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Author: Subject: Electronegativity...I don't get it!!
chasingit33
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[*] posted on 8-4-2009 at 18:25
Electronegativity...I don't get it!!


so I just stumbled upon this forum. And since I'm curious to test it out, and happened to be learning some introductory chem for school, I figured i would throw a question that I have out there (even though it is probably very simple).

But anyway:

I just read about electronegativity, and how H2O has a polar covalent bond because Oxygen is more electronegative than Hydrogen. The textbook goes on to say that CH4 (methane) is far LESS polar than H2O because there is less difference in electronegativity between C & H, in comparison to the electronegativity between O & H.

What is this based on? Intuitively, I suspected that electronegativity is based on how many electrons an atom needs to fill its valence shell, and since Oxygen needs 2 and Hydrogen only needs 1, Oxygen has greater electronegativity. However, Carbon needs 4 electrons, compared to Hydrogen's 1, in order to fill the valence shell. Therefore, shouldn't methane (CH4) have even greater electronegativity? and be more polar? (in comparison to H2O).

I imagine the answer has to do with Carbon being both 4 electrons away from filling its valence shell, and 4 electrons away from shedding its valence shell? Whereas Oxygen is clearly closer to filling its valence shell than to shedding it.

But then how does Oxygen compare to Sulfur (also 6 valence electrons) or Nitrogen and Phosphorous (both w/ 5 valence electrons)? Does Nitrogen and Phosphorous have even GREATER electronegativity? While Sulfur has EQUIVALENT electronegativity? (compared with Oxygen)

Thanks!



[Edited on 9-4-2009 by chasingit33]
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DJF90
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[*] posted on 8-4-2009 at 18:33


Electronegativity is a manifestation of Effective nuclear charge. If you look at basic "sheilding", it will make sense that as you move from left to right across a period the ENC increases, as there is a whole unit increase of nuclear charge at the nucleus but very little shielding added by the additional electron. Hence fluorine is greater than oxygen which is greater than nitrogen, as the higher ENC has more of a pull of the electrons and so the atoms are more electonegative.

As you move down a group there is a large increase in nuclear charge but there is also a whole new shell of electrons shielding the outer shell electrons from the nuclear charge. It turns out that the increase in shielding is more effective than the increase in nuclear charge, and so the ENC decreases down a group (i.e. Fluorine is more electronegative than chlorine ....than bromine .....than iodine. I hope this makes some sense.

Depending on your level of understanding in chemistry you could look at Slater's rules.

[Edited on 9-4-2009 by DJF90]
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[*] posted on 8-4-2009 at 18:48


Electronegativity is a function of atomic weight and the distance of the valence electrons to the nucleus.

Going down a group, electronegativity decreases, because the valence electrons are becoming farther and farther away and there is more shielding. For example, the halogen group: Most electroneg. F > Cl > Br > I Least electroneg.

Going across a period, electronegativity increases because atomic weight increases. Because the shielding remains pretty much constant across a period, as the atomic weight increases, there is more positive charge in the nucleus to pull in the electrons. Therefore, in your example, O is more electronegative than C.

[Edited on 4/9/2009 by Saerynide]




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chasingit33
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[*] posted on 8-4-2009 at 18:52


thanks for the replies
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[*] posted on 9-4-2009 at 16:57


Good question, its worth remembering also, when posing questions such at this to oneself, that these 'explanations' science poses are simply models we have that describe observations. They may, or may not be the way things actually are. If you find a discrepancy between what you are taught and what appears a logical observation it just may mean the model is weak is this way, not that the theory you are learning is wrong.
Teachers often forget this basic premise and when you ask then about a logical contradiction they try and bullshit you, don't be discouraged by this they are just bad teachers.




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chemoleo
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9-4-2009 at 17:12
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[*] posted on 9-4-2009 at 19:57


I couldn't help commenting on an little thing in your post. You said that methane is more polar than water, and while this is true, I think you meant that the C-H bonds in methane for example, are less polar than the O-H bonds in water! big difference ;)

This whole issue was made clearer to me thanks to a chem. teacher in highschool explaining the truth... Bonds aren't either polar or non-polar, they're all on a continuum.

The reason that a methane molecule is less polar than a water molecule, is because water is bent planar, and has a net dipole, whereas methane is symetrical and does not.




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[*] posted on 9-4-2009 at 22:46


Ramiel - I get a mixed impression from your post:
Quote:

You said that methane is more polar than water, and while this is true...

The reason that a methane molecule is less polar than a water molecule...


Methane is NON-POLAR, for the simple reason that although there is polarity in the C-H bonds, the H atoms are arranged tetrahedrally around the central carbon an so there is no net dipole.

Water is POLAR, because the large difference in electonegativity between O and H, and the fact that the two O-H bonds have some common direction (i.e. its a bent molecule) means that there is a net dipole.
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[*] posted on 9-4-2009 at 23:05


Right, I just suck at explaining :D



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