The energy released by the formation of two additional bonds more than compensates for the excitation energy required, energetically favoring the formation of four C-H bonds. The carbon atom can also bond to four hydrogen atoms by an excitation (or promotion) of an electron from the doubly occupied 2s orbital to the empty 2p orbital, producing four singly occupied orbitals. The carbon atom can use its two singly occupied p-type orbitals, to form two covalent bonds with two hydrogen atoms, yielding the singlet methylene CH 2, the simplest carbene. For a tetrahedrally coordinated carbon (e.g., methane CH 4), the carbon should have 4 orbitals with the correct symmetry to bond to the 4 hydrogen atoms.Ĭarbon's ground state configuration is 1s 2 2s 2 2p 2 or more easily read: Hybridisation describes the bonding of atoms from an atom's point of view. The p character or the weight of the p component is N 2λ 2 = 3/4. Since the electron density associated with an orbital is proportional to the square of the wavefunction, the ratio of p-character to s-character is λ 2 = 3. The ratio of coefficients (denoted λ in general) is √ 3 in this example. Quantum mechanics describes this hybrid as an sp 3 wavefunction of the form N(s + √ 3pσ), where N is a normalisation constant (here 1/2) and pσ is a p orbital directed along the C-H axis to form a sigma bond. For example, in methane, the C hybrid orbital which forms each carbon– hydrogen bond consists of 25% s character and 75% p character and is thus described as sp 3 (read as s-p-three) hybridised. Hybrid orbitals are assumed to be mixtures of atomic orbitals, superimposed on each other in various proportions. In heavier atoms, such as carbon, nitrogen, and oxygen, the atomic orbitals used are the 2s and 2p orbitals, similar to excited state orbitals for hydrogen. In the case of simple hybridization, this approximation is based on atomic orbitals, similar to those obtained for the hydrogen atom, the only neutral atom for which the Schrödinger equation can be solved exactly. Orbitals are a model representation of the behavior of electrons within molecules. The amount of p character or s character, which is decided mainly by orbital hybridisation, can be used to reliably predict molecular properties such as acidity or basicity. Hybridisation theory explains bonding in alkenes and methane. For drawing reaction mechanisms sometimes a classical bonding picture is needed with two atoms sharing two electrons. Hybridisation theory is an integral part of organic chemistry, one of the most compelling examples being Baldwin's rules. It gives a simple orbital picture equivalent to Lewis structures.
This concept was developed for such simple chemical systems, but the approach was later applied more widely, and today it is considered an effective heuristic for rationalizing the structures of organic compounds. Each hybrid is denoted sp 3 to indicate its composition, and is directed along one of the four C-H bonds. Pauling supposed that in the presence of four hydrogen atoms, the s and p orbitals form four equivalent combinations which he called hybrid orbitals. The angle between any two bonds is the tetrahedral bond angle of 109☂8' (approx. In reality, methane has four C-H bonds of equivalent strength. Pauling pointed out that a carbon atom forms four bonds by using one s and three p orbitals, so that "it might be inferred" that a carbon atom would form three bonds at right angles (using p orbitals) and a fourth weaker bond using the s orbital in some arbitrary direction. 9 Localized vs canonical molecular orbitalsĬhemist Linus Pauling first developed the hybridisation theory in 1931 to explain the structure of simple molecules such as methane (CH 4) using atomic orbitals.5 Hybridisation of hypervalent molecules.