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Chemical Bonding and Molecular Structure (Chapter 9). Ionic vs. covalent bonding Molecular orbitals and the covalent bond (Ch. 10) Valence electron Lewis dot structures octet vs. non-octet resonance structures formal charges VSEPR - predicting shapes of molecules

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Chemical bonding and molecular structure chapter 9 l.jpg
Chemical Bonding and Molecular Structure (Chapter 9)

  • Ionic vs. covalent bonding

  • Molecular orbitals and the covalent bond (Ch. 10)

  • Valence electron Lewis dot structures

  • octet vs. non-octet

  • resonance structures

  • formal charges

  • VSEPR - predicting shapes of molecules

  • Bond properties

    • electronegativity

  • polarity, bond order, bond strength

  • Bonding and structure (2)


    Rules for making lewis dot structures l.jpg

    — 2 for # of PAIRS

    Rules for making Lewis dot structures

    1. Count no. of valence electrons

    (- don’t forget to include the charge on molecular ions!)

    2. Place a bond pair (BP) between connected atoms

    3. Complete octets by using rest of e- as lone pairs (LP)

    4. For atoms with <8 e-, make multiple bonds to complete octets

    5. Assign formal charges : fc = Z - (#BP/2) - (#LP)

    Indicate equivalent (RESONANCE) structures

    6. Structures with smaller formal charges are preferred

    - consider non-octet alternatives (esp. for 3rd, 4th row)

    • OCTET RULE:#Bond Pairs + #Lone Pairs = 4

    • (except for H and atoms of 3rd and higher periods)

    #lone pairs at central atom in AXn = {(#e-) - 8*n}/2

    Bonding and structure (2)


    Sulfur dioxide so 2 l.jpg

    Rules 1-3

    O—S —O

    +

    +

    Sulfur Dioxide, SO2

    These equivalent structures

    are called:

    RESONANCE

    STRUCTURES.

    The proper Lewis structure

    is a HYBRID of the two.

    Each atom has OCTET . .

    . . . BUT there is a +1 and -1 formal charge

    Bonding and structure (2)


    So 2 2 l.jpg

    O = S = O

    SO2 (2)

    Alternate Lewis structure for SO2 uses 2 double bonds

    Sulfur does not obey OCTET rule

    BUT the formal charge = 0

    This is better structure than O=S+-O-

    since it reduces formal charge (rule 6).

    3rd row S atom can have 5 or 6 electron pairs

    NB: # of central atom lone pairs = (3*6 -8*2)/2 = 1 in both O=S+-O- and O=S=O structures

    Bonding and structure (2)


    Thiocyanate ion scn l.jpg

    A. S=C=N

    Calculated partial charges

    B. S=C - N

    C. S-C N

    -0.16

    -0.32

    -0.52

    Thiocyanate ion, (SCN)-

    Which of three possible resonance structures

    is most important?

    ANSWER:

    C > A > B

    Bonding and structure (2)


    Molecular geometry l.jpg

    6_VSEPR.mov

    MOLECULAR GEOMETRY

    Molecule adopts the shape that minimizes the electron pair repulsions.

    VSEPR

    • Valence Shell Electron Pair Repulsion theory.

    • Most important factor in determining geometry is relative repulsion between electron pairs.

    Bonding and structure (2)


    Slide7 l.jpg

    No. of e- Pairs

    Around Central

    Atom

    linear

    2

    F—Be—F

    180o

    F

    planar trigonal

    3

    B

    F

    F

    120o

    109o

    H

    4

    tetrahedral

    C

    H

    H

    H

    CAChe

    image

    Example

    Geometry

    Bonding and structure (2)


    Structure determination by vsepr l.jpg

    lone pair of electrons

    in tetrahedral position

    N

    H

    H

    H

    Structure Determination by VSEPR

    Ammonia, NH3

    There are 4 electron pairs at the corners of a tetrahedron.

    The ELECTRON PAIR GEOMETRY is tetrahedral.

    Bonding and structure (2)


    Vsepr ammonia l.jpg

    lone pair of electrons

    in tetrahedral position

    N

    H

    H

    H

    VSEPR - ammonia

    Ammonia, NH3

    Although the electron pair geometry is tetrahedral . . .

    . . . the MOLECULAR GEOMETRY

    — the positions of the atoms

    — is PYRAMIDAL.

