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
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How many sigma (σ) and pi ( ) bonds are present in the
structure of HCN?
A.
B.
C.
D.
σ
1
2
2
3
3
3
2
1
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Carbon forms a vast number of
covalently bonded compounds…
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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…
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What about the difference in energy
level?
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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?
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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…
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There are several combinations of s and p orbital hybrids
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Think about mixing paint…
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The red and white paint on the left represent the s and p orbitals…
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If you ‘mix’ the paint you make a new paint which has characteristic of both
colours…
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E.g. white = 1 s orbital, red = 3 p orbitals  dark pink which is closer to
the p orbital in character. (sp3 hybrid)
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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…!
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sp3 hybridisation
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When carbon forms four single bonds, it undergoes sp3
hybridisation producing 4 equal orbitals
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The four orbitals will orientate themselves at 109.5o 
tetrahedron
4 sigma bonds are formed
(sp3 overlap H)
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Visualisation of sp3
Hybridisation Animation
http://www.learnerstv.com/animation/animation
.php?ani=52&cat=chemistry
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sp2 hybridisation
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When carbon makes a double bond it undergoes sp2 hybridisation, producing 3
equal orbitals
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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!
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Visualisation of sp2
Hybridisation Animation
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sp hybridisation
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When carbon forms a triple bond, it undergoes sp hybridisation, producing two
equal orbitals.
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These orbitals orientate themselves at 180o, giving a linear shape.
Overlap of the two hybrid orbitals with other atomic orbitals forms two sigma
bonds.
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E.g. C2H2
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Each carbon has two unhybridised p orbitals that are orientated 90o to each
other. As these overlap sideways , two pi bonds form.
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Visualisation of sp
Hybridisation Animation
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Lone pairs can be involved in hybridisation too…
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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.
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Hybridisation can be used to predict
molecular shape
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Learn the relationship below for each electron domain
geometry:
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Tetrahedral  sp3 hybridised
Triangular planar  sp2 hybridised
Linear  sp hybridised
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Excellent Hybridisation Video - ChemistNATE
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