[irelalex]

Hi, I have some confusion about Phase and constructive and destructive interference

When building a picture of the molecular orbitals in methane, all of the atomic orbitals in 4 H and 1 C can be in phase, let's call them all positive. This would give them constructive interference and make up a part of the total bonding in CH4.


Now going to the first of the P orbitals, I don't understand how 2 of the 1s orbitals from H's can now be in a negative phase (in phase with the negative side of the p orbital) and the other 2 can be positive (in phase with other side of P orbital). I can't see how they are now different, as they are geometrically similar to each other (on either side of the P AO) and they were in similar (positive) phase during the 1s - 2s MO mentioned in the above paragraph.   How can they change phase?

Any help appreciated, cheers.

[irelalex]

I think I've solved my problem.
I see that the p(x) orbital has an angular node on the yz plane, and will hence sort of 'miss' the electron density of 2 of the hydrogen atoms.
Is this what gives them different phase?

[Enthalpy]

Better.

Just notice that each lobe of these molecular orbitals includes two hydrogen atoms, so all four hydrogens are included. It's a matter of orientation of these lobes: the "axis" passes between two hydrogens.

Similar discussion there
http://www.chemicalforums.com/index.php?topic=46440.0

Perhaps the best entry point is a drawing
http://www.users.csbsju.edu/~frioux/h2bond/MethaneMOBonding.pdf
second figure of the pdf, with hydrogen atoms at each second apex of the cube.

Notice the absence of sp³ hybridization, which pertained to an older theory. But since hybridization seems still useful to chemists for being more manageable, keep it in mind.