1. SHAPES STRUCTURES GEOMETRY
VALENCE SHELL ELECTRON PAIR REPULSION
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2. Valence shell electron pair repulsion, VSEPR, is a super- simple technique for predicting the geometry of atomic centres in small molecules and molecular ions:
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3. Model deals primarily with bond angles and molecular geometries
KEY TOOLS
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4. Atomic centers with VSEPR determined geometry can be joined together into molecular bodies like Cyclohexane and glucose:
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5. Cyclohexane Twist-boat shape conformation α-Glucose Boat shape
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6. This molecular building-block logic can be extended, enabling large biomolecular structures like DNA to be modelled and understood:
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7. Generate the VSEPR geometry of an atomic centre such as:
Carbon in methane, CH4
Nitrogen in ammonia, NH3
The VSEPR Technique
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8. The overall geometry of the atomic centre is determined by the mutual repulsion between the electron pairs of the total coordination number.
The effect can be replicated by holding 2, 3, 4, 5 or 6 balloons together:
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10. Methane, CH4, has a perfect tetrahedral bond angle of 109°. 28' (109
Methane, CH4, has a perfect tetrahedral bond angle of 109°.28' (109.47°), while the H-N-H bond angle of ammonia, NH3 :, is slightly less at 107
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12. The oxygen of water has two bonded electron pairs and two nonbonded "lone" electron pairs giving a total VSEPR coordination number of 4.
But the geometry is defined by the relationship between the H-O-H atoms and water is said to be "bent" or "angular" shape of 104.5°.
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14. 2 Balloons give a linear geometry 3 Balloons give a trigonal planar geometry 4 Balloons give a tetrahedral geometry 5 Balloons give a trigonal bipyramidal geometry 6 Balloons give an octahedral geometry
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Dr Seemal Jelani

15. Valence-shell electron-pair repulsion (VSEPR) theory
Assumes that each atom in a molecule will be positioned so that there is minimal repulsion between the valence electrons of that atom.
Valence-shell electron-pair repulsion (VSEPR) theory
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Dr Seemal Jelani

16. In simple molecules in which there are no nonbonding electrons, there are five basic shapes:
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17. All diatomic molecules are linear.
Molecules with two atoms around a central atom such as BF2 are linear because positioning the two attachments at opposite ends of the central atom minimizes electron repulsion.
LINEAR - Bond angle = 180
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18. Linear shape
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19. MX2 (where M is the central atom and X is are the bonding atoms).
Generic Formula:  
MX2 (where M is the central atom and X is are the bonding atoms).
Linear shape
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Dr Seemal Jelani

20. TRIGONAL PLANAR - Bond angle = 120
Molecules with three atoms around a central atom such as BF3 are Trigonal planar because electron repulsion is minimized by positioning the three attachments toward the corners of an equilateral triangle.
TRIGONAL PLANAR - Bond angle = 120
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21. MX3 (where M is the central atom and X is are the bonding atoms).
Generic Formula:  
MX3 (where M is the central atom and X is are the bonding atoms).
Trigonal planar
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23. TETRAHEDRAL - Bond angle = 109.5
Molecules with four atoms around a central atom such as CH4 are tetrahedral because electron repulsion is minimized by position the four attachments toward the corners of a tetrahedron.
TETRAHEDRAL - Bond angle = 109.5
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24. TETRAHEDRAL - Bond angle = 109.5
Generic Formula:  
MX4 (where M is the central atom and X is are the bonding atoms).
TETRAHEDRAL - Bond angle = 109.5
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25. TETRAHEDRAL - Bond angle = 109.5
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26. Molecules with Five atoms around a central atom such as PF5 are Trigonal bipyramidal.  Three of the attachments  are positioned in a Trigonal plane with 120 bond angles.  
Trigonal bipyramidal
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28. Bond angle within the equilateral plane= 120
Bond angle between equatorial and axial plane= 90
TRIGONAL BIPYRAMIDAL
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29. The remaining two attachments are positioned perpendicular (90) to the Trigonal plane at opposite ends of the central atom.  This arrangement of atoms minimizes electron repulsion.
Trigonal bipyramidal
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30. MX5 (where M is the central atom and X is are the bonding atoms).
Generic Formula:  
MX5 (where M is the central atom and X is are the bonding atoms).
TRIGONAL BIPYRAMIDAL
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31. TRIGONAL BIPYRAMIDAL
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32. OCTAHEDRAL - Bond angle = 90
Molecules with six atoms around a central atom such as SF6 are Octahedral.  Four of the attachments are positioned in a square plane with 90o bond angles.  
OCTAHEDRAL - Bond angle = 90
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34. OCTAHEDRAL - Bond angle = 90
The remaining two attachments are positioned perpendicular 90o to the square plane at opposite ends of the central atom.  This arrangement of atoms minimizes repulsion.
OCTAHEDRAL - Bond angle = 90
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35. OCTAHEDRAL - Bond angle = 90
Generic Formula:  
MX6 (where M is the central atom and X is are the bonding atoms).
OCTAHEDRAL - Bond angle = 90
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Dr Seemal Jelani

