[jasmine85]

Hi. I'm trying to understand the physical nature of chemical bonds, in particular with respect to how the physical shape of the parent molecule is affected by electron configurations.

I understand the oxygen atom has an electron configuration of 1s2 2s2 2p4, which would suggest two complete orbitals, and two half complete orbitals, in the outermost shell.

The two half complete orbitals give the oxygen a valency of 2, which provide bond points that are at an angle to one another, because of the two lone pairs.

My question is how do these complete/incomplete orbitals affect the shape of the orbitals? Do complete orbitals claim a greater solid angle "territory" than incomplete ones?

Also, what is the significance of p and s orbitals on this geometry? Four equal full orbitals would form a tetrahedral shape, and presumably incomplete orbitals distort the tetrahedron as described in the above paragraph, but how is this geometry affected by the fact we are dealing with both s and p orbitals? s orbitals being spherically symmetric presumably are unbiased and don't provide a corner to the tetrahedron? So is it right to think of it as a tetrahedron at all?

thanks for any help you can offer in describing/visualising this

Jas

[MrTeo]

Hi Jas! I'm a newbie of the forum just like you and I'll try to be as clear as possible...

First of all you have to consider that the orbitals of the 2nd level in the molecule of water are not common s and p orbitals as you seem to consider them but they're hybridized in 4 sp3 orbitals. This means that their shape is quite different from the usual p orbitals: the electron cloud is asymmetric and we could say that there's a 25% of s bond and a 75% of p bond. Due to this the new orbitals are disposed (according to the VSEPR) to minimize repulsion forces and they form the characteristic thetraedron we all know.

Now we can put the electrons on the 2nd level in the four new orbitals, following the usual rules (Hund and Pauli): we'll find two orbitals filled with electrons and other two with only one electron each: the half complete orbitals give, as you said, the valency to the oxygen while the other ones, the so-called "lone pairs" cause the distortion because their repulsion forces affect the usual angle of 109.5º reducing it to approximately 105º.

The conclusion is that when you analyze a molecule using the VSEPR theory you should always think about the structure that minimizes the forces between electrons as this is the most common form (we can't be completely sure about it because of the quantum mechanics but this doesn't matter much for now )

Here's an image of an sp3 orbital, hope it helps and sorry form my rusty English... I'm Italian ^^

[jasmine85]

Thank-you. This is really helpful