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The Blending of Orbitals
Hybridization The Blending of Orbitals 11 DP Chemistry (AHL)
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Lets look at a molecule of methane, CH4.
We have studied electron configuration notation and the sharing of electrons in the formation of covalent bonds. Lets look at a molecule of methane, CH4. Methane is a simple natural gas. Its molecule has a carbon atom at the center with four hydrogen atoms covalently bonded around it.
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Carbon ground state configuration
What is the expected orbital notation of carbon in its ground state? Can you see a problem with this? (Hint: How many unpaired electrons does this carbon atom have available for bonding?)
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Carbon’s Bonding Problem
You should conclude that carbon only has TWO electrons available for bonding. That is not not enough! How does carbon overcome this problem so that it may form four bonds?
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Carbon’s Empty Orbital
The first thought that chemists had was that carbon promotes one of its 2s electrons… …to the empty 2p orbital.
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However, they quickly recognized a problem with such
an arrangement… Three of the carbon-hydrogen bonds would involve an electron pair in which the carbon electron was a 2p, matched with the lone 1s electron from a hydrogen atom. A Problem Arises
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This would mean that three of the bonds in a methane
molecule would be identical, because they would involve electron pairs of equal energy. But what about the fourth bond…? Unequal bond energy
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The fourth bond is between a 2s electron from the
carbon and the lone 1s hydrogen electron. Such a bond would have slightly less energy than the other bonds in a methane molecule. Unequal bond energy #2
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This bond would be slightly different in character than
the other three bonds in methane. This difference would be measurable to a chemist by determining the bond length and bond energy. But is this what they observe? Unequal bond energy #3
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The simple answer is, “No”.
Measurements show that all four bonds in methane are equal. Thus, we need a new explanation for the bonding in methane. Chemists have proposed an explanation – they call it Hybridization. Hybridization is the combining of two or more orbitals of nearly equal energy within the same atom into orbitals of equal energy. Enter Hybridization
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In the case of methane, they call the hybridization
sp3, meaning that an s orbital is combined with three p orbitals to create four equal hybrid orbitals. These new orbitals have slightly MORE energy than the 2s orbital… … and slightly LESS energy than the 2p orbitals. sp3 Hybrid Orbitals
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sp3 Hybrid Orbitals Here is another way to look at the sp3 hybridization and energy profile…
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sp Hybrid Orbitals While sp3 is the hybridization observed in methane,
there are other types of hybridization that atoms undergo. These include sp hybridization, in which one s orbital combines with a single p orbital. Notice that this produces two hybrid orbitals, while leaving two normal p orbitals
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sp2 Hybrid Orbitals Another hybrid is the sp2, which combines two orbitals from a p sublevel with one orbital from an s sublevel. Notice that one p orbital remains unchanged.
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Methane building blocks
VSEPR
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3 sp Methane: Carbon 1s 2s 2px 2py 2pz sp3 sp3 sp3 sp3 Promote
Hybridize sp 3 x 109.5o z Methane: Carbon
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Formaldehyde CH2O H C O 120o H C O O H C H VSEPR
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2 sp Trigonal Planar 2s + 2px + 2pz Formaldehyde: Carbon 1s 2s 2px 2py
sp2 sp2 sp2 Promoted Hybridized sp 2 120o Trigonal Planar 120o 2s + 2px + 2pz Formaldehyde: Carbon
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2 sp Trigonal Planar 2s + 2px + 2pz Formaldehyde: Oxygen 1s 2s 2px 2py
sp2 sp2 sp2 Hybridized Lone Pairs sp 2 120o Trigonal Planar 120o 2s + 2px + 2pz Formaldehyde: Oxygen
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Formaldehyde 2 Lone Pairs bond Sigma bond
bonds are formed by the sideways overlapping of p orbitals (weaker) bonds are formed by the axial (end to end) overlapping of s or p orbitals (stronger)
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Hydrogen Cyanide HCN H C N H C N H C N
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H C N sp Linear 2s + 2px Hydrogen Cyanide: Carbon 1s 2s 2px 2py 2pz
sp sp Promoted Hybridized H C N sp Linear 2s + 2px Hydrogen Cyanide: Carbon
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H C N sp Linear 2s + 2px Hydrogen Cyanide: Nitrogen 1s 2s 2px 2py 2pz
sp sp Hybridized H C N sp Linear 2s + 2px Hydrogen Cyanide: Nitrogen
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Hydrogen Cyanide HCN C N Sigma bond H C N
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Phosphorus Pentachloride
PCl5 P Cl Cl Cl Cl Cl P Cl Cl P Cl Cl Cl Cl VSEPR
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3 sp d Trigonal Bipyrimidal Phosphorus Pentachloride: Phosphorus 2 3s
sp3d sp3d sp3d sp3d sp3d Neon 2 3s 3px 3py 3pz dxz dyz dxy dx2-y2 dz2 Hybridized 90o Promoted sp d 3 120o Trigonal Bipyrimidal 120o Phosphorus Pentachloride: Phosphorus
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Cl Cl Cl P Cl Cl
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