Have you learned much about atomic orbitals? and hyrbidization too?
Basically for Carbon:
___ ___ ___
2px 2py 2pz
__
2s
__
1s
Now, generally, the 1s will have 2 electrons in it (one spin up, one spin down). The 2s will also have 2 electrons, 1 spin up, one spin down.
since Carbon has 6 electrons, the remaining two will go into the 2p orbitals (for this case, we'll just say one goes into 2px spin up, and one goes into 2py spin up. According to all principles (Hund, Schrodinger, Pauli Exclusion etc). As you can see, Carbon will only have 2 unpaired electrons and thus can only attach to 2 other atoms right? Well, if you excite the carbon, the one of the 2s orbitals will be promoted and be in the 2pz, spin up. you now have 4 orbitals with only 1 electron each (eerily convenient, since it bonds to 4 hydrogen atoms, which also have 1 electron each). So there you have it. , you need to excite the electron to have 4 orbitals of one electron each,
Note that this alone is slightly wrong, since you would not have a tetrahedral atom, since the s orbitals are spherical, and p orbitals are dumbell shaped and at angles of 90 degrees to one another. to get the right geometry, the s and the 3 2p orbitals then combine to create 4 new orbitals which point to the 4 corners of a box, creating a tetrahedral shape. from this you have the right geometry and the right number of possible bonds.
|