chapt07 Covalent bonding

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1 Chapter 7 Covalent Bonding Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1

2 Change in electron density as two hydrogen atoms approach each other. 2

3 Energy of two hydrogen atoms as a function of the distance between their nuclei Attractive energies between oppositely charged particles (electron-proton ) slightly exceed the repulsive energies between particle of like charge (electron-electron, proton- proton) ( 電子與質子間相互吸引力大於電子間與質子間的斥力降低氫分子間靜電能量 ) When two hydrogen atoms cone together to form a molecule, the electrons are spread over the entire volume of the molecule instead of being confined to a particular atom. ( 當兩氫原子結合成分子後, 電子是分佈於整個分子體積而不是局限於某一特定原子, 將二原子上的 1s 軌域互相重疊所形成的分子將非常安定 ) 3

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5 7.1 Lewis Structures; the Octet rule Lewis pointed out that the electron configuration of the noble gases appears to be a particularly stable one. Nonmetal atoms, by sharing electrons to form an electron- pair bond, can acquire a stable noble-gas These structure (without the circles ) are referred to as Lewis structures. H H + H H H + F H F Using dots to represent electrons; the circles emphasize that the pair of electrons in the covalent bond can be considered to occupy the 1s orbital of either hydrogen atom. in the H 2 molecule has the electronic structure of the noble gas helium. With the electron configuration 1s 2. 5

6 7.1 Lewis Structures; the Octet rule Valence electrons ( 價電子 ) are the outer shell electrons of an atom. The valence electrons are the electrons that participate in chemical bonding Group e - configuration # of valence e - 1A ns 1 1 2A ns 2 2 3A ns 2 np 1 3 4A ns 2 np 2 4 5A ns 2 np 3 5 6A ns 2 np 4 6 7A ns 2 np

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8 A covalent bond is a chemical bond in which two or more electrons are shared by two atoms. Why should two atoms share electrons? F + F F F 7e - 7e - 8e - 8e - Lewis structure of F 2 single covalent bond lone pairs F F lone pairs lone pairs F F lone pairs single covalent bond 8

9 Lewis structure of water single covalent bonds H + O + H H O H or H O H 2e8e - - 2e - Double bond two atoms share two pairs of electrons O C O or O C O double 8e - 8ebonds - - double bonds Triple bond two atoms share three pairs of electrons N N or N N 8e - 8e - triple bond triple bond 9

10 octet rule( 八隅律 ) The principle that atoms in covalently bonded species tend to have noble-gas electronic structures. ( 共價鍵原子符合惰性氣體之電子組態 ) Nonmetals, except for hydrogen, achieve a noble- gas structure by sharing in an octet of electrons (eight). ( 非金屬原子除了氫原子以外藉共用電子獲得如惰性氣體具八個價電子 ) 10

11 octet rule( 八隅律 ) 建議第二週期中元素 C, N, O, F 都會遵守八隅律, 因其價殼軌域只能調節 8 個電子第二週期 B, Be 在其化合物中會有少於 8 個電子圍繞這些電子缺少的化合物反應性非常高第三週期以下的元素通常會滿足八隅體規則, 但也可以用其空的 d 軌域來超過八隅律規律當要劃出一分子的路易士結構時, 首先要滿足該原子之八隅體規則若在八隅律規則後還有剩餘電子, 則將其於置於具有 d 軌域的元素上 11

12 Writing Lewis Structures p Count the number of valence electrons 計算總價電子數 2. Draw a skeleton structure for the species, Joining atoms by single bonds. 原子之電子參與鍵結形成單鍵劃出化合物結構 3. Determine the number of valence electrons still available for distribution. ( 依上步驟決定分子內可供使用的電子數, 即為總價電子數扣掉鍵結電子數 ) 4. Determine the number of valence electrons required to fill out an octet for each atom (except H) 1. If the number of electrons available is equal to the number required, distribute the available electrons as unshared pairs, satisfying the octet rule for each atom. 若步驟 3 的可供分配電子數與步驟 4 需求電子數相同時, 滿足每個原子符合八隅律下填入未鍵結電子對 2. If the number of electrons available is less than the number required, the skeleton structure must be modified by changing single to multiple bonds. 若步驟 3 的可供分配之電子數少於步驟 4 需求電子數時, 則此結構可能含有多重鍵結, 多重鍵的形成儘限 C, N, O, S 等四個原子 12

