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The simplest molecules to consider are those formed from two elements A and B. The molecules of interest have the general molecular formula ABx where x.

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Presentation on theme: "The simplest molecules to consider are those formed from two elements A and B. The molecules of interest have the general molecular formula ABx where x."— Presentation transcript:

1 The simplest molecules to consider are those formed from two elements A and B.

2 The simplest molecules to consider are those formed from two elements A and B. The molecules of interest have the general molecular formula ABx where x =2, 3, … and A will be the central atom.

3 The simplest molecules to consider are those formed from two elements A and B. The molecules of interest have the general molecular formula ABx where x =2, 3, … and A will be the central atom. If x = 1, the molecule is linear by definition.

4 The simplest molecules to consider are those formed from two elements A and B. The molecules of interest have the general molecular formula ABx where x =2, 3, … and A will be the central atom. If x = 1, the molecule is linear by definition. Some regular geometric shapes that can be generated by molecules of the type ABx where x varies from 2 to 6 are:

5 x value shape name

6 x value shape name linear

7 x value shape name linear bent (or nonlinear)

8 x value shape name linear bent (or nonlinear) trigonal planar

9 x value shape name linear bent (or nonlinear) trigonal planar square

10 x value shape name tetrahedral

11 x value shape name tetrahedral No bonds in place yet.

12 x value shape name tetrahedral trigonal bipyramidal No bonds in place yet.

13 x value shape name tetrahedral trigonal bipyramidal

14 x value shape name octahedral

15 x value shape name octahedral No bonds in place yet.

16 x value shape name octahedral

17 Predicting Shapes Some examples

18 Predicting Shapes Some examples
Key to the first examples:

19 Predicting Shapes Some examples
Key to the first examples: There are NO lone pairs on the central atom.

20 1. Beryllium hydride, BeH2

21 1. Beryllium hydride, BeH2 In the gas phase the predicted Lewis structure is H : Be : H

22 1. Beryllium hydride, BeH2 In the gas phase the predicted Lewis structure is H : Be : H The only way for the two bond pairs to be farthest apart from each other is for them to lie in a straight line, so that the HBeH angle is 180o. Thus the molecule is linear.

23 2. Boron trifluoride, BF3

24 The lone pairs on the surrounding atoms do not impact the shape of the molecule, so focus on the bonding electrons:

25 The lone pairs on the surrounding atoms do not impact the shape of the molecule, so focus on the bonding electrons:

26 The lone pairs on the surrounding atoms do not impact the shape of the molecule, so focus on the bonding electrons: The most stable arrangement is one in which the three B F bonds point to the corners of an equilateral triangle.

27 The lone pairs on the surrounding atoms do not impact the shape of the molecule, so focus on the bonding electrons: The most stable arrangement is one in which the three B F bonds point to the corners of an equilateral triangle. Thus all three FBF angles are equal to 120o and all four atoms lie in the same plane.

28 The lone pairs on the surrounding atoms do not impact the shape of the molecule, so focus on the bonding electrons: The most stable arrangement is one in which the three B F bonds point to the corners of an equilateral triangle. Thus all three FBF angles are equal to 120o and all four atoms lie in the same plane. BF3 is therefore trigonal (triangular) planar.

29 3. Methane, CH4 The Lewis structure is:

30 3. Methane, CH4 The Lewis structure is: or

31 3. Methane, CH4 The Lewis structure is: or At first glance, it might appear that the bonding electron pairs will be farthest apart when the HCH bond angles are 90o.

32 3. Methane, CH4 The Lewis structure is: or At first glance, it might appear that the bonding electron pairs will be farthest apart when the HCH bond angles are 90o. If this were the case the molecule would be square planar.

33 However, the molecule adopts a non-planar geometry, in this case, it is tetrahedral.

34 However, the molecule adopts a non-planar geometry, in this case, it is tetrahedral. In a tetrahedral geometry, the HCH bond angles are 109o 28’, which represents a significant improvement over the square planar geometry.

