<< Chapter < Page Chapter >> Page >

Tetrahedral structure of methane

The dotted lines illustrate that the hydrogens form a tetrahedron about the carbon atom.
The same tetrahedron is formed by placing four points on a sphere as far apart from one another as possible.

We conclude that molecular geometry is determined by minimizing the mutual repulsion of the valence shellelectron pairs. As such, this model of molecular geometry is often referred to as the valence shell electron pair repulsion (VSEPR) theory . For reasons that will become clear, extension of this model impliesthat a better name is the Electron Domain (ED) Theory .

This model also accounts, at least approximately, for the bond angles of H 2 O and N H 3 . These molecules are clearly not tetrahedral, like C H 4 , since neither contains the requisite five atoms to form thetetrahedron. However, each molecule does contain a central atom surrounded by four pairs of valence shell electrons. We expect fromour Electron Domain model that those four pairs should be arrayed in a tetrahedron, without regard to whether they are bonding orlone-pair electrons. Then attaching the hydrogens (two for oxygen, three for nitrogen) produces a prediction of bond angles of109.5°, very close indeed to the observed angles of 104.5° in H 2 O and 107° in N H 3 .

Note, however, that we do not describe the geometries of H 2 O and N H 3 as "tetrahedral," since the atoms of the molecules do not form tetrahedrons, even if the valence shell electron pairs do. (It isworth noting that these angles are not exactly equal to 109.5°, as in methane. These deviations will be discussed later .)

We have developed the Electron Domain model to this point only for geometries of molecules with four pairs ofvalence shell electrons. However, there are a great variety of molecules in which atoms from Period 3 and beyond can have morethan an octet of valence electrons. We consider two such molecules illustrated in .

More molecular structures

First, P Cl 5 is a stable gaseous compound in which the five chlorine atoms are each bonded to the phosphorous atom. Experiments reveal that thegeometry of P Cl 5 is that of a trigonal bipyramid : three of the chlorine atoms form an equilateral triangle with the P atom in the center, and theother two chlorine atoms are on top of and below the P atom. Thus there must be 10 valence shell electrons around the phosphorousatom. Hence, phosphorous exhibits what is called an expanded valence in P Cl 5 . Applying our Electron Domain model, we expect the five valenceshell electron pairs to spread out optimally to minimize their repulsions. The required geometry can again be found by trying toplace five points on the surface of a sphere with maximum distances amongst these points. A little experimentation reveals that thiscan be achieved by placing the five points to form a trigonal bipyramid. Hence, Electron Domain theory accounts for the geometryof P Cl 5 .

Second, S F 6 is a fairly unreactive gaseous compound in which all six fluorineatoms are bonded to the central sulfur atom. Again, it is clear that the octet rule is violated by the sulfur atom, which musttherefore have an expanded valence. The observed geometry of S F 6 , as shown in , is highly symmetric: all bond lengths are identical and all bond angles are90°. The F atoms form an octahedron about the central S atom: four of the F atoms form a square with the S atom at the center, and the othertwo F atoms are above and below the S atom. To apply our Electron Domain model to understand this geometry, we must place six points,representing the six electron pairs about the central S atom, on the surface ofa sphere with maximum distances between the points. The requisite geometry is found, in fact, to be that of anoctahedron, in agreement with the observed geometry.

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
Ali
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
Google
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
hey
Giriraj
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
revolt
da
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
Bhagvanji
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
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
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
Santosh
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
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
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
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!





Source:  OpenStax, Concept development studies in chemistry. OpenStax CNX. Dec 06, 2007 Download for free at http://cnx.org/content/col10264/1.5
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Concept development studies in chemistry' conversation and receive update notifications?

Ask