    Bonding and structure (2)


    Ax n e m notation l.jpg
    AXnEm notation

    • a good way to distinguish between

    • electron pair and molecular geometries

    • is theAXnEmnotation

    • where:

    • A - atom whose local geometry is of interest (typically the CENTRAL ATOM)

    • Xn - n atoms bonded to A

    • Em - m lone pair electrons at A

    • NH3 is AX3E system  pyramidal

    • (NB this notation not used by Kotz)

    Bonding and structure (2)


    Vsepr water l.jpg

    ••

    H - O - H

    ••

    VSEPR - water

    Water, H2O

    2. Count BP’s and LP’s = 4

    3. The 4 electron pairs are at the corners of a tetrahedron.

    1. Draw electron dot structure

    The electron pair geometry is TETRAHEDRAL.

    Bonding and structure (2)


    Vsepr water 2 l.jpg

    ••

    H - O - H

    ••

    VSEPR - water (2)

    Although the electron pair geometry is TETRAHEDRAL . . .

    . . . the molecular geometry is bent.

    H2O - AX2E2 system - angular geometry

    Bonding and structure (2)


    Vsepr formaldehyde l.jpg

    O

    O

    C

    H

    H

    C

    H

    H

    Formaldehyde, CH2O

    VSEPR - formaldehyde

    1. Draw electron dot structure

    2. Count BP’s and LP’s:

    At Carbon there are 4 BP but . . .

    3. These are distributed in ONLY 3 regions.

    Double bond electron pairs are in same region.

    There are 3 regions of electron density

    Electron repulsion places them at the corners of a planar triangle.

    Both the electron pair geometry and the molecular geometry are PLANAR TRIGONAL  120o bond angles.

    H2CO at the C atom is an AX3 species

    Bonding and structure (2)


    Vsepr bond angles l.jpg

    6_CH3OH.mov

    VSEPR - Bond Angles

    Methanol, CH3OH

    H

    ••

    Define bond angles 1 and 2

    Angle 1 = H-C-H = ?

    Angle 2 = H-O-C = ?

    Answer:

    H—C—O—H

    ••

    H

    Angle 1

    Angle 2

    109o because both the C and O

    atoms are surrounded by 4 electron pairs.

    AXnEm designation ?

    at C

    at O

    AX4 = tetrahedral

    AX2E2 = bent

    Bonding and structure (2)


    Vsepr bond angles 2 l.jpg

    H

    N

    H—C—C

    ••

    1

    2

    H

    Acetonitrile, CH3CN

    VSEPR - bond angles (2)

    Angle 1 = ?

    109o

    Define bond angles 1 and 2

    Angle 2 = ?

    180o

    Why ? :

    The CH3 carbon is surrounded by 4 bond charges

    The CN carbon is surrounded by 2 bond charges

    AXnEm designation ?

    at CH3 carbon

    at CN carbon

    AX4 = tetrahedral

    AX2 = linear

    Bonding and structure (2)


    What about structures with central atoms that do not obey the octet rule l.jpg
    What about:STRUCTURES WITH CENTRAL ATOMS THAT DO NOT OBEY THE OCTET RULE ?

    PF5

    BF3

    SF4

    Bonding and structure (2)


    Geometry for non octet species also obey vsepr rules l.jpg
    Geometry for non-octet species also obey VSEPR rules

    Consider boron trifluoride, BF3

    The B atom is surrounded by only 3 electron pairs.

    Bond angles are 120o

    Molecular Geometry is

    planar trigonal

    BF3 is an AX3 species

    Bonding and structure (2)


    Compounds with 5 or 6 pairs around the central atom l.jpg

    6_VSEPR.mov

    Trigonal bipyramid

    90°

    F

    5 electron pairs

    F

    120°

    F

    P

    F

    F

    90°

    Octahedron

    F

    6 electron pairs

    F

    F

    F

    S

    F

    90°

    F

    Compounds with 5 or 6 Pairs Around the Central Atom

    AX5 system

    AX6 system

    Bonding and structure (2)


    Sulfur tetrafluoride sf 4 l.jpg

    ••

    ••

    F

    ••

    ••

    ••

    S

    F

    F

    ••

    ••

    F

    ••

    F

    F

    S

    F

    F

    F

    F

    S

    F

    F

    Sulfur Tetrafluoride, SF4

    Number of valence e- = 34

    No. of S lone pairs =

    {17 - 4 b.p. - 3x4 l.p.(F)}

    = 1 lone pair on S

    There are 5 (BP + LP)

    e- pairs around the S

    THEREFORE:

    electron pair geometry ?

    = trigonal bipyramid

    OR

    AX4E system. Molecular geometry ?

    Bonding and structure (2)


    Sulfur tetrafluoride sf 4 2 l.jpg

    F

    F

    S

    F

    equatorial

    F

    axial

    Sulfur Tetrafluoride, SF4 (2)

    90°

    120°

    Lone pair is in the equatorial position because it requires more room than a bond pair.

    Molecular geometry of SF4 is “see-saw”

    Q: What is molecular geometry of SO2 ?

