Diamond and Graphite

[Jazmin]

Hi everyone! i have two qusestion that desperatly need answering. I hope somebody out the can help with the answers

 

Outline the principles of a theory that is used to describe the shapes of simple covalent molecules. Use the theory to explain the shapes of the following molecules:

a) BH3 b)PCl3 c)SiH4 d)H2S

 

And my darlings;

 

In terms of structure and bonding explain the difference in physical properties between diamond and graphite.

 

A Big thank you to anybody that can help me!

x

[ecoli]

Outline the principles of a theory that is used to describe the shapes of simple covalent molecules. Use the theory to explain the shapes of the following molecules:

a) BH3 b)PCl3 c)SiH4 d)H2S

 

You'll want to do some research on Valence Shell electron pair repulasion, or VSEPR theory.

 

In terms of structure and bonding explain the difference in physical properties between diamond and graphite.

 

Both are made purely out of carbon, but how they bond affects them a LOT.

 

Here are the crystal structures of both: http://www.geo.ucalgary.ca/~tmenard/crystal/diamond.html

[RyanJ]

Hmm interesting question. I believe you want to look into something called [acr=Valence Shell Electron Pair Repulsion]VSEPR[/acr]

 

It sounds like a homework question so I'll point you too some links the rest it down too you

 

http://www.chem.purdue.edu/gchelp/vsepr/

http://en.wikipedia.org/wiki/VSEPR

 

Should both help you.

[woelen]

BH3

This is a non-existent molecule. There exists a molecule B2H6, which has a very special structure. I write it as H2B(-H-)2BH2.

 

The left two H-atoms and right two H-atoms are bonded to the B-atom using a normal bond, sharing two electrons (one from the B-atom and one from the H-atom).

The two H-atoms in the middle, however, are bonded by means of the so-called 2e3c bond, 2 electron 3 centered. For each of the H-atoms, one electron of a B-atom, and its own electron are shared by both B-atoms and the H-atom. Unfortunately I cannot draw over here, but the following is an attempt to draw the structure of B2H6. It does not resemble C2H6 at all, it uses a TOTALLY different bonding structure. The =|= stands for a type of bond, with 4 electrons in total (kind of double bond), but with the H-atoms associated with it as well. Google for "2 electron 3 centered" bond for more details.

 

H         H
\   H   / 
 B =|= B
/   H   \
H         H

 

Have a look at this page: http://en.wikipedia.org/wiki/Diborane

 

 

 

PCl3

P has 5 electrons in its outer shell. Three of them are shared with Cl, 2 remain as free electron pair. The two electrons repel the bonds to the Cl-atoms, so the molecule will have the three Cl-atoms on one side, with the Cl-atoms spanning a triangle with equal sides. I expect this molecule to have a distorted tetrahedral shape.

 

SiH4

Si has 4 electrons in its outer shell. All of them are single bonded to H-atoms. This molecule will be symmetric as much as possible and will form a tetrahedron, with the Si-atom inside, and the H-atoms on the vertexes.

 

H2S

S has 6 electrons in its outer shell. Two of them are used for bonding with H, the other two free electron pairs repel the bonds with the H-atoms. This molecule will have the shape of the letter V, but with a larger H-S-H angle. The two free electron pairs will be above and below the plane, in which the H-S-H atoms are.