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Bonding and Molecular Structure: Fundamental Concepts Valence e- and Bonding CovalentIonic Resonance & Exceptions to Octet Rule Bond Energy & Length Structure,

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Presentation on theme: "Bonding and Molecular Structure: Fundamental Concepts Valence e- and Bonding CovalentIonic Resonance & Exceptions to Octet Rule Bond Energy & Length Structure,"— Presentation transcript:

1 Bonding and Molecular Structure: Fundamental Concepts Valence e- and Bonding CovalentIonic Resonance & Exceptions to Octet Rule Bond Energy & Length Structure, Shape & Polarity of Compounds AP Notes Chapter 8

2 What is a Bond? A force that holds atoms together. A force that holds atoms together. Why? Why? We will look at it in terms of energy. We will look at it in terms of energy. Bond energy the energy required to break a bond. Bond energy the energy required to break a bond. Why are compounds formed? Why are compounds formed? Because it gives the system the lowest energy. Because it gives the system the lowest energy.

3 Covalent compounds? The electrons in each atom are attracted to the nucleus of the other. The electrons in each atom are attracted to the nucleus of the other. The electrons repel each other, The electrons repel each other, The nuclei repel each other. The nuclei repel each other. The reach a distance with the lowest possible energy. The reach a distance with the lowest possible energy. The distance between is the bond length. The distance between is the bond length.

4 Bond Formation Thus Hydrogen is Diatomic!

5 Covalent Character e-e- e-e-

6 E He + He He 2 Inter-nuclear Distance Why Isnt Helium Diatomic?....

7 F + F F 2 F + F F 2 2p ____ ____ ___ ___ ____ ____ 2p 2s ____ ____ 2s FF FF

8 Ionic Bonding An atom with a low ionization energy reacts with an atom with high electron affinity. An atom with a low ionization energy reacts with an atom with high electron affinity. The electron moves. The electron moves. Opposite charges hold the atoms together. Opposite charges hold the atoms together.

9 Li + Cl 1s 2 2s 1 [Ne] 3s 2 3p 5 2s ___ 3p _____ _____ ___ 1s _____ 3s _____ [Ne] Li + Cl 1s 2 2s 1 [Ne] 3s 2 3p 5 2s ___ 3p _____ _____ ___ 1s _____ 3s _____ [Ne]

10 Li + Cl 2s ___ 3P _____ _____ _____ 1s _____ 3s _____ [Ne] Li + Cl 2s ___ 3P _____ _____ _____ 1s _____ 3s _____ [Ne]

11 LiCl 2s ___3P _____ _____ _____ 1s _____ 3s _____ [Ne] LiCl 2s ___3P _____ _____ _____ 1s _____ 3s _____ [Ne]

12 Electronegativity Describes the relative ability of an atom within a molecule to attract a shared pair of electrons to itself.

13 Electronegativity Pauling electronegativity values, which are unit- less, are the norm.

14 Figure 9.9 – Kotz & Treichel Electronegativity Range from 0.7 to 4.0

15 Bond: A - B EN = | EN A - EN B | EN = | EN A - EN B |

16 Bond Character Ionic Bond - Principally Ionic Character Covalent Bond - Principally Covalent Character

17 Determining Principal Character of Bond covalentionic EN ~0~41.7

18 F - F EN = 0 Non-polar

19 N - O EN = |3.0 - 3.5| = 0.5 Slightly polar N O

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21 Ca - O EN = |1.0 - 3.5| = 2.5 Ionic Bond with some covalent character Ca O

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23 Electronegativity The ability of an electron to attract shared electrons to itself. The ability of an electron to attract shared electrons to itself. Pauling method Pauling method Imaginary molecule HX Imaginary molecule HX Expected H-X energy = H-H energy + X-X energy 2 Expected H-X energy = H-H energy + X-X energy 2 = (H-X) actual - (H-X) expected = (H-X) actual - (H-X) expected

24 Electronegativity is known for almost every element is known for almost every element Gives us relative electronegativities of all elements. Gives us relative electronegativities of all elements. Tends to increase left to right. Tends to increase left to right. decreases as you go down a group. decreases as you go down a group. Noble gases arent discussed. Noble gases arent discussed. Difference in electronegativity between atoms tells us how polar. Difference in electronegativity between atoms tells us how polar.

25 Electronegativity difference Bond Type Zero Intermediate Large Covalent Polar Covalent Ionic Covalent Character decreases Ionic Character increases

26 Dipole Moments A molecule with a center of negative charge and a center of positive charge is dipolar (two poles), A molecule with a center of negative charge and a center of positive charge is dipolar (two poles), or has a dipole moment. or has a dipole moment. Center of charge doesnt have to be on an atom. Center of charge doesnt have to be on an atom. Will line up in the presence of an electric field. Will line up in the presence of an electric field.

