13 Bonding: General Concepts. Types of chemical bonds. Covalent bonding Ex. H 2. Repulsions of nuclei and e s. Zero interaction at long distance

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1 13 Bonding: General Concepts Types of chemical bonds Covalent bonding Ex. 2 E (kj/mol) epulsions of nuclei and e s r r (nm) - bond length Two e s shared by two s: covalent bonding Zero interaction at long distance (repulsions between nuclei and e s are balanced by attraction forces)

2 Ionic bonding Ex. Na + Cl : ionic forces operate Coulomb attraction: charges 19 Q1Q = ( J nm) r E 2 Polar covalent bond Ex. Unequal sharing of e Charge distribution occurs Stronger attraction for e δ + δ ractional charge A dipole ( 偶極 )

3 Electronegativity The ability of an atom in a molecule to attract shared e s Linus Pauling ( ) BE A B Expected BE = (BE (BE AA + BEBB 2 = A -B ) actual A-B ) expected (Actually use (BE A-B ) expected = BE to avoid negative ) The greater EN difference, the greater value Define EN(A) EN(B) = Assign EN() = 4.0 AA BE BB luorine atom with the highest EN EN for all elements can be determined

4 EN difference for A B Bond type 0 covalent polar covalent large ionic (A +, B ) Increasing EN A B δ + δ igher EN Decreasing EN r (0.7) periodic table lowest highest Ex. - S- Cl covalent polar covalent

5 Bond polarity and dipole moments Molecules with a separate positive center and negative center: dipolar A dipole has a dipole moment (µ) Will orient in an electric field µ = Q distance (m) charge (coulomb) Unit: debye (D) 1D = C m

6 If - is ionic + µ = ( C)( m) = C m = 4.40 D In fact, - is not fully ionic Measured µ = 1.83 D with only partial charge (1.83 D)( C m/d) = (δ)( m) δ = C The fraction: has 42% ionic bonding (an oversimplified view) = δ + δ The full charge The partial charge

7 or polyatomic molecules δ + δ + δ net dipole The center of all positive atoms and the center of all negative atoms do not merge C B Cl Cl Cl Cl net dipole = zero Ex. -C Cl-Cl zero S S 2.5 zero C zero

8 Ions: Electron configurations and sizes Ionic compounds In general A M + non-metal Non-metal: gains e Metal: loses e main group metal reach noble gas configuration Ex. Ca: [Ar]4s 2 Ca 2+ : [Ar] : [e]2s 2 2p 4 2 : [e]2s 2 2p 6 = [Ne] Exceptions: Sn 2+, Sn 4+, Bi 3+, Bi 5+

9 Sizes of ions determines the structure and stability of ionic compounds Negative ions: larger than parent (gain e ) Positive ions: smaller than parent (lose e ) Trend: Down a group larger Ex. Li + Be (pm) Na + Mg 2+ Al 3+ S 2 Cl K + Ca 2+ Ga 3+ Se 2 Br b + Sr 2+ In 3+ Te 2 I

10 or isoelectronic ions: with the same # of e Ex. 2,, Na +, Mg 2+, Al 3+ : [Ne] config With the same # of e Size depends on nuclear charge (atomic #) Atomic # size

11 ormation of binary ionic compound Lattice energy M + (g) + X (g) M X(s) gaseous an ionic solid energy released = lattice E Exothermic: (+) Endothermic:( )

12 verall energy consideration Ex. Born-aber cycle Li(s) + 1 / 2 2 (g) 161 Li(g) 1 / 2 BE of 2 = 77 ionization E (g) electron affinity 328 Li(s) Li + (g) + (g) 1047 Unit: kj/mol At 298 K lattice E vap ioni BE EA LE rxn = = 617 kj/mol Too small Unable to compensate the loss Dominating factor

13 Energy diagram Li + (g)+ 1 / 2 2 (g) Li + (g)+ (g) Li + (g)+ (g) Li(g)+ 1 / 2 2 (g) 161 Li(s)+ 1 / 2 2 (g) kj Li(s)

14 Lattice E calculations Q Q2 Lattice E = k( 1 ) r charge of the ions opposite sign LE: (+) + Q 1, Q 2 LE r LE a constant characteristic of the solid r Ex. LE for Mg(s): 3916 kj/mol LE for Na(s): 923 verall Mg(s) + ½ 2 (g) Mg(s) Na(s) + ½ 2 (g) Na(s) Charge is higher = 602 kj/mol = 570 kj/mol Too many factors involved difficult to predict the rel. size

15 Mg(s) + ½ 2 (g) Mg(g) + ½ 2 (g) Mg(g) + ½ 2 (g) Mg 2+ (g) + ½ 2 (g) = 150 kj/mol = 2180 (high) Mg 2+ (g) + ½ 2 (g) Mg 2+ (g) + (g) = 247 Mg 2+ (g) + (g) Mg 2+ (g) + 2 (g) = 737 e - e repulsion

