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Mawiege

molecular polarity - help!

can someone please explain the concept of molecular polarity to me? I don't understand how in some cases, the resultant dipoles cancel out the individual dipoles, because sometimes it's pretty clear, but other times it just doesn't make sense to me..

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from what i know, there is no real scientific way to figure this out

what you should do is draw the direction of each of the bond polarities and from there deduce a net charge direction.

if there is one, then its polar. if there isn't one, then its non polar.

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Molecular polarity is the uneven distribution of electrons in a covalent bond due to a difference in their electronegativity values.

Just consider H-Cl, Cl is way more electronegative than H and as such, electron density will be highly concentrated towards the Cl than H, this will cause an even distribution of charge itself, the charge in Cl side will be slightly negative and the charge in H will be slightly positive. This is why it is considered a polar molecule, the direction of arrow will indicate the direction in which electron density is higher aka negative side, and this is called the net dipole moment. This polarity will cause the molecules to have stronger intermolecular forces than Van Der Waals (Van Der Waals molecules do not have permanent dipoles). This intermolecular force is called Dipole-Dipole Force.

If a molecule had bonds all around it, two opposing bonds will cancel out the effect of each other, just imagine it as if the central atom is the atom that other terminal atoms are grabbing. Take Cl-Cl as an example, in this molecule both atoms have the same electronegativity and the electrons will as a result be distributed equally. This molecule is said to be non polar.

Lets take a more complicate molecule which is H2O this molecule is V-Shaped in which O is the central atom and H2 are the terminal atoms, these terminal atoms have lower electronegativity than O, this will cause the electron density to be more attracted to O than H, Had there been two other Hydrogens bonded to the O at the opposite ends (H4O - this is a imaginary molecule) the net dipole moment would have canceled out. note that H2O is hydrogen bonded.

Basically you can tell if there is molecular polarity from the shape of molecule, if it was symmetrical, the molecule would be non-polar, if it was asymmetrical the molecule would be polar.

Tetrahedral - Non-polar

Trigonal pyramidal - polar

V-Shaped/bent - polar

Linear - non-polar

Trigonal planar - non-polar

Octahedral - non-polar

T-shaped - polar

Square Pyramidal - polar

Square planar - non-polar

Trigonal bipyramidal - non polar

Hope this helps

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