lone pair e-

[bgiorgi_3A]

I have a general question... do lone pair electrons play a role in determining if a molecule is polar or not? When I was in high school someone once told me that if the center atom had lone pairs that then the molecule would be polar, however that does not seem to be the case. For example XeF2 has three lone pairs around the central atom, yet it is a non polar molecule. Any explanation would be appreciated! Thanks:)

[Jenny Lee 2L]

Hello! Yes, lone pair e-s do play a role in determining if a molecule is polar or not. The only reason why this example, XeF2, is not polar is because the lone pairs actually create the two F molecules to be 180 degrees to each other, which creates the linear shape. Because the F molecules' dipoles cancel out, the XeF2 molecule is overall nonpolar. However, lone pair e-s definitely still play a role in molecules, even XeF2. After all, the lone pairs is what caused XeF2 to have a linear shape.

An example where a lone pair e-s create a polar molecule is H2O. These lone pairs help to create a bent shape, and because O is much more electronegative than the hydrogens, H2O is a polar molecule.

[Samudrala_Vaishnavi 3A]

I don't think that lone pairs play a role in determining the polarity of a molecule directly. Like, having more lone pairs doesn't correlate to having more or less polarity. However, they DO influence the shape of the molecule and whether the dipole moments cancel out each other when there are polar bonds between two atoms. If a lone pair is present and it repels that atoms so that their dipole moments aren't the opposite of each other, they can be are polar in nature. For instance, if we look at a structure with 34 valence electrons in which a 3rd period or above central atoms (empty d-shell) has four surrounding atoms, there will be a lone pair at the central atom which repels the surrounding atoms equally so they aren't opposites of each other and if there was a polar bond between the central and surrounding atom it would not cancel. If there was no lone pair at the central atom, the polar bond would be canceled out by the opposite polar bond since the shape isn't changed.

Sorry about the long explanation but I hope that helps.

[Vince Li 2A]

I get what the issue is. It's hard to really determine if a shape is polar and nonpolar just from looking at the lone pairs. In fact, I don't think there's any way to determine if the shape is polar or nonpolar just by looking at if the central atom has lone pairs. You have to identify the electron density geometry and the atom geometry in order to determine if it's nonpolar or polar. Once you find the molecular geometry, you will be able to decide its polarity.