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Observation 3: distortions from expected geometries

It is interesting to note that some molecular geometries( C H 4 , C O 2 , H C C H ) are exactly predicted by the Electron Domain model, whereas inother molecules, the model predictions are only approximately correct. For examples, the observed angles in ammonia and watereach differ slightly from the tetrahedral angle. Here again, there are four pairs of valence shell electrons about the central atoms.As such, it is reasonable to conclude that the bond angles are determined by the mutual repulsion of these electron pairs, and arethus expected to be 109.5°, which is close but not exact.

One clue as to a possible reason for the discrepancy is that the bond angles in ammonia and water are both less than 109.5°. Another is that both ammonia and water molecules have lone pair electrons, whereas thereare no lone pairs in a methane molecule, for which the Electron Domain prediction is exact. Moreover, the bond angle in water, withtwo lone pairs, is less than the bond angles in ammonia, with a single lone pair. We can straightforwardly conclude from theseobservations that the lone pairs of electrons must produce a greater repulsive effect than do the bonded pairs. Thus, inammonia, the three bonded pairs of electrons are forced together slightly compared to those in methane, due to the greater repulsiveeffect of the lone pair. Likewise, in water, the two bonded pairs of electrons are even further forced together by the two lone pairsof electrons.

This model accounts for the comparative bond angles observed experimentally in these molecules. The valenceshell electron pairs repel one another, establishing the geometry in which the energy of their interaction is minimized. Lone pairelectrons apparently generate a greater repulsion, thus slightly reducing the angles between the bonded pairs of electrons. Althoughthis model accounts for the observed geometries, why should lone pair electrons generate a greater repulsive effect? We must guessat a qualitative answer to this question, since we have no description at this point for where the valence shell electronpairs actually are or what it means to share an electron pair. We can assume, however, that a pair of electrons shared by two atomsmust be located somewhere between the two nuclei, otherwise our concept of "sharing" is quite meaningless. Therefore, the powerfultendency of the two electrons in the pair to repel one another must be significantly offset by the localization of these electronsbetween the two nuclei which share them. By contrast, a lone pair of electrons need not be so localized, since there is no secondnucleus to draw them into the same vicinity. Thus more free to move about the central atom, these lone pair electrons must have a moresignificant repulsive effect on the other pairs ofelectrons.

These ideas can be extended by more closely examining the geometry of ethene, C 2 H 4 . Recall that each H-C-H bond angle is 116.6° and each H-C-C bond angle is 121.7°, whereas the Electron Domain theoryprediction is for bond angles exactly equal to 120°. We can understand why the H-C-H bond angle is slightly less than120° by assuming that the two pairs of electrons in the C=C double bond produce a greater repulsive effect than do either ofthe single pairs of electrons in the C-H single bonds. The result of this greater repulsion is a slight "pinching" of the H-C-H bondangle to less than 120°.

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Concept development studies in chemistry. OpenStax CNX. Dec 06, 2007 Download for free at http://cnx.org/content/col10264/1.5
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