13 Ex:7.1 Draw Lewis structures of (a) The Hypochlorite ion, 次氯酸鹽離子 OCl - (1) The number of valence electrons is, 總價電子數 = 14 :O Cl : - (2) The skeleton structure is (3) 可供分配之電子數 [O-Cl] = 10 (4) 滿足每一原子符合八隅體所需之電子數 = 12 Ø Œº :O ø Cl œ ß - : - 13

14 (b) Methanol :CH 4 O (1) valence electrons (2) 分子結構 H H C O H H = 14 (3) 可供分配之電子數 = 4 (4) 滿足每一原子符合八隅體所需之電子數 4 H.. H C O.. H H 14

15 Ex:7.2 Draw Lewis structures of (a) SO 2 (1) valence electrons (2) 分子結構 [O-S-O] (3) 可供分配之電子數 = 18 :O Cl - : 18-4 = 14 (4) 滿足每一原子符合八隅體所需之電子數.... S.. : O.. O: = 16 (5) 可供分配之電子數少於符合八隅體所需電子數故有雙鍵 15

16 (b) N 2 (1) valence electrons (2) 分子結構 N-N 2 5 = 10 - Cl (3) 可供分配之電子數 :O : 10-2 = 8 (4) 滿足每一原子符合八隅體所需之電子數 12 (5) 可供分配之電子數少於符合八隅體所需電子數故有雙鍵或參鍵 :N N: 16

17 Write the Lewis structure of nitrogen trifluoride (NF 3 ). Step 1 N is less electronegative than F, put N in center Step 2 Count valence electrons N - 5 (2s 2 2p 3 ) and F - 7 (2s 2 2p 5 ) 5 + (3 x 7) = 26 valence electrons Step 3 Draw single bonds between N and F atoms and complete octets on N and F atoms. Step 4 - Check, are # of e - in structure equal to number of valence e -? 3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons F N F F

18 Write the Lewis structure of the carbonate ion (CO 3 2- ). Step 1 C is less electronegative than O, put C in center Step 2 Count valence electrons C - 4 (2s 2 2p 2 ) and O - 6 (2s 2 2p 4 ) -2 charge 2e (3 x 6) + 2 = 24 valence electrons Step 3 Draw single bonds between C and O atoms and complete octet on C and O atoms. Step 4 - Check, are # of e - in structure equal to number of valence e -? 3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons Step 5 - Too many electrons, form double bond and re-check # of e - O C O O 2 single bonds (2x2) = 4 1 double bond = 4 8 lone pairs (8x2) = 16 Total = 24 18

19 A resonance structure is one of two or more Lewis structures for a single molecule that cannot be represented accurately by only one Lewis structure. 1.Resonance forms do not imply different kinds of molecules with electrons shifting eternally between them. 2.Resonace can be anticipated when it is possible to write two or more Lewis structures that are about equally plausible. 3.Resonance forms differ only in the distribution of electrons, not in the arrangement of atoms S.. : O.. O: : O.. O: N.... : O.. O.. : N S.. : O.. O:.. - :.... O O.. : N - : Ọ. : : O : : Ọ. : 19

20 + - O O O - O + O O What are the resonance structures of the carbonate (CO 32 -) ion? - - O C O O C O - - O C O O O - O - 20

21 Formal Charge Two possible skeletal structures of formaldehyde (CH 2 O) H C O H An atom s formal charge is the difference between the number of valence electrons in an isolated atom and the number of electrons assigned to that atom in a Lewis structure. C f =X-(Y+Z/2) H H C O formal charge on an atom in a Lewis structure = total number of valence electrons in - the free atom total number of nonbonding electrons total number of bonding electrons ( ) The sum of the formal charges of the atoms in a molecule or ion must equal the charge on the molecule or ion. 21

