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2.2.16 hybridisation (HL)

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chem modelsofbondingandstructure

Structure 2.2.16 - hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals for bonding
concept

carbon forms 4 covalent bonds, despite its two singly occupied orbitals available for bonding:

the fact that carbon forms 4 covalent bonds indicates that this ground-state electron configuration changes during bonding.

excitation is when an electron is promoted within the atom from the 2s orbital to the vacant 2p orbital

the atom now has 4 singly occupied orbitals available for bonding. hund’s rule states that all electrons in singly occupied orbitals in the same sub-shell have parallel spin, so:

the amount of energy to excite the electron is compensated by the energy released on forming four bonds

the excited carbon has 1 singly occupied s orbital and 3 singly occupied p orbitals, which are at different energy levels. this means that if the orbitals were to participate in covalently bonding, the bonds would be unequal. however, has 4 identical carbon-hydrogen bonds, suggesting that the orbitals have been made equal.

in essence, unequal atomic orbitals within an atom mix to form new hybrid atomic orbitals which are the same as each other, but different from the original.

hybrid orbitals have different energies, shapes, and orientation in space and are able to form stronger bonds by allowing for greater overlap.

  1. orbitals are formed when 1 s orbital and 3 p orbitals are hybridised, producing 4 equal orbitals which are more p than s in character
  2. orbitals are formed when 1 s orbital and 2 p orbitals are hybridised, producing 3 equal orbitals
  3. orbitals are produced when 1 s orbital and 1 p orbital are hybridised, producing 2 equal orbitals

the different types of hybrid orbitals have different orientations.

hybridisation

when carbon forms four single bonds, it undergoes hybridisation. the 4 equal orbitals orientate themselves at 109.5, forming a tetrahedron. each hybrid orbital overlaps with the atomic orbital of another atom forming four sigma bonds

eg

hybridisation

when carbon forms a double bond, it undergoes hybridisation. the 3 equal hybrid orbitals orientate themselves at 120, forming a triangular planar shape. each hybrid orbital overlaps with a neighbouring atomic orbital, forming three sigma bonds.

eg

as the two carbon atoms approach each other, the p orbitals that have retained their dumbbell shape overlap sideways, forming a pi bond. thus, the double bond between carbon atoms consists of 1 sigma and 1 pi bond

hybridisation

when carbon forms a triple bond, it undergoes hybridisation. overlap of the two hybrid orbitals with other atomic orbitals forms two sigma bonds

eg

each carbon has two unhybridised p orbitals that are oriented at 90 to each other. as these combine laterally, two pi bonds form, representing four lobes of electron density. these coalesce into a cylinder of negative charge around the atom, making the molecule susceptible to attack by electrophilic reactants

lone pairs can also be involved in hybridisation. eg in , the non-bonding pair on the N atom resides in an hybrid orbital

hybridisation and molecular geometry

hybridisation can also be used to predict molecular geometry
sp3d and sp3d2

  • tetrahedral arrangement hybridised
  • triangular planar arrangement hybridised
  • linear arrangement hybridised


the lone pair resides in one of the sp3 hybrid orbitals

hybridisation in benzene

each of the six carbon atoms is hybridised, and forms three bonds with angles of 120, leaving one unhybridised electron on each atom with the dumbbell shape perpendicular to the plane of the ring.

instead of the p orbitals pairing to form discrete alternating bonds, the p orbitals overlap in both directions, spreading themselves out to evenly be shared by all six carbon atoms. this forms a delocalised electron cloud where electron density is concentrated in two donut-shaped rings above and below the plane of the ring

challenge questions
  1. molecules with expanded octets require more than 4 hybrid orbitals around the central atom. this involves the use of d orbitals in addition to s and p orbitals. what type of hybrid orbitals would be formed by the phosphorus atom in , and the sulfur atom in ?

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