    Bonding and structure (2)


    Bonding with hybrid atomic orbitals l.jpg

    6_CH4.mov

    Bonding with Hybrid Atomic Orbitals

    - Carbon prefers to make 4 bonds as in CH4

    4 C atom orbitals hybridize to form four

    equivalent sp3 hybrid atomic orbitals.

    But atomic carbon has an s2p2 configuration

    Why can it make more than 2 bonds ?

    Bonding and structure (2)


    Orbital hybridization l.jpg
    Orbital Hybridization

    BONDS SHAPE HYBRID REMAIN e.g.

    s2p2 

    2 linear {2 x sp & 2 p’s} C2H2

    3 trigonal {3 x sp2 & 1 p} C2H4 planar

    4 tetrahedral {4 xsp3 } CH4

    Bonding and structure (2)


    Multiple bonds s and p bonding in c 2 h 4 l.jpg

    ­

    ­

    ­

    ­

    ­¯

    ­

    ­

    p

    2s

    2p

    3 sp2

    orbital

    hybrid

    orbitals

    H

    H

    sp2

    120°

    C

    C

    H

    H

    Multiple Bondss and p Bonding in C2H4

    • The extra p orbital electron on each C atom overlaps the p orbital on the neighboring atom to form the p bond.

    C atom orbitals are COMBINED

    (= re-hybridized) to form orbitals

    better suited for BONDING

    • The 3 sp2 hybrid orbitals

    • are used to make the C-C

    • and two C-H  bonds

    6_C2H4-sg.mov

    6_C2H4-pi.mov

    6_C2H4.mov

    Bonding and structure (2)


    Consequences of multiple bonding l.jpg

    233

    E (kJ/mol)

    27

    -180 0 180

    C-C=C angle (o)

    Consequences of Multiple Bonding

    Restricted rotation around C=C bond in

    1-butene = CH2=CH-CH2-CH3.

    See Butene.Map in ENER_MAP in CAChe models.

    P. 475 - Photo-rotation

    about double bonds

    lets us see !!

    Bonding and structure (2)


    Bond properties l.jpg
    Bond Properties

    - bond order

    - bond length

    - bond strength

    - bond polarity

    - MOLECULAR polarity

    • What is the effect of bonding and structure on molecular properties ?

    Buckyball in HIV-protease, see page 107

    Bonding and structure (2)


    Bond order l.jpg

    triple, BO = 3

    H

    H

    and 2 p

    1

    s

    H

    C

    C

    C

    N

    double, BO = 2

    single

    and 1 p

    1

    s

    BO = 1

    1

    s

    Bond Order

    • the number of bonds between a pair of atoms.

    CH2CHCN

    Acrylonitrile

    Bonding and structure (2)


    Bond order 2 l.jpg

    Total #

    of e

    -

    pairs used for a type of bond

    Bond

    order

    =

    Total #

    of bonds of that type

    3 (e - pairs in N-O bonds)

    =

    Bond order in NO2-

    2 (N - O bonds)

    Bond Order (2)

    Fractional bond orders occur in molecules with resonance structures.

    Consider NO2-

    N-O bond order in NO2- = 1.5

    Bonding and structure (2)


    Bond order and bond length l.jpg

    110 pm

    745 kJ

    123 pm

    414 kJ

    Formaldehye

    Bond Order and Bond Length

    Bond order is related to two important bond properties:

    (a) bond strength

    as given by DE

    (b) Bond length

    - the distance between the nuclei of two bonded atoms.

    Bonding and structure (2)


    Bond length l.jpg
    Bond Length

    Molecule R(H-X)

    H- F 104 pm

    H- Cl 131 pm

    H- I 165 pm

    - depends on size of bonded atoms:

    - depends on bond order.

    Molecule R(C-O)

    CH3C- OH 141 pm

    O=C=O 132 pm

    C O 119 pm

    Bonding and structure (2)


    Bond strength l.jpg
    Bond Strength

    • Bond Dissociation energy (DE) - energy required to break a bond in gas phase.

    • See Table 9.5

    BOND STRENGTH (kJ/mol) LENGTH (pm)

    H—H 436 74

    C—C 347 154

    C=C 611 134

    CºC 837 121

    NºN 946 110

    The GREATER the number of bonds (bond order)

    the HIGHER the bond strength and the SHORTER the bond.

    Bonding and structure (2)


    Bond strength 2 l.jpg
    Bond Strength (2)

    Bond Order Length Strength

    HO—OH 1 149 pm 210 kJ/mol

    O=O 2 121 498 kJ/mol

    1.5 128 ?

    303 kJ/mol

    O3 (g)  3 O(g)

    HOW TO CALCULATE ?

