CHEMISTRY TASKDETERMINING THE MOLECULAR

SHAPE(USING VSEPR AND HYBRIDIZATION

THEORY)

Created by:

Name : Restu Waras Toto

Class : XI IPA 2

Number : 23

SMAN 1 BANTULJl. Wachid Hasyim Sumuran Palbapang Bantul

ACADEMIC YEAR

2011/2012MOLECULAR 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 1800 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.

Basic Shapes of Molecules. Linear

DETERMINING THE MOLECULAR SHAPE 1

Figure (a) Linear molecule with 2 and 3 atoms. The bonding forms a 1800 angle.

Planar triangular

Figure (b) Planar triangular molecule with 4 atoms. The bonding forms 1200 angle.

Tetrahedral

Figure (c) Tetrahedral molecule with 5 atoms. The bonding forms 109.50 angle.

Trigonial bipyramidal

Figure (d) Trigonal bipyramidal molecule with 6 atoms. The 3 equatorial bonds form an equatorial angle of 1200 and the 2 axial bonds form an axial angle of 900

with the equator.

DETERMINING THE MOLECULAR SHAPE 2

Octahedral

Figure (e) Octahedral molecule with 7 atoms. The bonding form a 900 angle.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.

1. USING ELECTRON DOMAIN THEORYTABLE 1 Steps to predict molecular shapes.

No Steps H2O molecules CO2 molecules1 Write down the Lewis structure

2 Determine the number of electron domains around the central atom. Note, double bond is considered as one domain. Distinguish bonding electron domain (BED) and non-bonding electron domain (NBED).

4 electron domains (2 BED and 2 NBED)

2 electron domains (2 BED and 0 NBED)

3 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

DETERMINING THE MOLECULAR SHAPE 3

4 Place the central atom at the center of the geometry. Draw lines connecting the central atom to the endpoints of the geometry.

5 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

6 Place the bonding electron domains and write down the respective atoms.

7 According to the Electron Domain Theory, the order of repulsion between bonding and non-bonding electron domains is as follows:NBED-NBED>NBED-BED>BED-BEDThis 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:

DETERMINING THE MOLECULAR SHAPE 4

8 Draw the molecular shape without the line connecting the non-bonding electron domains to the central atom.

9 The molecular shape can be determined using the following formula

AXnEm

Where:A = central atomX = atoms bonded to the central atomE = non-bonding electron domain (BED)n = number of BEDm = number of NBED

From the formula obtained, use table 1 to determine the molecular shape.

∴The shape of H20 molecule is planar V or non-linear

∴ The shape of CO2

molecule is linear

10 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.

Table 2 The following table contains a summary on how to use the AXnEm formula

to predict molecular shapes.

Electron domains around central atom

Number of BED

(n)

Number of NBED

(m)AXnEm

Molecular shape

Bond angle Structure Example

2 2 0 AX2 Linear 1800 BeCl2, BeF2, CO2

3

3 0 AX3

Trigonal planar

(Triangular planar)

1200 BCl3, BF3, SO3

2 1 AX2E Bent <1200 SO2, SnI2, GeF2, NO2

−¿¿

DETERMINING THE MOLECULAR SHAPE 5

4

4 0 AX4 Tetrahedral 109.50 CH4, CCl4

3 1 AX3ETrigonal

pyramidal<109.50 NH3, NF3

2 2 AX2E2

Bent/V-shaped or non-linear

<109.50 H20

5

5 0 AX5Trigonal

bipyramidal

1200

(Equ-Equ)1800

(Ax-Ax)900 (Ax-

Equ)

PCl5, PF6

4 1 AX4ESeesaw

(Distorted tetrahedral)

<1200

(Equ-Equ)

900 (Ax-Equ)

SF4

3 2 AX3E2 T-shaped

1800

(Ax-Ax)900 (Ax-

Equ)ClF3

2 3 AX2E3 Linear1800

(Ax)XeF2, I 3

−¿¿

6

6 0 AX6 Octahedral 900 SF6

5 1 AX5E Square pyramidal

900 (Ax-Equ)

BrF5, XeOF4

DETERMINING THE MOLECULAR SHAPE 6

4 2 AX4E2Square planar

900 XeF4

3 3 AX3E3 - - - -2 4 AX2E4 - - - -

**Note: - There are only 3 shapes ofmolecule with 6 electron domains around the central atom although

theoretically there can be more,- Ax = Axial, Equ = Equatorial.

2. USING HYBRIDIZATION THEORY

The shape of molecules can be predicted using domain theory. But, this theory could not explain how the molecules get their shape. As an example, domain theory states that methane (CH4) is tetrahedron-shaped molecule with 4 equivalent C- H bonding. The fact of experiment appropiate with this prediction.

At basic stage, carbon (Z = 6) has electron configuration below :

6C : 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.

6C : 1s2 2s2 2p2

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 CH4 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

DETERMINING THE MOLECULAR SHAPE 7

them with one atom hydrogen form CH4. 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

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