35 However, the molecule adopts a non-planar geometry, in this case, it is tetrahedral. In a tetrahedral geometry, the HCH bond angles are 109o 28’, which represents a significant improvement over the square planar geometry. The tetrahedral geometry gets the bond pairs as far apart as possible, and minimizes the electrostatic repulsion between the bond pair electrons.

36 However, the molecule adopts a non-planar geometry, in this case, it is tetrahedral. In a tetrahedral geometry, the HCH bond angles are 109o 28’, which represents a significant improvement over the square planar geometry. The tetrahedral geometry gets the bond pairs as far apart as possible, and minimizes the electrostatic repulsion between the bond pair electrons. Hence the shape of CH4 is tetrahedral.

37 tetrahedral

38 4. Phosphorous pentachloride, PCl5

39 4. Phosphorous pentachloride, PCl5
The Lewis structure is

40 4. Phosphorous pentachloride, PCl5
The Lewis structure is Just with the bonds shown we have

41 The only way to minimize the electrostatic repulsion among the bond pairs is to arrange the P Cl bonds so that the PCl5 molecule becomes trigonal bipyramidal.

42 The only way to minimize the electrostatic repulsion among the bond pairs is to arrange the P Cl bonds so that the PCl5 molecule becomes trigonal bipyramidal. trigonal bipyramidal

43 The only way to minimize the electrostatic repulsion among the bond pairs is to arrange the P Cl bonds so that the PCl5 molecule becomes trigonal bipyramidal. trigonal bipyramidal No bonds in place yet.

44 The only way to minimize the electrostatic repulsion among the bond pairs is to arrange the P Cl bonds so that the PCl5 molecule becomes trigonal bipyramidal. trigonal bipyramidal

45 5. sulfur hexafluoride, SF6

46 5. sulfur hexafluoride, SF6
The Lewis structure is

47 5. sulfur hexafluoride, SF6
The Lewis structure is Just with the bonds shown we have

48 The octahedral shape provides the most stable arrangement.

49 The octahedral shape provides the most stable arrangement.
No bonds present yet.

50 The octahedral shape provides the most stable arrangement.

51 More examples – The case of lone pairs on the central atom

52 More examples – The case of lone pairs on the central atom
Key to the following examples:

53 More examples – The case of lone pairs on the central atom
Key to the following examples: There are lone pairs on the central atom.

54 More examples – The case of lone pairs on the central atom
Key to the following examples: There are lone pairs on the central atom. Complications arise if the molecule contains lone pairs on the central atom in addition to bond pairs.

55 More examples – The case of lone pairs on the central atom
Key to the following examples: There are lone pairs on the central atom. Complications arise if the molecule contains lone pairs on the central atom in addition to bond pairs. There are now three types of electrostatic repulsions present in the molecule.

56 More examples – The case of lone pairs on the central atom
Key to the following examples: There are lone pairs on the central atom. Complications arise if the molecule contains lone pairs on the central atom in addition to bond pairs. There are now three types of electrostatic repulsions present in the molecule. (i) bond pair – bond pair repulsions

57 More examples – The case of lone pairs on the central atom
Key to the following examples: There are lone pairs on the central atom. Complications arise if the molecule contains lone pairs on the central atom in addition to bond pairs. There are now three types of electrostatic repulsions present in the molecule. (i) bond pair – bond pair repulsions (ii) lone pair – lone pair repulsions (if present)

58 More examples – The case of lone pairs on the central atom
Key to the following examples: There are lone pairs on the central atom. Complications arise if the molecule contains lone pairs on the central atom in addition to bond pairs. There are now three types of electrostatic repulsions present in the molecule. (i) bond pair – bond pair repulsions (ii) lone pair – lone pair repulsions (if present) (iii) lone pair – bond pair repulsions

59 As a general rule, the repulsion decreases as follows:
lone pair – lone pair > lone pair – bond pair > repulsion repulsion

60 As a general rule, the repulsion decreases as follows:
lone pair – lone pair > lone pair – bond pair > bond pair – bond pair repulsion repulsion repulsion

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