APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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Chemical Bonding: Shapes of Molecules or Molecular Geometries (Special Tutorial) Lewis structures are useful. However, we need simple model for predicting shapes of molecules. Example Consider the molecule chlorofluoromethane (CH2FCl) There are two Lewis Dot Structure for this molecule

=> or If you're good at visualizing, you can see that two molecules depicted are identical (Cl is red, F is green) and superimposible

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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For CHBrFCl, there are two distict structures with different properties. Isomers are molecules with same formula but different structures and properties.

or Notice that these can not be superimposed. Flat representations produced in Lewis structures do not give a complete picture. Molecules are not generally flat but exist in three dimensions.

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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Valence Shell Electron Pair Repulsion Theory (VSEPR) This theory states that electron pairs in valence shell about an atom repel one another and take up positions around atomic core so that they are a maximum distance apart. VESPR theory focuses not just on electron pairs but on electron groups consisting of a bonded pair, lone pair, a single unpaired electron or a double bond. Each is treated as one electron group when determining shape.

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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The positions of minimum energy for valence shell electron groups centered around the nucleus are illustrated below, with each balloon representing an electron group.

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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Electron group geometry (electron group distribution)

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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Each molecule can be represented by general formula AXnEy

� A represents central atom

� X represents atom bonded to A. Bond could be single, double or triple bond. It makes no difference to the scheme.

� E represents non-bonded electron domain (lone pair). Example � Methane, CH4 ,is AX4 molecule- meaning that 4 H atoms are

bonded to the central C atom and there are no lone pairs

� Ammonia, NH3 ,is AX3E molecule- meaning that 3 H atoms bonded to central N atom and there is a single lone pair of electrons.

� Both of these molecules have four electron pair domains.

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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The following outlines various shapes molecules would exhibit 2 valence shell electron pairs (AX2 or 2,2,0)

BeCl2

LINEAR

This molecule is linear. The Be does not fill its octet shell in this situation. To do so would put large negative charge on it and a positive charge on chlorine atoms. This is impossible because chlorine atoms are far more electronegative than beryllium atoms.

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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3 valence shell electron pairs (AX3 or 3,3,0)

BF3 Trigonal Planar shape

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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4 valence shell electron pairs

CH4

AX4

(4,4,0)tetrahedral

NH3 AX3E (4,3,1)

trigonal pyramidal

H2O

AX2E2

(4,2,2)bent or angular

HF AX1E3 or just AXE3

linear

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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Note

� In each of examples shown above, electron pair-domain geometry is tetrahedral. However, the molecular shape is not always so.

� In the case of CH4, the molecule is actually tetrahedral in shape with a perfect tetrahedral angle of 109.5º.

� Next two examples have lone pairs which occupy a larger domain volume (push more on the bonding pairs) and reduce the bond angle to less than 109.5º.

� The last case, HF, is simply a linear diatomic molecule. There is no bond angle.

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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5 valence shell electron pairs

PCl5 AX5

(5,5,0)trigonal bipyramidal

SF4

AX4E (5,4,1)

See Saw or

disphenoidal

ClF3

AX3E2

(5,3,2)T-shaped

XeF2 AX2E3 Linear

(5,2,3)

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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� For five electron group molecules, electron-domain geometry is always trigonal bipyramidal.

� The first molecule, PCl5, has ideal angles of 90 and 120 degrees for axial and equatorial bonds, respectively.

� SF4 has one lone pair and four bonding pairs. Lone pair will preferentially locate itself in an equatorial position since that position has only two other pairs of electrons within 90 degrees while an axial position would have three. The molecule has see-saw (disphenoidal) shape. Bond angles less than ideal angles of 90 and 120 degrees.

� ClF3 has two lone pairs which locate themselves in equatorial positionsThis molecule is T-shaped with bond angles of less than 90 degrees.

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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6 valence shell electron pairs

SF6 AX6

(6,6,0)Octahedral

ClF5 AX5E (6,5,1)

Square Pyramidal

XeF4 AX4E2

(6,4,2)Square Planar

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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� In above examples, the electron-domain geometry is octahedral and in the case of SF6, so is the shape.

� Molecule ClF5 has one lone pair and five bonding pairs but since all positions in the octahedral geometry are equivalent, it doesn't matter which position the lone pair takes. The molecule is square pyramidal.

� In the case of XeF4, the two lone pairs will locate themselves on opposite sides of the square planar molecule.

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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Dipole Moments and Predicting the Polarity of Molecules � Electronegativity values used to determine bonding type

(covalent, ionic or polar covalent)

� In molecule HF, electron density shifts towards the more electronegative F atom making F end of molecule more electron rich and slightly negative (red) while H end of molecule becomes electron poor and slightly positive (blue).

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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� Quantitatively, polarity of a bond (extent if charge displacement) is measured using dipole moment (μ, mu), where for polar bond between atoms A and B

A

�+ --- B

�-

μ = Q x R where Q =the fraction of charge on an electron (� of 1.6 x 10

-19 C)

that has shifted towards B, the more electronegative atom R =bond length (m) Dipole moment unit in debyes, D, where 1 D=3.336 x 10

-30C.m

e.g. Suppose that in molecule of H–Cl, Q = 2.70 x 10

-20 C,

experimentally determined by finding μ and R, which is 17% charge on an electron. Means that shift of electron cloud density towards Cl has given Cl an equivalent additional charge of 17% of charge on electron. Bond is 17% ionic.

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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� Molecules having a dipole moment are said to be polar molecules while those that don’t are called non polar molecules.

� To be able to determine if a molecule is polar, both the shape of the molecule and electronegativity values must be known.

� The dipole moment of entire molecule is made up of bond moments (the individual dipole moments in the bonds) Each bond moment is a vector quantity and the vector sum of all of the bond moments gives the dipole moment of the entire molecule.

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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Consider the molecule NH3

� Electron density shifts towards nitrogen (higher

electronegativity) and away from hydrogen

� Electron density of lone pair produces dipole

� Vector sum shows resultant molecular dipole

� This molecule is polar with N end of molecule, negative, and hydrogen end of molecule, positive.

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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Consider the substance NF3

� Electron density shifts towards fluorine (higher

electronegativity) and away from nitrogen

� Electron density of lone pair produces dipole upwards

� Vector sum shows resultant molecular dipole is down indicating that the three N – F bond moments acting down offset the contribution of the lone pair acting up

� This molecule is polar with N end of molecule, positive, and fluorine end of molecule, negative.

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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Example Predict whether each of the following molecules has a dipole moment: (a) IBr, (b) BF3, (c) CH2Cl2

APSC 131 – Chemical Bonding 4-04 –Shapes of Molecules : VSEPR theory

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Suggested Homework Read Petrucci 395 -406 Study examples 10-10 and 10-11 Problems Chapter 10: 53, 55, 59, 61, 63, 69 Draw a Lewis structure and give the shape of each of the following. Is the molecule polar or non polar?

� F2O (angular, polar) SCl4 (seesaw, polar)

� NF3 (trigonal pyramid, polar) XeCl2 (linear, non polar)

� CH4 (tetrahedral, non polar) ClI3 (T-shape, polar)

� BF3 (trigonal planar, non polar) TeCl4 (seesaw, polar)

� BeH2 (linear, non polar) H2S (angular, polar)

� XeI4 (square planar, non polar) AsCl5 (trigonal bipyram)

� PI5 (trigonal bipyramid, non polar)

� SbCl5 (trigonal bipyramidal, non polar)

� BrF5 (square base pyramid, polar)