36. OCTAHEDRAL - Bond angle = 90
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37. Three Regions of Electron Density
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40. SHAPES WITH LONE-PAIRS
There are seven shapes for molecules with one or more pairs of nonbonding electrons.
SHAPES WITH LONE-PAIRS
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41. BENT (ANGULAR or V-SHAPED)
Molecules with two atoms and one or two pairs of nonbonding electrons around a central atom such as H2O are bent
BENT (ANGULAR or V-SHAPED)
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42. BENT (ANGULAR or V-SHAPED)
If there is one pair of nonbonding electrons angle is less than 109 if there are two pairs of nonbonding electrons the angle is less 107
BENT (ANGULAR or V-SHAPED)
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43. BENT (ANGULAR or V-SHAPED)
Generic Formula:  
MXE2 (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons).
BENT (ANGULAR or V-SHAPED)
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44. BENT (ANGULAR or V-SHAPED)
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45. Molecules with three atoms and one pair of nonbonding electrons around a central atom such as NH3 are Trigonal pyramidal.  
TRIGONAL PYRAMIDAL
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46. These molecules are basically tetrahedral molecules with one of the attached atoms replaced by a pair of nonbonding electrons.  
TRIGONAL PYRAMIDAL
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47. The force of repulsion of these electrons makes the bond angle between the attached atoms less than  For example, in NH3, the H-N-H bond is
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48. Generic Formula:
 MX3E (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons).
TRIGONAL PYRAMIDAL
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Dr Seemal Jelani

49. TRIGONAL PYRAMIDAL
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50. Molecules with four atoms and one pair of nonbonding electrons around a central atom such as SF4 are seesaw- shaped.  
SEESAW-SHAPED
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51. These molecules are basically Trigonal bipyramidal molecules with one of the Equatorial- positioned atoms (in the Trigonal plane) replaced by a pair of nonbonding electrons.  
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52. SEESAW-SHAPED (DISTORTED TETRAHEDRAL)
This leaves the two axial- positioned atoms and two of the equatorial-positioned atoms in the shape of a seesaw or a teeter- totter.
SEESAW-SHAPED (DISTORTED TETRAHEDRAL)
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53. Generic Formula:  
MX4E (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons)
SEESAW-SHAPED
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54. SEESAW-SHAPED
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55. Molecules with three atoms and two pairs of nonbonding electrons around a central atom such as ClF3 are T-shaped.  
T-SHAPED
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56. Molecules with three atoms and two pairs of nonbonding electrons around a central atom such as ClF3 are T-shaped.  
T-SHAPED
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57. This leaves the two axial- positioned atoms and one of the equatorial-positioned atoms in a T-shape.
T-SHAPED
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58. Generic Formula: T-SHAPED
 MX3E2 (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons).
T-SHAPED
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59. T-SHAPED
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60. Molecules with two atoms and three pairs of nonbonding electrons around a central atom such as XeF2 are linear  
LINEAR
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61. These molecules are basically Trigonal bipyramidal molecules with all three of the equatorial- positioned atoms (in the Trigonal plane) each replaced by a pair of nonbonding electrons
Linear
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62. This leaves only the two axial- positioned atoms which are still 180 from each other on opposite ends of the central atom.
LINEAR
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63. Generic Formula:  
MX2E3 (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons)
LINEAR
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64. LINEAR
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65. Molecules with five atoms and one pair of nonbonding electrons around a central atom such as BrF5 are square pyramidal
SQUARE PYRAMIDAL
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66. These molecules are basically octahedral molecules with one of the attached atoms replaced by a pair of nonbonding electrons  
SQUARE PYRAMIDAL
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67. This leaves four atoms in a plane as a square base and one atom positioned perpendicular (90)  to this plane.
SQUARE PYRAMIDAL
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68. Generic Formula:  
MX5E (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons)
SQUARE PYRAMIDAL
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69. SQUARE PYRAMIDAL
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70. Molecules with four atoms and two pairs of nonbonding electrons around a central atom such as XeF4 are square planar.  
SQUARE PLANAR
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71. These molecules are basically octahedral molecules with two of the attached atoms opposite each other around the central atom each replaced by a pair of nonbonding electrons.  This leaves four atoms in a square plane.
SQUARE PLANAR
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72. Generic Formula:  
MX4E2 (where M is the central atom, X is are the bonding atoms, and E are nonbonding pairs of electrons)
SQUARE PLANAR
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73. SQUARE PLANAR
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