Beginner's question
Herman Jurjus
In a typical high school level chemistry book, one can read that
the H2O molecule is a little magnetic dipole, while CO2 is not.
This is usually illustrated with a picture showing how the H-atoms are
located asymmetrically around the O atom, while the CO2 molecule
is perfectly symmetrical.
But what accounts for this difference?
Is it because of the smaller relative sizes of the atoms involved?
Or the ratio of the masses? Or something else?
It's also said that the H atoms are located in an angle of 105 degrees.
Where does this angle come from? How does one calculate that?
Is all of this a simplification of much more complex behavior?
If yes, what's really going on?
Same question for NH3 (why are the three H atoms not located more
symmetrically, 'on the equator'?)
More generally: are there good online sources that address questions
like these?
Many thanks in advance.
--
Cheers,
Herman Jurjus
Ian Gay
That should be 'electric dipole', not magnetic.
For a simple explanation of molecular shapes, google for VSEPR.
Ian
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dlzc
On Nov 10, 12:45 am, Herman Jurjus <hjur...@hetnet.nl> wrote:
> In a typical high school level chemistry book,
> one can read that the H2O molecule is a little
> magnetic dipole, while CO2 is not. This is
> usually illustrated with a picture showing how
> the H-atoms are located asymmetrically
> around the O atom, while the CO2 molecule
> is perfectly symmetrical.
A result of the orbitals in the atoms, where the electrons are shared,
to form a molecule.
> But what accounts for this difference?
Orbitals. Described by the schroedinger equation (where we can obtain
solutions).
http://en.wikipedia.org/wiki/Atomic_orbital
> Is it because of the smaller relative sizes of the
> atoms involved?
No, it is described by the orbitals or shells filled by the electrons
that are used in the binding.
> Or the ratio of the masses? Or something else?
Number of charges.
> It's also said that the H atoms are located in an
> angle of 105 degrees. Where does this angle
> come from? How does one calculate that?
You know the average bond length (by other experiment), then determine
how much energy is stored in spinning those type of molecules.
> Is all of this a simplification of much more
> complex behavior?
It is the measured / observed result of a complex behavior. Most of
which we have working models for.
> If yes, what's really going on?
We don't have access to Reality, all we can do is measure and make
inferences.
> Same question for NH3 (why are the three H
> atoms not located more symmetrically, 'on the
> equator'?)
Same answer. "Atomic size" doesn't really kick in until you get
beyond calcium, which will allow additional "oxidation states" to
start appearing.
> More generally: are there good online sources
> that address questions like these?
Wikipedia is a good starting point. Launching off into its references
would be the required next step.
David A. Smith
Martin Brown
> Hi all,
>
> In a typical high school level chemistry book, one can read that
> the H2O molecule is a little magnetic dipole, while CO2 is not.
> This is usually illustrated with a picture showing how the H-atoms are
> located asymmetrically around the O atom, while the CO2 molecule
> is perfectly symmetrical.
> But what accounts for this difference?
The way electron wavefunctions interact. CO2 has double bonds and all
the bonding electrons are used up in that molecule so it is linear.
CO2 is O=C=O
H2O is "O"
/ \
H H
In crude form ASCII art. Two single bonds to hydrogen and two lone pairs
of electrons that repel each other more strongly than the hydrogens
do. So the hydrogens are squashed together slightly.
> Is it because of the smaller relative sizes of the atoms involved?
> Or the ratio of the masses? Or something else?
>
> It's also said that the H atoms are located in an angle of 105 degrees.
> Where does this angle come from? How does one calculate that?
It is a distortion of the tetrahedral angle which 109.5 which would be
the natural bond angle for the symmetrical structure of methane CH4.
>
> Is all of this a simplification of much more complex behavior?
Yes.
> If yes, what's really going on?
Molecular orbitals as a solution of Schrodingers equation. But for the
purposes of a handwaving understanding of the approximate rules of the
game try Wiki on "Lone pairs"
>
> Same question for NH3 (why are the three H atoms not located more
> symmetrically, 'on the equator'?)
They are as symmetrically disposed as they can manage consistent with a
minimum energy structure. There is one lone pair without a hydrogen.
>
> More generally: are there good online sources that address questions
> like these?
Wiki isn't bad on this sort of stuff. eg
http://en.wikipedia.org/wiki/Lone_pair#Angle_Changes
As are some of the introductory university chemistry sites.
It is sadly all too rare to see a real question about chemistry here :(
Regards,
Martin Brown
erschro...@gmail.com
The O in water has 2 lone pairs; this makes the 2 bonding pairs lie at
an angle of around 104 degrees, so their dipoles are not opposite each
other and do not cancel each other out. In carbon dioxide, the O does
not have any lone pairs, so the 2 bonding pairs are at 180 degrees to
each other and thus their bond dipoles cancel.
Salmon Egg
Herman Jurjus <hju...@hetnet.nl> wrote:
I saw some of the responses so far. I do not think they were helpful. I
do not think I can be very helpful either at the high school level. Even
before I was in high school, I was plagued by similar lack of
understanding. For now, the best approach may be just to believe your
elders who wrote the books. If your intellect is truly ahead of your
cohorts', you can learn much by yourself. Certainly, that was the way
taken by a very few great scientists and mathematicians who did not have
access to the best educations.
Most of you questions will be answered when you understand quantum
chemistry and physics. That may not occur even in college. It might have
to wait until graduate level studies. Meanwhile, the best advice I saw
given was to investigate appropriate sections in Wikipedia. You might
have to jump from place to place and repeat sections. The Scientific
American may have suitable articles from time to time. Two old books
that may be useful are approximately 1) Kaufman, Quantum Chemistry; 2)
Pauling, The Nature of the Chemical Bond.
Good luck.
Bill
--
As the years go by, dying just before having to fill out a tax return has merit.
Anonymous
> the H2O molecule is a little magnetic dipole, while CO2 is not.
> This is usually illustrated with a picture showing how the H-atoms are
> located asymmetrically around the O atom, while the CO2 molecule
> is perfectly symmetrical.
To make things more complicated (and correct), water is also
symmetrical, but with "C_2v" (read: "see two vee") symmetry.
http://www.phys.ncl.ac.uk/staff/njpg/symmetry/Water/water0.html
CO2 has "D_infinity-h" (I can't draw infinity in ASCII; read
"dee infinity aitch") symmetry.
http://en.wikipedia.org/wiki/Molecular_symmetry
Historically, there was an interplay of experiment and theory
to both determine and explain these structures. VSEPR provides
a useful description and explanation of structure, up to a
point. Quantum is better but harder to work with (compute;
solve). Take it one step at a time, grasp a new idea and
then add to that.
Herman Jurjus
Many thanks to all respondents.
They /were/ very helpful.
(I'm no longer a high school student, btw - but when it comes to
chemistry that's my current level.)
--
Cheers,
Herman Jurjus