makes a compound more or less likely to be polar, and $\ce{AlCl3}$ has a much greater difference between the two elements' electronegativities. So why isn't it more likely to be polar of the two?
It is because $\ce{AlCl3}$ has a trigonal planar geometry (leading to no permanent dipole moment) and $\ce{AsCl3}$ has a pyramidal geometry (leading to a permanent dipole moment).