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In genetics, hybridisation is the process of mixing different species or varieties of organisms.
In molecular biology hybridisation is the process of joining two complementary strands of DNA.
In chemistry, hybridisation is a linear combination of atomic orbitals of approximately the same energy, yielding hybridised or hybrid orbitals. Hybridised orbitals are of the same energy and have a special geometrical arrangement.
The hybridisation theory has been introduced due to the inability of the valence bond theory to explain the existence of molecules such as methane (CH4).
The electron configuration of carbon is 1s² 2s² 2px¹ 2py¹, or, in graphical terms:
<math>C\quad
<math>
The valence bond theory would predict, based on two half-filled orbitals (2px and 2py), that C forms two covalent bonds. CH2, however, does not exist at all, and this theory cannot explain the experimentally proved existence of CH4.
During bond formation, energy is released, which enables one electron from 2s to go into the 2pz orbital (excited state), yielding the following arrangement of electrons:
<math>C^{*}\quad\frac{\uparrow\downarrow}{1s}\;\frac{\uparrow\,}{2s}\;\frac{\uparrow\,}{2p_{x}}\frac{\uparrow\,}{2p_{y}}\frac{\uparrow\,}{2p_{z}}<math>
Ordinarily, 2s and 2p orbitals do not have the same energy; however, experiments have shown that all bonds formed in CH4 are the same. So too do these orbitals have the same energy. One may speak of sp³ hybrid atomic orbitals. Hybridisation is sp³ because the energy level is s¹ + p¹ + p¹ + p¹; adding up the above numbers yields sp³ (read 's p three').
In CH4, 4 sp³ hybridised orbitals are overlapped frontally by hydrogen's s orbitals, yielding 4 sigma (σ) bonds. The shape is tetrahedral.
translates into
Similarly, other C-compounds and other molecules may be explained, for example ethene (C2H4). The two carbon atoms are sp² hybrised; the 2pz¹ orbital is not hybridised. Two hybridised orbitals of each C-atom frontally overlap with the s orbitals of H-atoms, one hybridised orbital frontally overlaps the other C-atom's hybridised orbital (thus, 5 sigma bonds), and 2pz orbitals have side overlapping, yielding a pi (π) bond. The shape of each part of the molecule is trigonal planar.
Hybridisation helps to explain molecule shape.
This works if there are no lone electron pairs on the central atom. If there are, they should be counted in the Xi number. For example, in water (H2O), the oxygen atom has two bonds with H and two lone electron pairs (as can be seen with the valence bond theory as well from the electronic configuration of oxygen), which means there are 4 such 'elements' on O. The model molecule is, then, AX4: sp³ hybridisation is utilised, and the electron arrangement of H2O is tetrahedral. The shape, however, is non-linear bent, since lone electron pairs are not visible, and also because repulsions must be taken into account. The HOH angle is round about 104.5 degrees.
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