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Polarity Question?

If the central atom has a lone pair does it make the whole molecule Polar? Despite the fact that the dipoles do not reinforce (which would make it non polar). For example in SF4.

Also if a molecule has one different atom, shouldn't that make it polar?
As in the case of PClO (where the oxygen is the different atom). Or does it not matter if the atoms is different as long as the dipoles do not have a general direction.
(edited 8 years ago)
The dipoles "appear" if there is a difference in electronegativity hence why things like HF and HCl have dipole-dipole intermolecular forces and F is highly electronegative and H is much less electronegative. Molecules like F2 don't have dipole-dipole because there is no difference in electronegativity. Hope this helps :-)
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Avatar for abitpissy
abitpissy
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Original post by BrandonF1407
The dipoles "appear" if there is a difference in electronegativity hence why things like HF and HCl have dipole-dipole intermolecular forces and F is highly electronegative and H is much less electronegative. Molecules like F2 don't have dipole-dipole because there is no difference in electronegativity. Hope this helps :-)


I think you didn't quite understand my question(s). What is the significance of a lone pair of electrons on the polarity of the molecule? In the case of SF4 Without the lone pair it would be unpolar as there is no general direction of the dipole. (Or at least that how I interpreted it. )
(edited 8 years ago)
Ahhh I see now. umm well because a lone pair of electrons has a greater electron density as the electrons aren't "spread out" over a covalent bond, and as the do effect the position of the bonds in the molecule and therefore the position of the other atoms, they could has an effect on the direction of the dipole. However, I think they just affect the magnitude of the dipole moment as you have to look at the strength and direction of the dipole moment seperatly.

Overall, they significantly effect the magnitude of the moment due to the fact the electrons are more localised in a lone pair than in a covalent bond however if there is no overall direction of the dipole moment then this could cause the lone pair to not have as great effect on the overall dipole moment.
Reply 4
Avatar for abitpissy
abitpissy
OP
14
Original post by BrandonF1407
Ahhh I see now. umm well because a lone pair of electrons has a greater electron density as the electrons aren't "spread out" over a covalent bond, and as the do effect the position of the bonds in the molecule and therefore the position of the other atoms, they could has an effect on the direction of the dipole. However, I think they just affect the magnitude of the dipole moment as you have to look at the strength and direction of the dipole moment seperatly.

Overall, they significantly effect the magnitude of the moment due to the fact the electrons are more localised in a lone pair than in a covalent bond however if there is no overall direction of the dipole moment then this could cause the lone pair to not have as great effect on the overall dipole moment.


Okay, seems to make sense. Thanks for the clarification. Does a lone pair generally mean that the molecule is polar?

Lone pair-bond pair repulsion is greater than bond pair-bond pair repulsion after all. This probably results in unsymmetrical shape, therefore it is more likely to be polar. Am I correct in thinking so?
Kind of, I think lone-pairs have a greater effect on molecular shape rather than dipole moments. The easiest example is the difference between CO2 and H2O and in CO2 each Oxygen has 2 lone pairs which stabilise a linear 180 degree bond angle while in H2O the 2 lone pairs are on the Oxygen which puss the 2 Hydrogens down to form a smaller bond angle with a "bent" or "angular" shape :-)

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