27 How It is drawn H - F + -

28 Which Molecules Have Them? Any two atom molecule with a polar bond. Any two atom molecule with a polar bond. With three or more atoms there are two considerations. With three or more atoms there are two considerations. There must be a polar bond. There must be a polar bond. Geometry cant cancel it out. Geometry cant cancel it out.

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31 Ionic Radii -- Cations

32 Ionic Radii -- Anions

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34 Molecular Polarity Vector Sum of Bond Polarities

35 Br Mg MgBr 2 Mg - Br EN = |1.2 - 2.8| = 1.6 Covalent BOND w/much ionic character, BUT NON-POLAR molecule

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39 Lewis Structures

40 The most important requirement for the formation of a stable compound is that the atoms achieve noble gas e - configuration

41 Valence Shell Electron Pair Repulsion Model (VSEPR) The structure around a given atom is determined principally by minimizing electron-pair repulsions

42 VSEPR Electron pairs Bond Angles Underlying Shape 2180° Linear 3120° Trigonal Planar 4109.5° Tetrahedral 5 90° & 120° Trigonal Bipyramidal 6 90°Octagonal

43 LEWIS STRUCTURES 1 : draw skeleton of species 2 : count e - in species 3 : subtract 2 e - for each bond in skeleton 4 : distribute remaining e -

44 Distinguish Between ELECTRONIC Geometry & MOLECULAR Geometry

45 CH 4 Electronic geometry: tetrahedral Molecular geometry: tetrahedral Bond angle = 109.5 0

46 H3O+H3O+ Electronic geometry: tetrahedral Molecular geometry: trigonal pyramidal Bond angle ~ 107 0

47 H2OH2O Electronic geometry: tetrahedral Molecular geometry: bent Bond angle ~ 104.5 0

48 NH 2 - Electronic geometry: tetrahedral Molecular geometry: bent Bond angle ~ 104.5 0

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50 Octet Rule holds for connecting atoms, but may not for the central atom.

51 BaI 2 Electronic geometry: linear Molecular geometry: linear Bond angle =180 0

52 BF 3 Electronic geometry: trigonal planar Molecular geometry: trigonal planar Bond angle =120 0

53 PF 5 Electronic geometry: trigonal bipyramidal Molecular geometry: trigonal bipyramidal Bond angle = 120 0 & 90 0

54 SF 4 Electronic geometry: trigonal bipyramidal Molecular geometry: see-saw Bond angle = 120 0 & 90 0

55 ICl 3 Electronic geometry: trigonal bipyramidal Molecular geometry: T-shape Bond angle <= 90 0

56 I3-I3- Electronic geometry: trigonal bipyramid Molecular geometry: linear Bond angle = 180 0

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58 PCl 6 - Electronic geometry: octahedral Molecular geometry: octahedral Bond angle = 90 0

59 BrF 5 Electronic geometry: octahedral Molecular geometry: square pyramidal Bond angle ~ 90 0

60 ICl 4 - Electronic geometry: octahedral Molecular geometry: square planar Bond angle = 90 0

61 Actual shape Electron Pairs Bonding Pairs Non- Bonding Pairs Shape 220linear 330trigonal planar 321bent 440tetrahedral 431trigonal pyramidal 422bent

62 Actual Shape Electron Pairs Bonding Pairs Non- Bonding Pairs Shape 550trigonal bipyrimidal 541See-saw 532T-shaped 523linear

63 Actual Shape Electron Pairs Bonding Pairs Non- Bonding Pairs Shape 660Octahedral 651Square Pyramidal 642Square Planar 633T-shaped 621linear

64 What happens when there are not enough electrons to satisfy the central atom?

65 EXAMPLES Ethene Acetic Acid OxygenNitrogen

66 RESONANCE & FORMAL CHARGE

67 Resonance Sometimes there is more than one valid structure for an molecule or ion. Sometimes there is more than one valid structure for an molecule or ion. NO 3 - NO 3 - Use double arrows to indicate it is the average of the structures. Use double arrows to indicate it is the average of the structures. It doesnt switch between them. It doesnt switch between them. NO 2 - NO 2 - Localized electron model is based on pairs of electrons, doesnt deal with odd numbers. Localized electron model is based on pairs of electrons, doesnt deal with odd numbers.

