question archive A single bond consists of a σσ bond, which is defined as the head-on overlap between two compatible orbitals
A single bond consists of a σσ bond, which is defined as the head-on overlap between two compatible orbitals
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A single bond consists of a σσ bond, which is defined as the head-on overlap between two compatible orbitals.
Because hybrid orbitals are made to match the symmetry of the incoming (i.e. not-yet-bonded) atom's atomic orbital for the sake of being able to bond at all (which is usually if not always "totally symmetric about the internuclear axis"), they always have to overlap head-on.
Thus, a hybrid orbital has to make a σσ bond.
Not just sp3sp3, but any hybrid orbital. Even in a triple bond, like in acetylene (H−C≡C−HH-C≡C-H), the ππ bonds are made by the pxpx and pypy orbitals (or any qualified equivalent sidelong orbital overlap), while the σσ bonds are made with the hybrid orbitals, which consist of only the pzpz and ss orbitals.
Here, the zz axis is defined as the internuclear axis, whereas the xx and yy axes are vertical and towards us, respectively.
As an alternative, consider ethene (H2C=CH2H2C=CH2), where the double bonds lie on the xyxy-plane in the plane of the screen/paper, and the zz axis protrudes outward towards us.
If we consider the sp2sp2 hybridization of carbon in ethene, carbon actually uses three sp2sp2 hybrid orbitals: one each to σσ bond with the hydrogens, and one to σσ bond with the other carbon. (You would get that if you drew the molecule with all single bonds and no double bonds.)
It made each hybrid orbital by mixing the 2s2s, 2px2px, and 2py2py orbitals together and distributing their angular positions evenly on the xyxy-plane since the original 2p2p orbitals used to construct them were actually aligned in the xx and yy directions, respectively. (The notation sp2sp2 comes from mixing one ss and two pp orbitals together to get 33%33% ss character and 66%66% pp character.)
Hence, the σσ bonds of ethene (shown in yellow) lie on a single plane.
In addition, we have the 2pz2pz atomic orbital (perpendicular to the plane of the molecule, in purple) from each carbon that is used to ππ bond (sidelong overlap) with the other carbon, thus constructing the second component of the double bond (one σσ + one ππ bond = one double bond).
It was not used in the orbital hybridization, and it remains as a different, incompatible orbital (with respect to the 2px2px and 2py2py) for σσ bonding within the molecule. The only thing it can do at this point is ππ bond because it is oriented precisely to do so.
