Makalah Bentuk2 Molekul
Makalah Bentuk2 Molekul
Makalah Bentuk2 Molekul
SMAN 1 BANTUL
Jl. Wachid Hasyim Sumuran Palbapang Bantul
ACADEMIC YEAR
2011/2012
MOLECULAR SHAPE
Molecules that consists of only 2 atoms are linear in shape. However, for molecules with 3 atoms or more, the shape varies. The basic for understanding molecular shape is the Lewis structure. It provides information on the bonding and non-bonding electron pairs around central atom. These pairs of valence electrons undergo electrostatic force because of their charge. With this understanding, in 1970 R.G. Gillespie proposed VSEPR (Valence Shell Electron Pair Repulsion) . Theory that states: Pairs of electron that are negatively charged will keep their distance as far apart as possible, thus minimizing the repulsion between them
Figure 1. The position both (valence) electron pairs in 180 0 gives minimum repulsion. The VSEPR theory was further developed for molecules with multiple bonds where these bonds were treated as electron pairs that are contained in an electron domain. Later, the concept of electron domain was also applied to non -bonding pairs around the central atom. Consequently, electron domain can be distinguished into: Bonding Electron Domain (BED) Contains pairs of bonding electron Non-Bonding Electron Domain (NBED) Which contains pairs of non -bonding electrons.
Figure (a) Linear molecule with 2 and 3 atoms. The bonding forms a 180 0 angle.
Planar triangular
Figure (b) Planar triangular molecule with 4 atoms. Thebonding forms 120 0 angle.
Tetrahedral
Figure (c) Tetrahedral molecule with 5 atoms. The bonding forms 109.5 0 angle.
Trigonial bipyramidal
Figure (d) Trigonal bipyramidal molecule with 6 atoms. The 3 equatorial bonds form an equatorial angle of 120 0 and the 2 axial bonds form an axial angle of 90 0 with the equator.
Octahedral
Figure (e) Octahedral molecule with 7 atoms. The bonding form a 90 0 angle.
DETERMINING THE MOLECULAR SHAPE
Later on, the definition of electron domain was also extended for single bonds. In addition, the number of electrons in a domain was not limited anymore, meaning that the more electrons there are, the greater the repulsion will be. Eventually, the extension of the VSEPR theory bred another name of the theory, called the Electron Domain Theory. There is also another way to find a molecular shapes. The second way is Hybridization Theory. The Hybridization Theory will be explained after Electron Domain Theory.
Determine thenumber of electron domains around the central atom. Note, double bond 4 electron domains (2 is considered as one domain. BED and 2 NBED) Distinguish bonding electron domain (BED) and non-bonding electron domain (NBED). Determine the basic geometry based on the number of electron domains. 2 electron domains = linear 3 electron domains = planar triangular 4 electron domains = tetrahedral 5 electron domains = trigonal bipyramid 6 electron domains = octahedral Place the central atom at the center of the geometry. Draw lines connecting the central atom to the endpoints of the geometry.
Place the non-bonding electron domains at the endpoints of the geometry. (NBED requires more space compared to BED) Notes: 1. For trigonal bipyramid geometry, the NBED will occupy an equatorial position. 2. For tetrahedral geometry, the first NBED may occupy any position but the second must occupy the opposing position Place the bonding electron domains and write down the respective atoms.
According to the Electron Domain Theory, the order of repulsion between bonding and non-bonding electron domains is as follows: NBED-NBED>NBED-BED>BEDBED This means that NBEDs tend to push the BEDs closer thus reducing the bond angle. Note the order of repulsion strength for BED is based on theumber of its atoms as follows:
Draw the molecular shape without the line connecting the non-bonding electron domains to the central atom.
The molecular shape can be determined using the following formula AXnEm Where: A = central atom X = atoms bonded to the central The shape of H 20 atom molecule is planar V or E = non-bonding electron non-linear domain (BED) n = number of BED m = number of NBED From the formula obtained, use table 1 to determine the molecular shape. The molecular shapes can also be drawn like those given in Table 1. Note that the drawings of the multiple bonds and single bonds are identical.
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Table 2 The following table contains a summary on how to use the AXnEmformula to predict molecular shapes. Electron domains around central atom 2
AXnEm
Molecular shape
Bond angle
Structure
Example
AX2
180 0
3 3 2
AX3
120 0
AX2E
<120 0
4 4 3
AX4
Tetrahedral
109.5 0
CH4, CCl 4
AX3E
Trigonal pyramidal
<109.5 0
NH3, NF3
AX2E2
Bent/Vshaped or non-linear
<109.5 0 120 0 (EquEqu) 180 0 (Ax-Ax) 900(AxEqu) <120 0 (EquEqu) 900(AxEqu) 180 0 (Ax-Ax) 900(AxEqu) 180 0 (Ax)
H20
AX5
Trigonal bipyramidal
PCl 5, PF6
4 5
AX4E
SF4
AX3E2
T-shaped
ClF3
AX2E3
Linear
XeF2,
AX6
Octahedral
900
SF6
AX5E
Square pyramidal
900(AxEqu)
BrF5, XeOF4
4 3 2
**Note:
2 3 4
Square planar -
900 -
XeF4
- There are only 3 shapes ofmolecule with 6 electron domains around the central atom although theoretically there can be more, - Ax = Axial, Equ = Equatorial.
: 1s2
2s2
2p2
With those configuration above, carbon only has an ability to make 2 covalent bondings. Because carbon can form 4 covalent bondings, we can assume that 1 electron from 2s orbital is promoted to 2p orbital so that the carbon has 4 single electrons in orbital 2s and 2p. : 1s2 2s2 2p2 6C
Hybridized, become:
6C
: 1s2
2s2
2p2
Though, the four electrons are not equivalent, one electron at 2s orbital and three electrons at 2p orbital, so it can not explain why carbon in CH 4 can form four equivalent covalent bondings. To explain it, we can say that when carbon forms covalent bonding with hydrogen atoms, 2s orbital and the three 2p orbital experience hybridization proccess forming equivalent orbitals. The hybridized orbitals are denoted by sp3 which states the origin place, those are 1 from orbital s and 3 from orbital p. Hybridization not also as regard as energy level, but also the orbital shapes. Now, carbon atom with 4 hybrid orbital sp3 can form 4 covalent bonding, each of them with one atom hydrogen form CH 4. So, hybridization is the fusion of orbitals from different energy level to be orbitals with equal energy level. The number of hybrid orbitals are the same with the number of orbitals involved in the hybridization proccess. The number of orbitals are the same with the number of electron domain in the molecule.
The type of hybridization Origin orbitals Hybrid orbitals Shape of orbital Picture
s,p
sp
Linear
s,p,p
sp2
Trigonal planar
s,p,p,p
sp3
Tetrahedral
s,p,p,p,d
sp3d
Trigonal bipyramidal
s,p,p,p,d,d
sp3d2
Octahedral