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The concept that lone pair electrons produce a greater repulsive effect than do bonded pairs can be used tounderstand other interesting molecular geometries. Sulfur tetrafluoride, S F 4 , is a particularly interesting example, shown in .

Molecular structure of sf4

Note that two of the fluorines form close to a straight line with the central sulfur atom, but the other two areapproximately perpendicular to the first two and at an angle of 101.5° to each other. Viewed sideways, this structurelooks something like a seesaw.

To account for this structure, we first prepare a Lewis structure. We find that each fluorine atom issingly bonded to the sulfur atom, and that there is a lone pair of electrons on the sulfur. Thus, with five electron pairs around thecentral atom, we expect the electrons to arrange themselves in a trigonal bipyramid, similar to the arrangement in P Cl 5 in . In this case, however, the fluorine atoms and the lone pair could be arranged in twodifferent ways with two different resultant molecular structures. The lone pair can either go on the axis of the trigonalbipyramid ( i.e. “above” the sulfur) or on the equator of the bipyramid ( i.e. “beside” the sulfur).

The actual molecular structure in shows clearly that the lone pair goes on the equatorial position. This can be understood if weassume that the lone pair produces a greater repulsive effect than do the bonded pairs. With this assumption, we can deduce that thelone pair should be placed in the trigonal bipyramidal arrangement as far as possible from the bonded pairs. The equatorial positiondoes a better job of this, since only two bonding pairs of electrons are at approximately 90° angles from thelone pair in this position. By contrast, a lone pair in the axial position is approximately 90° away from three bondingpairs. Therefore, our Electron Domain model assumptions are consistent with the observed geometry of S F 4 . Note that these assumptions also correctly predict the observeddistortions away from the 180° and 120° angles which would be predicted by a trigonalbipyramidal arrangement of the five electron pairs.

Review and discussion questions

Using a styrofoam or rubber ball, prove to yourself that a tetrahedral arrangement provides the maximumseparation of four points on the surface of the ball. Repeat this argument to find the expected arrangements for two, three, five,and six points on the surface of the ball.

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Explain why arranging points on the surface of a sphere can be considered equivalent to arranging electron pairsabout a central atom.

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The valence shell electron pairs about the central atom in each of the molecules H 2 O , N H 3 , and C H 4 are arranged approximately in a tetrahedron. However, only C H 4 is considered a tetrahedral molecule. Explain why these statements arenot inconsistent.

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Explain how a comparison of the geometries of H 2 O and C H 4 leads to a conclusion that lone pair electrons produce a greaterrepulsive effect than do bonded pairs of electrons. Give a physical reason why this might be expected.

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Explain why the octet of electrons about each carbon atom in ethene, C 2 H 4 , are not arranged even approximately in a tetrahedron.

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Assess the accuracy of the following reasoning and conclusions:

A trigonal bipyramid forms when there are five electron domains. If one ED is a lone pair, then the lone pairtakes an equatorial position and the molecule has a seesaw geometry. If two EDs are lone pairs, we have to decide among thefollowing options: both axial, both equatorial, or one axial and one equatorial. By placing both lone pairs in the axial positions,the lone pairs are as far apart as possible, so the trigonal planar structure is favored.

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Assess the accuracy of the following reasoning and conclusions:

The Cl-X-Cl bond angles in the two molecules are identical, because the bond angle is determined by the repulsion ofthe two Cl atoms, which is identical in the two molecules.

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Questions & Answers

what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
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Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
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
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
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.
Bharti
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
Daniel
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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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