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20 Oct 97Bonding and structure (2)1 Chemical Bonding and Molecular Structure (Chapter 9) Ionic vs. covalent bonding Molecular orbitals and the covalent.

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Presentation on theme: "20 Oct 97Bonding and structure (2)1 Chemical Bonding and Molecular Structure (Chapter 9) Ionic vs. covalent bonding Molecular orbitals and the covalent."— Presentation transcript:

1 20 Oct 97Bonding and structure (2)1 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

2 20 Oct 97Bonding and structure (2)2 OCTET RULE: #Bond Pairs + #Lone Pairs = 4 (except for H and atoms of 3rd and higher periods) Rules for making Lewis dot structures 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) # of PAIRS — 2 for # of PAIRS 1. Count no. of valence electrons (- don’t forget to include the charge on molecular ions!) #lone pairs at central atom in AX n = {(#e-) - 8*n}/2

3 20 Oct 97Bonding and structure (2)3 Sulfur Dioxide, SO 2 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 + —— + Rules 1-3  O—S —O

4 20 Oct 97Bonding and structure (2)4 SO 2 (2) Alternate Lewis structure for SO 2 uses 2 double bonds NB: # of central atom lone pairs = (3*6 -8*2)/2 = 1 in both O=S + -O - and O=S=O structures O = S = O 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

5 20 Oct 97Bonding and structure (2)5 A. S=C=N Thiocyanate ion, (SCN) - Which of three possible resonance structures is most important? Calculated partial charges ANSWER: C > A > B C. S-C N B. S=C - N

6 20 Oct 97Bonding and structure (2)6 VSEPR V alence S hell E lectron P air R epulsion theory. Most important factor in determining geometry is relative repulsion between electron pairs. MOLECULAR GEOMETRY Molecule adopts the shape that minimizes the electron pair repulsions. 6_VSEPR.mov

7 20 Oct 97Bonding and structure (2)7 H H H H tetrahedral 109 o C 4 F 120 o planar trigonal FF B 3 Geometry Example No. of e- Pairs Around Central Atom 180 o linear 2F—Be—F CAChe image

8 20 Oct 97Bonding and structure (2)8 Structure Determination by VSEPR There are 4 electron pairs at the corners of a tetrahedron. lone pair of electrons in tetrahedral position H H H N The ELECTRON PAIR GEOMETRY is tetrahedral. Ammonia, NH 3

9 20 Oct 97Bonding and structure (2)9 Although the electron pair geometry is tetrahedral... VSEPR - ammonia Ammonia, NH 3... the MOLECULAR GEOMETRY — the positions of the atoms — is PYRAMIDAL. lone pair of electrons in tetrahedral position H H H N

10 20 Oct 97Bonding and structure (2)10 AX n E m notation a good way to distinguish between electron pair and molecular geometries is the AX n E m notation where: A - atom whose local geometry is of interest (typically the CENTRAL ATOM) X n - n atoms bonded to A E m - m lone pair electrons at A NH 3 is AX 3 E system  pyramidal (NB this notation not used by Kotz)

11 20 Oct 97Bonding and structure (2)11 2. Count BP’s and LP’s = 4 3. The 4 electron pairs are at the corners of a tetrahedron. The electron pair geometry is TETRAHEDRAL. H - O - H VSEPR - water Water, H 2 O 1. Draw electron dot structure

12 20 Oct 97Bonding and structure (2)12... the molecular geometry is bent. VSEPR - water (2) Although the electron pair geometry is TETRAHEDRAL... H - O - H H 2 O - AX 2 E 2 system - angular geometry

13 20 Oct 97Bonding and structure (2)13 2. Count BP’s and LP’s: At Carbon there are 4 BP but 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  120 o bond angles. C HH O VSEPR - formaldehyde Formaldehyde, CH 2 O 1. Draw electron dot structure C HH O AX 3 species H 2 CO at the C atom is an AX 3 species

14 20 Oct 97Bonding and structure (2)14 AX n E m designation ? at C at O Define bond angles 1 and 2 Angle 1 = H-C-H = ? Angle 2 = H-O-C = ? Answer: VSEPR - Bond Angles H H Angle 2 H—C—O—H Angle 1 6_CH3OH.mov Methanol, CH 3 OH 109 o because both the C and O atoms are surrounded by 4 electron pairs. AX 4 = tetrahedral AX 2 E 2 = bent

