Hybridisation (HL) (HL IB Chemistry)

What is hybridisation?

• The ground state of the electrons in a carbon atom is 1s²2s²2p²
• This can be represented using a spin diagram as shown:

Ground state of carbon

Orbital spin diagram for carbon in the ground state

• This electronic structure would imply that carbon forms two covalent bonds using the unpaired 2p electrons
• Since the 2s electrons are paired there would be no reason for them to be involved in bonding
• However studies of carbon compounds show that carbon typically forms four covalent bonds that are all equal in energy
• This puzzle has been explained using the theory of bond hybridisation
• A half full p-subshell has a slightly lower energy than a partially filled one. The difference in energy between the 2s and 2p subshells is small, so an electron can fairly easily be promoted from the 2s to the 2p giving the new arrangement:

Excited state of carbon

Orbital spin diagram for carbon in the excited state

• The 2s and 2p subshells blend together and form four new hybrid orbitals (called sp³ orbitals, after the merger of an s and 3 p orbitals)
• This would give four unpaired electrons of equal energy, capable of forming four covalent bonds.

sp³ hybrid orbitals

Orbital spin diagram for carbon showing sp³ hybrid orbitals

• The theory of Quantum mechanics shows that the shape of a 1s orbital is spherical and a p orbital is dumbbell or figure-of-eight shaped
• There are three p orbitals all at right angles to each other, known as p_x, p_y and p_z

The shape of s and p orbitals

The s orbital is spherical and the three dumbbell-shaped p orbitals lie at right angles to each other

What is sp³ hybridisation?

• Four hybrid orbitals are produced when the 2s and three 2p orbitals blend together
• These hybrids have ¼ s character and ¾ p character so they have a club shape reminiscent of an enlarged p orbital
• The four sp³ hybrid orbitals space themselves out at 109.5^o forming a tetrahedron
• This is the resolution of the structure seen when carbon forms single bonds, such as would be found in methane

sp³ hybridised orbitals

4 x sp³ hybrid orbitals are formed from one s orbital and three p orbitals

• The sp³ orbitals merge with the s orbitals in hydrogen forming four equal sigma bonds
• It is not just bonding pairs of electrons that are accommodated in hybrid orbitals - lone pairs can also be present
• The domain geometry of ammonia is tetrahedral due to sp³ hybrid orbitals where three bonding pairs and one lone pair are found

What is sp² hybridisation?

• Three hybrid orbitals are produced when the 2s and two 2p orbitals blend together
• These hybrids have ⅓ s character and ⅔ p character
• The three sp² hybrid orbitals space themselves out at 120^o forming a trigonal planar geometry
• This is the resolution of the structure seen when carbon forms two single bonds and a double bond with another carbon in alkenes

sp² hybridised orbitals

3 x sp² hybrid orbitals are formed from one s orbital and three p orbitals

• In the case of carbon, the sp² orbitals merge with the s orbitals in hydrogen and the sp² of an adjacent carbon, forming three equal sigma bonds
• The double bond is created by the sideways (lateral) overlap of the unhybridised p-orbitals
• This bonding arrangement can also occur between a double bonded carbon and oxygen so is typically seen in the carbonyl group

What is sp hybridisation?

• Two hybrid orbitals are produced when the 2s and one 2p orbital blend together
• These hybrids have ½ s character and ½ p character
• The two sp hybrid orbitals space themselves out at 180^o forming linear geometry
• This is the resolution of the structure seen when carbon forms one single bonds and a triple bond with itself in alkynes

sp hybridised orbitals

2 x sp hybrid orbitals are formed from one s orbital and two p orbitals

• In the case of carbon, the sp orbital merges with the s orbital in hydrogen and the sp of an adjacent carbon, forming two equal sigma bonds
• The triple bond is created by the sideways overlap of two pairs of the unhybridised p-orbitals, set at right angles to each other