Saturday 11 December 2010

Chemistry : Shape of molecules

Valence Shell Electron Pair Repulsion (VSEPR) Theory:
1)       e- pairs in central atom tends to get as far as possible to minimize electrostatic repulsion between e- pairs in valence shell
2)       Both bonded and lone pair is involved.
The order of repulsion: LP-LP > LP-BP > BP-BP (LP: lone pair, BP: bond pair)
Wedge-shaped bonding lines: solid wedge implies the atom is in front of the plane, while a dotted one implies the atom is behind the plane.
1)       Linear (2BP), e.g. CO2 (O=C=O), BeCl2, ethyne (H-C≡C-H)
2)       Trigonal planar (3BP), e.g. BF3
3)       V-shaped (2BP-1LP), one of the BP replaced by LP, bond angle slightly smaller than 3BP. e.g., NO2-, SO2, O3
4)       Tetrahedral (4BP), e.g. CH4, CH2X2, PO43-, SO42-, ClO4-, NH4+ (one of them is dative covalent bond)
5)       Trigonal pyramidal (3BP-1LP), one of the BP replaced by LP, bond angle slightly smaller than 4BP. e.g., NH3, PCl3, H3O+
6)       V-shaped (2BP-2LP), two of the BP replaced by LP, the plane of BP is perpendicular to the plane of LP. e.g., H2O. OF2, NH2-
7)       Trigonal bipyrmidal (5BP), three of them on the triangular plane and the remaining two facing in opposite sides, perpendicular to the plane. e.g., PCl5.
8)       Linear (2BP-3LP): the LP are on the equatorial triangular plane. This is a special case for XeF2.
9)       Octahedral (6BP), e.g., SF6, complexes
10)   Square pyramidal (5BP-1LP), e.g., ClF5, BrF5
11)   Square planar (4BP-2LP), e.g., XeF4, [Cu(NH3)4]2+
Hybridization theory: sp3 forms a tetrahedral shape around the central atom and the sp2 forms a trigonal planar shape.
Some atoms can break the octet rule. For example, S under O.N. of +6, forms 6 bond like SO42-, P can form 5 bonds like PCl5, and Cr under O.N. of +6 like [CrO4]2-. Xe forms bond with halides without eliminating its electron on the outermost shell, and it can form up to 4 bonds. e.g., XeF2 and XeF4.
Carbon structure
1)       Diamond – each C connected to four C
2)       Graphite – hexagonal planar layered structure, each layer is connected by weak Wan der Waals’ force
3)       SiC (Silicon carbide) – each Si is attached to four C and each C is attached to four Si
4)       SiO2 (quartz) – each Si is connected to four O and each O is connected to two Si
5)       Fullerene group: macromolecules formed by C.
6)       Buckminsterfullerene (C60) has some special properties:
-Semi-conductor (delocalized π-bond gives delocalized e-), “Paper battery”
-Medical capsule (capturing compounds inside it, e.g., He@C60
-Dissolves in toluene to gives pink sol.
-Addition reaction is possible
-K3C60 can be a superconductor

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