Hybridization (College Board AP Chemistry)

• Covalent bonds are formed when atomic orbitals overlap that together contain a pair of electrons
• Hybridization occurs when atomic orbitals (s and p) are combined to form hybrid orbitals
• Hybrid orbitals are still localized in the individual atoms, but their shapes and energies are different
• The hybridization of valence orbitals is important to explain the molecular geometry of different carbon compounds

sp³ hybridization

• Carbon ground state electron configuration is defined by 1s²2s²2p²
• It is represented by the following box diagram

Box Diagram in Ground State

Orbital box diagram for carbon in the ground state

• By looking at the box diagram, there are two unpaired electrons which implies that only two covalent bonds can be formed
• The 2s electrons are paired so there is no reason that suggest that they will be involved in bonding
• The study of carbon compounds shows that the carbon atom tend to form four covalent bonds
• Since, 2s and 2p subshells are close in energy, one electron of the 2s subshell can be promoted to the 2p subshell giving the following new arrangement

Box Diagram in Excited State

Orbital box diagram  for carbon in when a 2s electron is promoted to the 2p subshell

• Therefore, the s orbital from the 2s subshell and the three orbitals from the 2p subshell will merge together to form four hybrid orbitals
  • It is important to remember the shapes of the orbital involved in the hybridization
  • An s orbital has an spherical shape
  • A p orbital is dumbbell or figure-of-eight shaped
    • There are three p orbitals depending on their orientation in the xyz plane
    • They are perpendicular between themselves

s and p Orbitals

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

• This new hybrid orbitals will be of equal energy and this explain how four covalent bonds are formed
• They are named sp³ hybrid orbitals, because it is the combination of one s orbital and three p orbitals

sp³ hybrid orbitals

Orbital box diagram for carbon showing sp3 hybrid orbitals

• These new hybrid orbitals have 1⁄4 s character and ¾ p character. Therefore, their shape would have a club shape, like an enlarged p orbital
• The four sp³ hybrid orbitals can be arranged in space within a tetrahedron. Therefore, this is the origin of the 109.5° angle of the molecular geometry
• The carbon atom in methane shows sp³
• The sp³ orbitals in the carbon atom, overlaps with the s orbital of hydrogen forming four equal covalent bonds

Formation of sp³ Hybridized Orbitals

 

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

• It is not just bonding pairs of electrons that can be accommodate in the hybrid orbitals, lone pairs can also be present
  • One of the clearest examples is the ammonia molecule. The nitrogen atom is sp³ hybridized, but three of the hybrid orbitals are used to arrange bonding pairs and one of them for a lone pair

sp² hybridization

• sp² hybrid orbitals are formed when one s orbital and two p orbitals are combined together
• One of the p orbitals of the 2p subshell remains unchanged after hybridization
• The sp² orbitals have the same energy, while the p orbital from the 2p subshell is higher in energy than the hybridized orbitals
• These new hybrid orbitals have ⅓ s character and ⅔ p character
• The three sp² hybrid orbitals can be arranged in space within a trigonal planar geometry. Therefore, this is the origin of the 120° angle of the molecular geometry
• The carbon atom in alkenes shows sp² hybridization when it forms two single bonds and a double bond with another carbon
• The sp² orbitals in the carbon atom overlap with the s orbital of two hydrogen atoms and one hybrid sp² orbital of the adjacent carbon forming three equal sigma bonds
• The double bond is created when the unhybridized p-orbitals from adjacent carbons sideways overlap
  • Sideways overlap is a lateral interaction between the p-orbitals of two adjacent atoms

Formation of sp² Hybridized Orbitals

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

sp hybridization

• sp hybrid orbitals are formed when one s orbital and one p orbital are combined together
• Two of the p orbitals of the 2p subshell remains unchanged after hybridization
• The sp orbitals have the same energy, while the two p orbitals from the 2p subshell is higher in energy than the hybridized orbitals
• These new hybrid orbitals have ½ s character and ½ p character
• The two sp hybrid orbitals can be arranged in space forming a linear geometry. Therefore, this is the origin of the 180° angle of the molecular geometry
• The carbon atom in alkynes shows sp hybridization when it forms one single bond and a triple bond with another carbon
• The sp orbitals in the carbon atom overlap with the s orbital of the hydrogen atom and one hybrid sp² orbital of the adjacent carbon forming two equal sigma bonds
• The triple bond is created when the two unhybridized p-orbitals from adjacent carbons sideways overlap
  • Sideways overlap is a lateral interaction between two adjacent atoms

Formation of sp Hybridized Orbitals

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