    Hrxn = {3xHf(O) - Hf(O3)} = {3x249.2 - 142.7} = 605 kJ/mol

    2 O-O bonds in O3  DE (O3) = 605/2 = 302.5 kJ/mol

    Bonding and structure (2)


    Bond polarity l.jpg
    Bond Polarity

    HCl is POLAR because it has a positive end and a negative end (partly ionic).

    Polarity arises because Cl has a greater share of the bonding electrons than H.

    Calculated charge by CAChe:

    H (red) is +ve (+0.20 e-)

    Cl (yellow) is -ve (-0.20 e-).

    (See PARTCHRG folder in MODELS.)

    Bonding and structure (2)


    Bond polarity 2 l.jpg
    Bond Polarity (2)

    • Due to the bond polarity, the H—Cl bond energy is GREATER than expected for a “pure” covalent bond.

    BOND ENERGY

    “pure” bond 339 kJ/mol calculated

    real bond 432 kJ/mol measured

    Difference 92 kJ/mol.

    This difference is the contribution of IONIC bonding

    It is proportional to the difference in

    ELECTRONEGATIVITY, c.

    Bonding and structure (2)


    Electronegativity c l.jpg
    Electronegativity, c

    c is a measure of the ability of an atom in a molecule to attract electrons to itself.

    Concept proposed by

    Linus Pauling (1901-94)

    Nobel prizes:

    Chemistry (54), Peace (63)

    See p. 425; 008vd3.mov (CD)

    Bonding and structure (2)


    Slide35 l.jpg

    Electronegativity, c

    Figure 9.7

    • F has maximum c.

    • Atom with lowest c is the center atom in most molecules.

    • Relative values of c determines BOND POLARITY (and point of attack on a molecule).

    Bonding and structure (2)


    Bond polarity36 l.jpg
    Bond Polarity

    Which bond is more polar ? (has larger bond DIPOLE)

    O—H O—F

    c H

    2.1

    O F

    3.5 4.0

    c(A) - c(B)3.5 - 2.1

    Dc 1.4

    3.5 - 4.0

    0.5

    (O-H) > (O-F)

    Therefore OH is more polar than OF

    Also note that polarity is “reversed.”

    Bonding and structure (2)


    Molecular polarity l.jpg
    Molecular Polarity

    • Molecules—such as HCl and H2O— can be POLAR (or dipolar).

    • They have a DIPOLE MOMENT.

    • Polar molecules turn to align their dipole with an electric field.

    Bonding and structure (2)


    Predicting molecular polarity l.jpg

    Symmetric molecules

    Predicting molecular polarity

    A molecule will be polar ONLY if

    a) it contains polar bonds

    AND

    b) the molecule is NOT “symmetric”

    Bonding and structure (2)


    Molecular polarity h 2 o l.jpg
    Molecular Polarity: H2O

    Water is polar because:

    a) O-H bond is polar

    b) water is non-symmetric

    The dipole associated with polar H2O

    is the basis for absorption of microwaves

    used in cooking with a microwave oven

    Bonding and structure (2)


    Carbon dioxide l.jpg

    -0.73 +1.46 -0.73

    Carbon Dioxide

    • CO2 is NOT polar even though the CO bonds are polar.

    • Because CO2 is symmetrical the BOND polarity cancels

    The positive C atom is why water attaches to CO2

    CO2 + H2O  H2CO3

    Bonding and structure (2)


    Slide41 l.jpg

    HBF2 is polar

    BF3 is NOT polar

    Molecular Polarity in

    NON-symmetric molecules

    B—F, B—H bonds polar

    molecule is NOT symmetric

    Atom Chg. 

    B +ve 2.0

    H +ve 2.1

    F -ve 4.0

    B +ve

    F -ve

    B—F bonds are polar

    molecule is symmetric

    Bonding and structure (2)


    Fluorine substituted ethylene c 2 h 2 f 2 l.jpg
    Fluorine-substituted Ethylene: C2H2F2

    C—F bonds are MUCH more polar than C—H bonds.

    (C-F) = 1.5, (C-H) = 0.4

    CIS isomer

    • both C—F bonds on same side

       molecule is POLAR.

    TRANS isomer

    • both C—F bonds on opposite side

       molecule is NOT POLAR.

    Bonding and structure (2)


    Chemical bonding and molecular structure chapter 943 l.jpg
    Chemical Bonding and Molecular Structure (Chapter 9)

    • Ionic vs. covalent bonding

    • Molecular orbitals and the covalent bond (Ch. 10)

    • Valence electron Lewis dot structures

    • octet vs. non-octet

    • resonance structures

    • formal charges

    • VSEPR - predicting shapes of molecules

    • Bond properties

      • electronegativity

  • polarity, bond order, bond strength

  • Bonding and structure (2)


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