15 20 Oct 97Bonding and structure (2)15 AX n E m designation ? at CH 3 carbon at CN carbon Define bond angles 1 and 2 VSEPR - bond angles (2) Acetonitrile, CH 3 CN Angle 1 = ? H 1 H—C—C 2 H N Why ? : The CH 3 carbon is surrounded by 4 bond charges The CN carbon is surrounded by 2 bond charges Angle 2 = ? AX 4 = tetrahedral AX 2 = linear 109 o 180 o

16 20 Oct 97Bonding and structure (2)16 What about: STRUCTURES WITH CENTRAL ATOMS THAT DO NOT OBEY THE OCTET RULE ? BF 3 SF 4 PF 5

17 20 Oct 97Bonding and structure (2)17 Geometry for non-octet species also obey VSEPR rules The B atom is surrounded by only 3 electron pairs. Bond angles are 120 o Molecular Geometry is planar trigonal BF 3 is an AX 3 species Consider boron trifluoride, BF 3

18 20 Oct 97Bonding and structure (2)18 F F F F F Trigonal bipyramid 120  90  P 5 electron pairs F F F Octahedron 90  F FF S 6 electron pairs Compounds with 5 or 6 Pairs Around the Central Atom 6_VSEPR.mov AX 5 system AX 6 system

19 20 Oct 97Bonding and structure (2)19 There are 5 (BP + LP) e- pairs around the S THEREFORE: electron pair geometry ? F F F F S Sulfur Tetrafluoride, SF 4 F F F F S Number of valence e- = 34 No. of S lone pairs = { b.p. - 3x4 l.p.(F)} = 1 lone pair on S = trigonal bipyramid AX 4 E system. Molecular geometry ? F F F F S OR

20 20 Oct 97Bonding and structure (2)  90  F F F F S Sulfur Tetrafluoride, SF 4 (2) axial equatorial Molecular geometry of SF 4 is “see-saw” Q: What is molecular geometry of SO 2 ? Lone pair is in the equatorial position because it requires more room than a bond pair.

21 20 Oct 97Bonding and structure (2)21 Bonding with Hybrid Atomic Orbitals 4 C atom orbitals hybridize to form four equivalent sp 3 hybrid atomic orbitals. equivalent sp 3 hybrid atomic orbitals. 6_CH4.mov But atomic carbon has an s 2 p 2 configuration Why can it make more than 2 bonds ? - Carbon prefers to make 4 bonds as in CH 4

22 20 Oct 97Bonding and structure (2)22 Orbital Hybridization BONDSSHAPEHYBRID REMAINe.g. 2linear {2 x sp &2 p’s}C 2 H 2 3trigonal {3 x sp 2 & 1 p}C 2 H 4 planar 4tetrahedral {4 xsp 3 } CH 4 s 2 p 2 

23 20 Oct 97Bonding and structure (2)23 Multiple Bonds  and  Bonding in C 2 H 4 The extra p orbital electron on each C atom overlaps the p orbital on the neighboring atom to form the  bond. p orbital 3 sp 2 hybrid orbitals       2p 2s C atom orbitals are COMBINED (= re-hybridized) to form orbitals better suited for BONDING The 3 sp 2 hybrid orbitals are used to make the C-C and two C-H  bonds 6_C2H4-sg.mov 6_C2H4.mov H H C HH sp  C 6_C2H4-pi.mov

24 20 Oct 97Bonding and structure (2)24 Consequences of Multiple Bonding Restricted rotation around C=C bond in 1-butene = CH 2 =CH-CH 2 -CH E (kJ/mol) C-C=C angle ( o ) P Photo-rotation about double bonds lets us see !! See Butene.Map in ENER_MAP in CAChe models.

25 20 Oct 97Bonding and structure (2)25 Bond Properties What is the effect of bonding and structure on molecular properties ? Buckyball in HIV-protease, see page bond order - bond length - bond strength - bond polarity - MOLECULAR polarity

26 20 Oct 97Bonding and structure (2)26 H H H C CN C Bond Order the number of bonds between a pair of atoms. single BO = 1 1  triple, BO = 3 1  and 2  double, BO = 2 1  and 1  CH 2 CHCN Acrylonitrile

