Chapter 4- Chemical bonding Flashcards
Resonance structures
When 2 or more lewis structures can be drawn OR when there is more than one possible position for a multiple pi bond
Resonance hybrid
Is the form of the molecule that is the more stable than either one of it’s resonance structure and it’s said to have delocalised bonds.
Examples of molecules that have resonance structures (3)
+ Benzene
+ Carbonate ion
+ Ozone
Formal charge
The charge that an atom would carry if ALL the bonding electrons were split equally between the 2 atoms joined by the bond.
→ FC = (No. of valence e-) - (1/2 No. of bonding e- + No. of lonepair e-)
The sum of the formal charge = ?
The charge on the ion
The lower the number of atoms with formal charges?
The more stable the structure
→ why resonance structures that have the least number of atoms carrying a FC is preferred
How to figure out formal charge?
The nuclear charge (charge on the nucleus) - the number of electrons in the the atom (bonding, non-bonding and in inner orbital)
Bond order
The average number of bonds of the different resonance structures
What happens to the charge carried by the atoms in a resonance structure?
It is also averaged
VSEPR
Valence Shell Electron Pair Repulsion
4 electron domains that are all bonded has the shape and bond angle of? + example
+ Tetrahedral
+ 109.5
E.g = Methane
3 electron domains are bonded and 1 lone pair has the shape and bond angle of? + example
+ Triangular Pyramidal
+ 107
E.g = Ammonia NH3
2 bonding electron domains and 2 non-bonding pairs has the shape and bond angle of? + example
+ Bent/ Non-linear
+ 104
E.g = Water
When there are 2 electron domains with NO non-bonding pair what is the shape and the bond angle? + example (not serious)
+ Linear
+ 180
E.g = Beryllium Chloride, CO2
3 electron domains bonded and NO non-bonding pair of electrons?
+ Trigonal Planar
+ 120
E.g = Boron Fluoride, Methanal
3 electron domains bonded and 1 non-bonding pair of electrons?
+ Non-linear (V-shaped)
+ 117
E.g = Sulfur dioxide
Bonding in Diamond (GIANT covalent structure)
+ C atom joined by 4 other
+ Tetrahedral arrangement
+ sp3 hybridisation
+ 109.5
Bonding in Silicon dioxide (GIANT covalent structure)
+ Si bonded to 4 other Oxygens
Bonding in Graphite (GIANT covalent structure)
+ 2 covalent network in 2 dimensions
+ only has dispersion forces betw/ the 2 sheets of C atoms = weak forces = can easily slide over e.o = lubricant
+ C bonded to 3 others forming a hexagon
+ 120 bond angle + sp2 hybridization
+ all atoms contribute one electron to the delocalised pi-bond
+ Delocalised electrons allow it to conduct electricity in 2D
C60/ Buckminsterfullerene/ Fullerene (GIANT covalent structure)
+ Spherical mol.
+ made of 5 and 6 membered C ring
+ C bonded by sigma-bond to 3 other carbons
+ sp2 hybridization
+ LITTLE delocalization of unpaired electrons = cuz surface of sphere = not planar → electrons can’t easily flow from one C60 to the next
How is the electrical conductivity of Fullerene?
Better than diamond but worse than graphite
What species does C60 behave as?
+ electron deficient mol. that readily accepts electrons from reducing agents to form anions with a variety of charges
What type of reactions do fullerene undergoe?
Addition reactions
Unlike diamond = fullerenes are molecular so what properties do they have that differ from graphite and diamond?
+ Dissolve in non-polar solvent
+ low m.p
Graphene
2D material = single sheet of graphite structure
Delocalised bonds
When sideways interactions of p-orbitals to form a pi-bond can involve more than 2 atoms → p-orbitals can extend over more than 2 nuclei so electrons are hared by a number of atoms
Why does delocalization make the molecule more stable?
It gives the species a lower potential energy than it would if it was composed of normal double and single bonds
How is Stratospheric Ozone formed?
+ Photodissociation of O2 molecules caused by UV light from sun to produce O ATOMS.
Mechanism for Photodissociation of O2
O2 (g) → (UV light) 2O• (g)
Oxygen atoms are v. reactive and combine with O2 mol. to form Ozone:
O2 (g) + O• (g) → O3 (g)
UV light also causes photodissociation of Ozone itself. State the mechanism.
O3 (g) → (UV light) O2 + O• (g)
O3 (g) + O• → 2O2 (g)
This results in an equilibrium concentration of Ozone in the Stratosphere
Is the UV absorbed by Ozone of longer or shorter wavelength than absorbed by Oxygen molecule?
Longer wavelength and less energetic → still v. damaging and increases risk of skin cancer in human
CFCs (Chlorofluorocarbons) undergo photo dissociation to yield chlorine atoms. What do chlorine atoms do?
Catalyse the decomposition of ozone
NO also catalyses decomp of ozone. How is NO produced?
Combination of Oxygen and Nitrogen in air at high temp of jet engines
5 regions of high electron density gives what shape according to VSEPR and what bond angle?
Trigonal bipyramidal
90° and 120°
5 e- domains w/ 1 non-bonding pair = shape and bond angles
saw horse
90° and 117°
5 e- domains w/ 2 non-bonding pair = shape and bond angles
T-shaped
90°
5 e- domains w/ 3 non-bonding pair = shape and bond angles
Linear
180°
6 regions of high electron density gives what shape according to VSEPR and what bond angle?
Octahedral
90°
6 e- domains w/ 1 non-bonding pair = shape and bond angles
Square pyramid
88°
6 e- domains w/ 2 non-bonding pair = shape and bond angles
Square planar
90°
Hybridization
The combining of atomic orbitals to form new orbitals of equal energy