22 Formal Charge 形式電荷 1. 在路易士結構中標示形式電荷, 只是有助於瞭解價電子的來龍去脈 ; 形式電荷並不表示真正的電荷 2. 對於中性分子, 所有的原子的形式電荷總和為零 ; 對於離子分子, 其形式電荷總和為離子的電荷 3. Lewis structures 傾向於形式電荷較低者 4. 當形式電荷的分佈類似時, 傾向於將負的形式電荷置於陰電性較高的原子上 Which is the most likely Lewis structure for CH 2 O? H C O H H H 0 0 C O 22

23 -1 +1 H C O H C 4 e - O 6 e - 2H 2x1 e - 12 e - 2 single bonds (2x2) = 4 1 double bond = 4 2 lone pairs (2x2) = 4 Total = 12 formal charge on an atom in a Lewis structure = total number of valence electrons in - the free atom total number of nonbonding electrons total number of bonding electrons ( ) formal charge on C formal charge on O = 4-2-½x 6 = -1 = 6-2-½x 6 = +1 23

H H 0 0 C O C 4 e - O 6 e - 2H 2x1 e - 12 e - 2 single bonds (2x2) = 4 1 double bond = 4 2 lone pairs (2x2) = 4 Total = 12 formal charge on an atom in a Lewis structure = total number of

24 H H 0 0 C O C 4 e - O 6 e - 2H 2x1 e - 12 e - 2 single bonds (2x2) = 4 1 double bond = 4 2 lone pairs (2x2) = 4 Total = 12 formal charge on an atom in a Lewis structure = total number of valence electrons in - the free atom total number of nonbonding electrons total number of bonding electrons ( ) formal charge on C formal charge on O = 4-0-½x 8 = 0 = 6-4-½x 4 = 0 24

25 Odd-Electron Molecules Exceptions to the Octet Rule NO N 5e - O 6e - 11e - N O The Expanded Octet (central atom with principal quantum number n > 2) SF 6 S 6e - 6F 42e - 48e - F F F S F F F 6 single bonds (6x2) = lone pairs (18x2) = 36 Total = 48 25

26 The Incomplete Octet Exceptions to the Octet Rule BeH 2 Be 2e - 2H 2x1e - 4e - H Be H B 3e - BF 3 3F 3x7e - F B F 24e - F 3 single bonds (3x2) = 6 9 lone pairs (9x2) = 18 Total = 24 26

27 Exception to the Octet Rule: Expanded Octets Largest class of molecules to violate the octet rule consists of species in which the central atom is surroundedy more than four pairs of valence electrons. cl cl cl P cl Cl cl 27

28 The geometry 7-2 Molecular Geometry (1) Diatomic molecule Cl 2 Cl-Cl (simply, Two points define a straight line.) (2) Molecules containing three or more atoms, the geometry is not so obvious. The angles between bonds, called bond angles. YX 2 Linear, bond angle of X-Y-X Bent, bond angle less than

29 7-2 Molecular Geometry Valence shell electron pair repulsion (VSEPR) model: The valence electron pairs surrounding an atom repulsion one another. Consequently, the orbitals containing those electron pairs are oriented to be as far apart as possible. 中心原子的電子對配置方式是為了要減少電子對之間的靜電排斥力 ; 不論共用或未共用電子對都會互相排斥, 分子中的電子對趨向使斥力減至最小, 亦即電子對彼此間距離愈遠愈穩定 1. 雙鍵及參鍵之間的電子對可視同單鍵 2. 若分子有兩種以上共振結構將 VSEPR 應用在任何一種即可, 且不必顯示形式電荷 29

30 Valence shell electron pair repulsion (VSEPR) model: 利用 VSEPR 模型預測分子結構的規則 : 1. 決定該分子之 Lewis structure 2. 對於具有共振結構的分子, 任取其中之一來預測分子構造 3. 計算出中心原子上之電子對的數目 4. 計算電子對的數目時, 每一多重鍵仍視為單鍵結電子對 5. 藉由降低電子間的斥力來決定電子對的排列方式, 如圖未鍵結電子對比鍵結電子對需要更多的空間 30

31 Valence shell electron pair repulsion (VSEPR) model: 中心原子的電子對配置方式是為了要減少電子對之間的靜電排斥力 Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AX linear linear B B 31

