4.7 Hybridisation (HL)

Hybridisation

Hybridisation is the mixing of atomic orbitals in a particular atom to produce a new set of orbitals (the same number as originally) that have characteristics of the original orbitals and are better arranged in space for covalent bonding. The hybrid orbitals are degenerate.

Nature of science: The need to regard theories as uncertain

Hybridization in valence bond theory can help explain molecular geometries, but is limited. Quantum mechanics involves several theories explaining the same phenomena, depending on specific requirements.

Hybridisation in ethene (C2H4)

The arrangement of atoms around each C atom is trigonal planar, and the molecule is planar overall. Of the 3 p orbitals on each C atom, one is not in the same plane as the H atoms or the other C atom - this p orbital is not involved in hybridization. Mixing the other 2 p orbitals and one s orbital produces 3 sp2 orbitals pointing towards the corners of an equilateral triangle. This leaves one p orbital, containing one electron, perpendicular to the single bond framework on each C atom. The p orbitals are involved in the formation of the pi component of the double bond. The double bond has two different components - a sigma bond, which results from the head-on overlap of two sp2 orbitals, and a pi bond which arises from the side-on overlap of parallel p orbitals.

Determining the hybridization of an atom in a molecule or ion

The hybridization scheme adopted in a molecule depends on the shape of the molecule: 2 electron domains - Linear - sp 3 electron domains - Trigonal planar - sp2 4 electron domains - Tetrahedral - sp3

Hybridisation in ethyne (C2H2)

The linear shape means that one s and one p orbital on each C atom are hybridized to produce two sp hybrid orbitals at 180° to each other. These are used to form sigma bonds between the C atoms and also to the H atoms. This leaves two p orbitals at 90° to the C-H bonds that are not involved in the hybridization. These p orbitals overlap side-on to produce two pi bonds.

Hybridisation in methane (CH4)

C has the outer shell electron configuration 2s(2) 2p(2). One of the electrons in the 2s orbital is promoted to the 2p sub-shell to give 4 unpaired electrons. In order to form methane, the four atomic orbitals on carbon (1 s and 3 p) mix to give four sp3 hybrid orbitals, which point to the vertices of a tetrahedron. Methane contains only single bonds between C atoms. These are sigma bonds formed when sp3 hybrid orbitals on the carbon atom overlap head-on with the s orbitals of each H atom.