[chatnoir]

I am unclear upon the concept which what determines if a molecule is polar. My teacher has previously told me that we can only determine that from the shape, which if there is a net-dipole, a molecule is polar. So if there is a lone pair and the shape of the molecule is unbalanced(dipole does not cancel out), can a molecule(such as ph3) with non polar covalent bond be polar?

[mbckenny]

You should think about whether a molecule is polar based on electronegativy.

With H-F for example, we know F is an extremely electonegative molecule and so it will withdraw the electrons in the H-F bond towards itself, thus making the molecule polar.

This electronegativity is therefore what determines whether a molecule is polar.

For your second query, PH3 is infact polar, PH3 is very much like NH3 and as you know, Nitrogen is electronegative and due to the lone pair of electrons making the molecule non planar (important as BF3 for example is non polar), this lack of symmetry in the molecule allows the molecule to be polar as the electron distribution is uneven.

Hope this helps,

Mike

[dazza95]

No, unless you change whats in the bond,
It can change if you add something with more electrons,