68 EXAMPLES Nitrate ion Ozone

69 FORMAL CHARGE the charge assigned to an atom in a molecule or polyatomic ion FC atom = Family# - [LPE + ½(BE)] Sum FCs atoms = ion charge Closer sum FCs is to zero more stable

70 Formal Charge For molecules and polyatomic ions that exceed the octet there are several different structures. For molecules and polyatomic ions that exceed the octet there are several different structures. Use charges on atoms to help decide which. Use charges on atoms to help decide which. Trying to use the oxidation numbers to put charges on atoms in molecules doesnt work. Trying to use the oxidation numbers to put charges on atoms in molecules doesnt work.

71 Formal Charge The difference between the number of valence electrons on the free atom and that assigned in the molecule. The difference between the number of valence electrons on the free atom and that assigned in the molecule. We count half the electrons in each bond as belonging to the atom. We count half the electrons in each bond as belonging to the atom. SO 4 -2 SO 4 -2 Molecules try to achieve as low a formal charge as possible. Molecules try to achieve as low a formal charge as possible. Negative formal charges should be on electronegative elements. Negative formal charges should be on electronegative elements.

72 Assignment of e - 1. Lone pairs belong entirely to atom in question 2. Shared e - are divided equally between the two sharing atoms

73 The sum of the formal charges of all atoms in a species must equal the overall charge on the species.

74 A useful equation (happy-have) / 2 = bonds (happy-have) / 2 = bonds POCl 3 P is central atom POCl 3 P is central atom SO 4 -2 S is central atom SO 4 -2 S is central atom SO 3 -2 S is central atom SO 3 -2 S is central atom PO 4 -2 P is central atom PO 4 -2 P is central atom SCl 2 S is central atom SCl 2 S is central atom

75 Exceptions to the octet BH 3 BH 3 Be and B often do not achieve octet Be and B often do not achieve octet Have less than and octet, for electron deficient molecules. Have less than and octet, for electron deficient molecules. SF 6 SF 6 Third row and larger elements can exceed the octet Third row and larger elements can exceed the octet Use 3d orbitals? Use 3d orbitals? I 3 - I 3 -

76 Exceptions to the octet When we must exceed the octet, extra electrons go on central atom. When we must exceed the octet, extra electrons go on central atom. ClF 3 ClF 3 XeO 3 XeO 3 ICl 4 - ICl 4 - BeCl 2 BeCl 2

77 If nonequivalent Lewis structures exist, the one(s) that best describe the bonding in the species has...

78 FAVORED LEWIS STRUCTURES 1. formal charges closest to zero 2. negative formal charge is on the most electronegative atom is on the most electronegative atom

79 EXAMPLES Carbon dioxide Thiocyanate ion Sulfate ion

80 BOND ENERGY & LENGTH

81 Bond Energies E = (Bonds Broken) – (Bonds Made)

82 Bonds form between atoms because bonded atoms exhibit a lower energy. Thus, energy is required to break bonds and energy is released when bonds are formed.

83 Bond Order = # bonds to a specific set of elements C-C the BO=1 C=C the BO=2 C C the BO=3 Fractions are possible

84 COVALENT BONDS Bond Dissociation Energy Table 9.9 (text)

85 Bond Energy (kJ/mol) H-F 565 H-Cl 432 H-Br 366 H-I 299

86 Bond Energy (kJ/mol) Cl-Cl 242 Br-Br 193 I-I 151

87 Bond Energy (kJ/mol)

88 Bond Energy (kJ/mol)

89 Use bond energies to predict H c for acetylene (C 2 H 2 ).

90 0 Energy Internuclear Distance

91 0 Energy Internuclear Distance

92 0 Energy Internuclear Distance

93 0 Energy Internuclear Distance

94 0 Energy Internuclear Distance Bond Length

95 0 Energy Internuclear Distance Bond Energy

96 Bond: Energy Length (kJ/mol) (pm)

97 Bond: Energy Length (kJ/mol) (pm)

98 Binary Ionic Compounds metal (s) + non-metal (g) ---> salt (s)

99 Lattice Energy Energy change occurring when separated gaseous ions are packed together to form an ionic solid M + (g) + NM - (g) --> M-NM

100 What is the lattice energy of NaCl(s)? Na + (g) + Cl - (g) ---> NaCl(s)

101 Lattice Energies LiCl 834 : BeCl 2 3004 NaCl 769 : MgCl 2 2326 KCl 701 : CaCl 2 2223 Li 2 O 2799 : BeO 4293 Na 2 O 2481 : MgO 3795 K 2 O 2238 : CaO 3414

102 LE = Lattice Energy Where: k = proportionality constant dependent on structure of solid and on electron configuration of the ions Where: Q 1 & Q 2 = charges on the ions Where: r = the shortest distance between the centers of the cation and anion


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