Question about covalent bonding

[dttom]

How can we determine the shape of a molecule? I know electrons in a molecule should be separated as far as possible but how we can predict this?may be this involve some mathematical skills.

[insane_alien]

most bond angles are the same in most cases for certain structures. all you have to do is make a model with those bond angles and you'll end up with the rough shape of the molecule

[mezarashi]

It's pretty hard to generalize a rule for all molecules. When you have four or five different species involved in many different hybrid bonding. If it were such an exact science, we probably wouldn't need the atomic-width microscopes to probe nano structures. There is a general theory however which is quite meaningless in complex systems (due to its mathematical complexity), that is the valence shell electron repulsion theory. It's works with simple compounds like CH4. It says that the electrons will arrange themselves geometrically such that the repulsions between them is at a minimum as so system is at its lowest energy possible.

[dttom]

The repulsion theory states that the outermost elections will separate as far as possible, right? In case of CO2, is the shape of the molecules linear? How about water molecule, why it has a V-shape?

[Leison]

in water there's repulsion between lone pairs of oxygen atom but in CO2 there's no lone pair of C.

[dttom]

That means the repulsion from the lone pair electrons affect the position of the bond? Can you explain it further?

[insane_alien]

a lone pair will act almost like a another bond(its repulsive force is slightly greater) so just think of it as a bond to an imainary atom.

[Leison]

That means the repulsion from the lone pair electrons affect the position of the bond? Can you explain it further?

 

repulsive force :

lone pair-lone pair > lone pair-bond pair > bond pair-bond pair

 

due to high repulsion betn 2 lone pairs of O ,the 2 bond pairs of H2O slightly contracts resulting a angle slightly less than the normal sp3 hybridized molecule like methane.

[dttom]

And there's another question related to covalent bond, that is, which type of covalent bond is stronger, single, double or triple covalent bond? Nitrogen atoms in its molecule are held by triple covalnet bond, compare with oxygen, nitrogen takes part less in chemical reactions, does it mean triple bond is stronger than double bond and so on? But why double bonds in organic compounds are weaker than single bond?

[The Thing]

Ah, you mixed up bond strength and reactivity (stability).

It is true that triple bonds are stronger than double bonds, which in turn are stronger than single bonds.

However, bond reactivity is another thing. The C-C triple bond (dunno how to get the three lines) is stronger than the C=C double bond, and it will be proven once you check any table of bond dissociation energy.

However, the triple bond has a region of greater density of electrons between the atoms than the double bond, and when electron-seekers approach the greater electron density allows more vulnerability to their attack.

So the overall message is: dissociation of a molecule by simply pulling the atoms apart (the bond strength) is different from attack from something that has a great affinity for electrons (reactivity).

 

Edit: A quick search and here are the single, double and triple bonds (carbon to carbon) in organic compounds and their respective bond energy (in kJ/mol)

 

Single, occurs in alkanes, with energy of 347.

Double, occurs in alkenes, with energy of 614

Triple, occurs in alkynes, with energy of 839