27 20 Oct 97Bonding and structure (2)27 Bond order = Total # of e- pairs used for a type of bond Total # of bonds of that type Bond Order (2) Fractional bond orders occur in molecules with resonance structures. Consider NO 2 - Bond order in NO 2 - = 3 (e - pairs in N-O bonds) 2 (N - O bonds) N-O bond order in NO 2 - = 1.5

28 20 Oct 97Bonding and structure (2)28 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. 745 kJ 414 kJ 123 pm 110 pm Formaldehye

29 20 Oct 97Bonding and structure (2)29 Bond Length - depends on size of bonded atoms: MoleculeR(H-X) H- F104 pm H- Cl131 pm H- I165 pm - depends on bond order. MoleculeR(C-O) CH 3 C- OH141 pm O=C=O132 pm C  O119 pm

30 20 Oct 97Bonding and structure (2)30 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 C—C C=C C  C N  N The GREATER the number of bonds (bond order) the HIGHER the bond strength and the SHORTER the bond.

31 20 Oct 97Bonding and structure (2)31 Bond Strength (2) BondOrderLengthStrength HO—OH 1149 pm210 kJ/mol O=O kJ/mol ? HOW TO CALCULATE ?  H rxn = {3x  H f (O) -  H f (O 3 )} = {3x } = 605 kJ/mol 2 O-O bonds in O 3  DE (O 3 ) = 605/2 = kJ/mol O 3 (g)  3 O(g) 303 kJ/mol

32 20 Oct 97Bonding and structure (2)32 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.)

33 20 Oct 97Bonding and structure (2)33 Due to the bond polarity, the H—Cl bond energy is GREATER than expected for a “pure” covalent bond. Bond Polarity (2) BONDENERGY “pure” bond339 kJ/mol calculated real bond432 kJ/mol measured ELECTRONEGATIVITY, . Difference 92 kJ/mol. This difference is the contribution of IONIC bonding It is proportional to the difference in

34 20 Oct 97Bonding and structure (2)34 Electronegativity,   is a measure of the ability of an atom in a molecule to attract electrons to itself. Concept proposed by Linus Pauling ( ) Nobel prizes: Chemistry (54), Peace (63) See p. 425; 008vd3.mov (CD)

35 20 Oct 97Bonding and structure (2)35 F has maximum . Atom with lowest  is the center atom in most molecules. Relative values of  determines BOND POLARITY (and point of attack on a molecule). Electronegativity,  Figure 9.7

36 20 Oct 97Bonding and structure (2)36 Bond Polarity  (A) -  (B)  1.4  H 2.1 O F Also note that polarity is “reversed.” Which bond is more polar ? (has larger bond DIPOLE) O—HO—F  (O-H) >  (O-F) Therefore OH is more polar than OF

37 20 Oct 97Bonding and structure (2)37 Molecular Polarity Molecules—such as HCl and H 2 O— can be POLAR (or dipolar). They have a DIPOLE MOMENT. Polar molecules turn to align their dipole with an electric field.

38 20 Oct 97Bonding and structure (2)38 Predicting molecular polarity A molecule will be polar ONLY if a) it contains polar bonds AND b) the molecule is NOT “symmetric” Symmetric molecules

39 20 Oct 97Bonding and structure (2)39 Molecular Polarity: H 2 O Water is polar because: a) O-H bond is polar b) water is non-symmetric The dipole associated with polar H 2 O is the basis for absorption of microwaves used in cooking with a microwave oven

40 20 Oct 97Bonding and structure (2)40 Carbon Dioxide CO 2 is NOT polar even though the CO bonds are polar. Because CO 2 is symmetrical the BOND polarity cancels The positive C atom is why water attaches to CO 2 CO 2 + H 2 O  H 2 CO

41 20 Oct 97Bonding and structure (2)41 B—F, B—H bonds polar molecule is NOT symmetric B—F bonds are polar molecule is symmetric Molecular Polarity in NON-symmetric molecules B +ve F -ve Atom Chg.  B +ve 2.0 H +ve 2.1 F -ve 4.0 BF 3 is NOT polar HBF 2 is polar

42 20 Oct 97Bonding and structure (2)42 Fluorine-substituted Ethylene: C 2 H 2 F 2 CIS isomer both C—F bonds on same side  molecule is POLAR. C—F bonds are MUCH more polar than C—H bonds. TRANS isomer both C—F bonds on opposite side  molecule is NOT POLAR.  (C-F) = 1.5,  (C-H) = 0.4

43 20 Oct 97Bonding and structure (2)43 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


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