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Author Topic: Polarity rules. Are these foolproof  (Read 476 times)

Offline Morgan Lane

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Polarity rules. Are these foolproof
« on: 23/11/2015 23:22:28 »
Hi,
I was studying elecronegativity and polarity today and came up with 3 rules to figure out if a molecule is polar or non polar.

1. ALL molecules that contain lone pairs are polar.

2. ALL symmetrical molecules without lone pairs are non polar

3. In a molecule where there are odd numbers of different elements surrounding the 'main' atom in a molecule, the molecule would be polar.

Are these theories correct?


 

Offline chiralSPO

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Re: Polarity rules. Are these foolproof
« Reply #1 on: 24/11/2015 01:04:11 »
1. False. For example, carbon dioxide (which has four lone pairs, two on each oxygen) is a nonpolar molecule.

2. This depends what you mean by symmetrical. The standard non-technical definition of "symmetrical" is having a mirror plane, but obviously this cannot be correct because water is mirror-plane-symmetrical and is highly polar. If a molecule has an inversion center* it cannot be polar.

3. False. For instance boron trifluoride (BF3) which is planar and has three fluorine atoms surrounding the central boron atom, is nonpolar.

*An inversion center is a point at the center of a molecule (or any other geometric form, abstract or concrete) for which every atom in the molecule has a partner atom that is the same distance from the center, but in exactly the opposite direction. If there are any atoms without a matching atoms on the other side of the center of the molecule, then the molecule does not have an inversion center (unless the unmatched atom is at the center). (https://en.wikipedia.org/wiki/Molecular_symmetry#Elements)

Any molecule without an inversion center must be polar (even if its dipole moment is so close to 0 that it is considered "non-polar," like pentane)
 

Offline Morgan Lane

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Re: Polarity rules. Are these foolproof
« Reply #2 on: 24/11/2015 13:50:50 »
By the first rule, the lone pairs refer to those or lack thereof in the 'main' atom in the compound

The third rule is for example if you have one hydrogen and three oxygens then the compound would be polar whereas if you had 2 of each it wouldn't be etc
 

Offline chiralSPO

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Re: Polarity rules. Are these foolproof
« Reply #3 on: 24/11/2015 19:29:44 »
By the first rule, the lone pairs refer to those or lack thereof in the 'main' atom in the compound

The third rule is for example if you have one hydrogen and three oxygens then the compound would be polar whereas if you had 2 of each it wouldn't be etc

A few points:

Most compounds don't have a "main" atom, for instance glycine (https://en.wikipedia.org/wiki/Glycine), which is polar, and benzene (https://en.wikipedia.org/wiki/Benzene) which is non-polar.

There are also non-polar molecules with lone pairs on the central "main" atom, like xenon difluoride (XeF2 https://en.wikipedia.org/wiki/Xenon_difluoride), which has 3 lone pairs on Xe.

The single rule I mentioned in my first response (iff a molecule has an inversion center it is nonpolar) is sufficient to determine whether any molecule is polar or not.
 

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Re: Polarity rules. Are these foolproof
« Reply #3 on: 24/11/2015 19:29:44 »

 

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