[steph_r]

Hello everyone

Im having trouble understanding:

1) The ways in which bonding and molecular shape determine the polarity of a molecule ; and

2) How the polarity of a molecule determines the type of intermolecular force present.

Anybodys help would be greatly appreciated

Thank you in advance

[shelanachium]

A big subject, but let's get you started.

1 If two elements differ greatly in electronegativity (electron-attracting power) the bond between them will be polar, with the less electrongative element carrying a positive charge, and the more electronegative a negative one. Consider the molecule HF. F is much more electronegative than H, so this molecule is highly polar, with a partial +ve charge on H and a -ve charge on F.

But now consider BF3. This is a planar trigonal molecule. The B-F bonds are at least as polar as the HF bonds in hydrogen fluoride, yet the molecule overall is not polar, because of its high symmetry. The same applies to linear CO2, tetrahedral CF4 and octahedral SF6.

Other polyatomic molecules with polar bonds have lower symmetry and therefore are polar overall, like bent H2O and SO2.

2 Polar molecules, having positive and negative 'ends' attract each other quite strongly, as the negative end of one attracts the positive end of the other. So polar molecules have higher melting and boiling-points than non-polar ones. Compare linear and therefore non-polar CO2 (sublimes -76) with bent, polar SO2 (boils -10); or non-polar SF6 (sublimes -64) and polar SF4 (bp -40). (SF4 is not a regular tetrahedron so is polar, look up its structure).

Hope this gets you started.

[steph_r]

Thank you!!
It has really helped