16 Partial ionic character of covalent bonds In the gas phase: no true ionic bond Polar covalent bond δ + δ percent ionic character = measured dipole moment calculated dipole moment 100% X + Y

17 Ex. NaCl(g): 75% In the solid form: multiple ion interactions help to stabilize the ionic form In general: > 50% ionic solid Ambiguity: N 4 + Cl, Na 2 S 4 covalent bonded group Definition of ionic compound conducts electric current when melted

18 The covalent bond: A model Ex. C 4 C + 4 requires 1652 kj C 4 is more stable than the discreet atoms by 1652 kj Consider forces exist between C and 4 bonding Consider localized C bonds 1652 To break each bond = = 413 kj 4

19 Ex. C 3 Cl C Cl requires 1578 kj 3 C bonds, 1 C Cl bond (413) = 339 kj/mol Strength of C Cl bond Assumption: bond strength does not vary with structures The covalent bond model Considering electrons are localized between two atoms

20 Covalent bond energies and chemical reactions act 1 act 2 C 4 (g) C 3 (g) + (g) 435 kj/mol C 3 (g) C 2 (g) + (g) 453 C 2 (g) C(g) + (g) 425 C (g) C(g) + (g) 339 total = 1652 Average: 413 C- BE -CBr kj/mol -CCl C C 2 C BEs are structurally dependent Average BEs are used

21 Bond types e s shared single 2 double 4 triple 6 Ex. BE (kj/mol) bond length (Å) C C C=C C C

22 Bond energy and enthalpy Ex. 2 (g) + 2 (g) 2(g) 2 2 2, 2 identical Bond E difference = = ΣD (bonds broken) ΣD (bonds formed) Bond E (+) = D - + D - 2D - = (565) = 544 kj Calculated from f : = 2( 271) = 542 kj f o ()

23 Ex. C 4 (g) + 2Cl 2 (g) (g) C 2 Cl 2 (g) + 2(g) + 2Cl(g) 4C- 2Cl-Cl 2-2C- 2-2-Cl 2C-Cl = 1194 kj cf. from o f 1126 kj Conclusion calculated from BE works well Note: Average BEs are used = E + P V at constant P Calculation from BE does not consider PV work Works better for gas phase reaction (intermolecular interaction smaller)

24 The localized electron bonding model (Valence bonding model) A covalent bond is formed between two atoms sharing electrons using atomic orbitals Pairs of e s localized on an atom: lone pairs Pairs of e s localized between atoms: bonding pairs

25 Lewis structures Describe the arrangement of valence electrons G. N. Lewis ( ) observed: In most stable compounds the atoms achieve noble gas electron configuration Ex. KBr Br : [Kr] K + : [Ar] Lewis structure K Br no valence e

26 Molecules with covalent bonds Duet rule or first row atoms:, e : e: (1s 2 ) ctet rule Second row nonmetals : C, N,, Ne Ex (2s 2 2p 6 ) Total: 8 e s (valence e )

27 Ex. C 2 :C: Total: 16 C Does not obey octet rule Trial and error C = C

28 Ex. CN ne extra charge = 10 e C N Not good C N Not good C N Not good C N Good C N

29 Ex. N 2 10 e N N Ex. N 3 8 e C 4 8 e N = 10 e isoelectronic with CN N C N C N + C 4 32 e C C

30 esonance Ex. N 3 24 e s N N Predict: act: a short double bond two long single bonds Equal length (between a single and a double) A new theory must be formulated

31 Actually there are three possible structures: N N N We can consider the real structure as the avg. of the three a resonance hybrid Any single structure can not represent the real structure Each Lewis structure is called resonance structure (exists only on paper) epresented as N N N double headed arrow (not )

32 nly allow electrons to move (nuclei stay at the same position) Must be proper Lewis structures Ex. Carbon can not have five bonds

33 Exceptions to the octet rule Less than 8 B nly 6 e ow about B act: B is very reactive toward electron rich molecules N + B N B

34 More than 8 S 12 e Classical explanation use of low lying empty d orbitals (3d for sulfur) pposite opinion In sulfur, E of 3d orbitals is still too high than 3s, 3p Concept of hyperconjugation 2+ S 2+ S etc.

35 PCl (5 7) = 40 e Cl P Cl Cl Cl Cl 10 e Allow the central atom to expand its valency I 3 (3 7) + 1 = 22 I I I 10 e ICl (4 7) + 1 = 36 e Cl I Cl Cl Cl 10 e

36 Ex. Xe (3 6) = 26 e Xe obeys BeCl (2 7) = 16 e Cl Be Cl nly 4 e Cl 3 (28 e ) Cl 10 e ncl 2 Cl n Cl 8 + (2 7) = 22 e 10 e

37 dd-electron molecules Nitric oxide N = 11 N N N Not good A radical with unpaired e *Keep the number of unpaired e the lowest C C C C Not a viable resonance structure N N oxidized by 2 2 in the air 5 + 2(6) = 17 N N N N

38 ormal charge Q: Structure of S 4 2 (32 e ) 2 S What about 2 2 S or S 10 e 12 e ow to decide?

