chasingit33
Harmless
Posts: 3
Registered: 8-4-2009
Member Is Offline
Mood: No Mood
|
|
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]
|
|
|
DJF90
International Hazard
Posts: 2266
Registered: 15-12-2007
Location: At the bench
Member Is Offline
Mood: No Mood
|
|
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]
|
|
|
Saerynide
National Hazard
Posts: 954
Registered: 17-11-2003
Location: The Void
Member Is Offline
Mood: Ionic
|
|
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]
"Microsoft reserves the right at all times to monitor communications on the Service and disclose any information Microsoft deems necessary to...
satisfy any applicable law, regulation or legal process"
|
|
|
chasingit33
Harmless
Posts: 3
Registered: 8-4-2009
Member Is Offline
Mood: No Mood
|
|
thanks for the replies
|
|
|
Panache
International Hazard
Posts: 1290
Registered: 18-10-2007
Member Is Offline
Mood: Instead of being my deliverance, she had a resemblance to a Kat named Frankenstein
|
|
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.
|
|
|
chemoleo
|
Thread Moved
9-4-2009 at 17:12 |
Ramiel
Vicious like a ferret
Posts: 484
Registered: 19-8-2002
Location: Room at the Back, Australia
Member Is Offline
Mood: Semi-demented
|
|
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.
Caveat Orator
|
|
|
DJF90
International Hazard
Posts: 2266
Registered: 15-12-2007
Location: At the bench
Member Is Offline
Mood: No Mood
|
|
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.
|
|
|
Ramiel
Vicious like a ferret
Posts: 484
Registered: 19-8-2002
Location: Room at the Back, Australia
Member Is Offline
Mood: Semi-demented
|
|
Right, I just suck at explaining 
Caveat Orator
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
