Title: Lesson 11 Hybridisation - Sigma and Pi Bonds Learning Objectives: Understand the formation of hybrid orbitals Identify the hybridisation of atoms Understand the causes and effects of hybridisation

Refresh How many sigma (σ) and pi () bonds are present in the structure of HCN? σ  A. 1 3 B. 2 3 C. 2 2 D. 3 1

Carbon forms a vast number of covalently bonded compounds… Electron configuration 1s22s22px12py1 If covalent bonds require sharing of one electrons from each atom, how can carbon make four bonds when it only has 2 unpaired electrons in the p subshell? During bonding the lowest energy or ground state electron configuration changes. Excitation occurs when an electron is promoted from the 2s to the vacant 2p orbital…  4 singly occupied orbitals! The energy needed to achieve this is compensated by the energy released when forming four bonds…

What about the difference in energy level? If Carbon can form 4 bonds from the four singly occupied orbitals in the s and the p sub shells, won’t the bonds be unequal in energy since the energy in the s and p subshells are not equal? Methane has four identical bonds – so that means that orbitals mix to form hybrid atomic orbitals which are identical to each other but different from their originals… There are several combinations of s and p orbital hybrids

Think about mixing paint… The red and white paint on the left represent the s and p orbitals… If you ‘mix’ the paint you make a new paint which has characteristic of both colours… E.g. white = 1 s orbital, red = 3 p orbitals  dark pink which is closer to the p orbital in character. (sp3 hybrid) E.g. white = 1 s orbital, red = 2 p orbitals  lighter pink which is closer to the p orbital in character. (sp2 hybrid) The hybrid paints that are produced are all equal, just like the hybrid orbitals that are produced…!

sp3 hybridisation When carbon forms four single bonds, it undergoes sp3 hybridisation producing 4 equal orbitals The four orbitals will orientate themselves at 109.5o  tetrahedron 4 sigma bonds are formed (sp3 overlap H)

Visualisation of sp3 Hybridisation Animation http://www.learnerstv.com/animation/animation.php?ani=52&cat=chemistry

sp2 hybridisation When carbon makes a double bond it undergoes sp2 hybridisation, producing 3 equal orbitals These equal orbitals orientate themselves at 120o  triangular planar shape Each hybrid orbital on each Carbon atom overlaps with the neighbouring orbital  three sigma bonds The unhybridized p orbital on each carbon overlap sideways  1 pi bond 1 sigma and 1 pi bond between the two carbons  double bond!

Visualisation of sp2 Hybridisation Animation

sp hybridisation When carbon forms a triple bond, it undergoes sp hybridisation, producing two equal orbitals. These orbitals orientate themselves at 180o, giving a linear shape. Overlap of the two hybrid orbitals with other atomic orbitals forms two sigma bonds. E.g. C2H2 Each carbon has two unhybridised p orbitals that are orientated 90o to each other. As these overlap sideways , two pi bonds form.

Visualisation of sp Hybridisation Animation

Lone pairs can be involved in hybridisation too… The examples seen all use orbitals with bonding electrons in the hybridisation process. Non bonding pairs of electrons can also take part in hybridisation. E.g. Ammonia, NH3, the non bonding pair on the N atoms resides in the sp3 orbital. The nitrogen has three unpaired p electrons, but by mixing the 2s and 2p orbitals, we can create four sp3 hybrid orbitals. Three of these can form covalent bonds with hydrogen  NH3. The fourth sp3 hybrid orbital contains the lone pair. In acidic solutions these cab co-ordinate with a hydrogen ion, forming the ammonium ion NH4+. The lone pair electrons give rise to a charge cloud that takes up space like any other orbital.

Hybridisation can be used to predict molecular shape Learn the relationship below for each electron domain geometry: Tetrahedral  sp3 hybridised Triangular planar  sp2 hybridised Linear  sp hybridised Excellent Hybridisation Video - ChemistNATE