39 Method: Estimate the charge xidation state: 2 S +6 The concept of formal charge ver-estimated nly useful for bookkeeping Compare: number of e in a neutral atom A number of e in a bonded atom B When A = B no charge A > B (+) A < B ( )

40 Define ormal charge = A B Assign lone pair e s to the atom completely Divide shared e s equally Ex. S 2 or A = 6 B = 6 + 2/2 = 7 shared e s lone pair e s ormal charge on = 6 7 = 1 or S: A = 6 B = 0 + 8/2 = 4 ormal charge on S = 6 4 = +2 shared e s lone pair e s

41 1 1 S verall charge = (+2) + 4( 1) S S 2 Single bonded : 1 Double bonded : 6 [4 + 4/2] = 0 S: 6 (10/2) = S ok

42 1 0 S (12/2) = 0 There are resonance structures 1 S 1 S 1 1 Another school of thought: Keeping the octet rule is more important A better structure due to minimum formal charge S 1 1 Note: they are all resonance structures 1 S etc.

43 Which is correct? act: Bond length of the real molecule S bond is shorter than a normal single bond Yes: double bonded S shorter Yes: single bonded S shorter because of the charge attraction Dispute continues

44 Guidelines or 2nd row elements: never exceed octet rule ormal charges as low as possible Negative charge on more electronegative atoms 1 B +1 Not good Ex. Xe (6) = 26 +3: Xe 1 1: Xe 1 1 Xe Xe 0 0

45 Additional rule for resonance: Nonequivalent structures do not make equal contributions Generally speaking more stable a structure higher contribution to the hybrid Ex. C 2 C C 4 + 2(6) = Low contribution 1. Positive charge on 2. Charge separation Note: ormal charge is only a closer estimation of actual charge A correct Lewis structure may not accurately account for the observed properties

46 Application A carbocation C nly 6 e, does not fulfill octet rule highly electron deficient highly reactive A carbon radical C nly 7 e, does not fulfill octet rule highly reactive

47 A carbanion C An oxygen radical ulfill octet rule very basic reactive Stronger bond Neutral ighly reactive Amine and ammonium ion - hydrogen atom abstraction + N amine N ammonium ion urther rxn

48 A stabilized carbocation C C nly 6 e Although with positive charge on oxygen, fulfills octet rule for every atom A better resonance structure Called oxonium ion C + + C Although contributes less, reflects positive character on carbon C

49 Molecular structures: The VSEP model eal structures are three dimensional Understand the 3-D structure understand properties and functions A simple model: Valence shell electron pair repulsion (VSEP) model (useful for nonmetals) The main postulation: minimizing electron pair repulsions Bonding and nonbonding pairs Ex. BeCl 2 Cl Be Cl : linear (two pairs) As far away as possible

50 B 3 B 120 o Trigonal planar (three pairs) C 4 C o Tetrahedral (four pairs) Cf. C 90 o N 3 N Square planar (not good) Based on the position of atoms: trigonal pyramid 2 A bent structure (four pairs)

51 Bond angles lone pairs C o N 107 o o Explanation lone pair is closer to the nucleus exerts large repulsion towards the other electron pairs N squeezed

52 ive pairs: PCl 5 Cl Cl Six pairs: Cl P Cl Cl PCl 6 Trigonal bipyramidal Cl Cl Cl P Cl Cl Cl ctahedral Summary: # of e pairs arrangement 2 linear 3 trigonal planar 4 tetrahedral 5 trigonal bipyramidal 6 octahedral

53 Ex. Xe 4 Xe Six pairs: octahedral arrangement Xe as far away as possible Square planar shape

54 Ex. I 3 I I I ive pairs: trigonal bipyramidal arrangement I I I 90 o, 90 o, 180 o I I I 90, 90, 120 I I I 120, 120, 120 Best (no 90 o repulsions) linear

55 Treat double or triple bonds as one effective pair Ex. N 3 pairs S 2 S S 3 pairs Bent structure In fact -S- ~120 o with little distortion long pair is satisfied with 120 o

56 More complicated cases Methanol C 3 C C Carbocation C C: 3 pairs trigonal planar C Carbanion C C: 4 pairs tetrahedral C pyramidal

H-H bond length Two e s shared by two Hs: covalent bonding. Coulomb attraction: Stronger attraction for e Fractional charge A dipole

H-H bond length Two e s shared by two Hs: covalent bonding. Coulomb attraction: Stronger attraction for e Fractional charge A dipole 8 Bonding: General Concepts Types of chemical bonds Covalent bonding Ex. 2 E (kj/mol) Repulsions of nucleus and e s r 0 458 0.074 r (nm) Zero interaction at long distance - bond length Two e s shared by