32 Cl Be Cl 2 0 atoms lone pairs bonded to central central atom atom 32

Class # of atoms bonded to central atom # lone pairs on central atom AX 2 2 0 linear linear AX 3

33 Class # of atoms bonded to central atom # lone pairs on central atom AX linear linear AX VSEPR Arrangement of electron pairs triangular planar Molecular Geometry triangular planar 33

Class # of atoms bonded to central atom # lone pairs on central atom AX 2 2 0 linear linear AX 3 3 0 VSEPR

35 Class # of atoms bonded to central atom # lone pairs on central atom AX linear linear AX VSEPR Arrangement of electron pairs trigonal planar Molecular Geometry trigonal planar AX tetrahedral tetrahedral 35

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Class # of atoms bonded to central atom # lone pairs on central atom AX 2 2 0 linear linear AX 3 3 0 VSEPR Arrangement of electron pairs

37 Class # of atoms bonded to central atom # lone pairs on central atom AX linear linear AX VSEPR Arrangement of electron pairs trigonal planar Molecular Geometry trigonal planar AX tetrahedral tetrahedral AX triangular bipyramid triangular bipyramid 37

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Class # of atoms bonded to central atom # lone pairs on central atom AX 2 2 0 linear linear AX 3 3 0 VSEPR Arrangement of electron pairs trigonal planar

39 Class # of atoms bonded to central atom # lone pairs on central atom AX linear linear AX VSEPR Arrangement of electron pairs trigonal planar Molecular Geometry trigonal planar AX tetrahedral tetrahedral AX trigonal bipyramidal trigonal bipyramidal AX octahedral octahedral 39

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42 Effect of Unshared Pairs on Molecular geometry lone-pair vs. lone pair repulsion lone-pair vs. bonding > > pair repulsion bonding-pair vs. bonding pair repulsion 42

Class # of atoms bonded to central atom # lone pairs on central atom AX 3 3 0 AX 2 E 2 1 VSEPR

43 Class # of atoms bonded to central atom # lone pairs on central atom AX AX 2 E 2 1 VSEPR Arrangement of electron pairs triangular planar triangular planar Molecular Geometry triangular planar bent 43

44 VSEPR Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AX tetrahedral tetrahedral AX 3 E 3 1 triangular pyramidal triangular pyramidal 44

45 VSEPR Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AX tetrahedral tetrahedral AX 3 E 3 1 tetrahedral triangular pyramidal AX 2 E tetrahedral bent H O H 45

Class # of atoms bonded to central atom # lone pairs on central atom AX 5 5 0 AX 4 E 4 1 VSEPR Arrangement of

46 Class # of atoms bonded to central atom # lone pairs on central atom AX AX 4 E 4 1 VSEPR Arrangement of electron pairs trigonal bipyramidal trigonal bipyramidal Molecular Geometry trigonal bipyramidal distorted tetrahedron 46

47 Class Fig7.8 Molecular geometries for molecules with expanded octets # of atoms bonded to central atom AX AX 4 E 4 1 AX 3 E and unshared electron pairs. VSEPR # lone pairs on central atom Arrangement of electron pairs trigonal bipyramidal trigonal bipyramidal trigonal bipyramidal Molecular Geometry trigonal bipyramidal distorted tetrahedron T-shaped F F Cl F 47

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49 VSEPR Class # of atoms bonded to central atom # lone pairs on central atom AX AX 4 E 4 1 AX 3 E AX 2 E Arrangement of electron pairs trigonal bipyramidal trigonal bipyramidal trigonal bipyramidal trigonal bipyramidal Molecular Geometry trigonal bipyramidal distorted tetrahedron T-shaped linear I I I 49

50 VSEPR Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AX octahedral octahedral AX 5 E 5 1 octahedral square pyramidal F F F F Br F 50

51 VSEPR Class # of atoms bonded to central atom # lone pairs on central atom Arrangement of electron pairs Molecular Geometry AX octahedral octahedral AX 5 E 5 1 octahedral AX 4 E octahedral square pyramidal square planar F F Xe F F 51

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54 Predicting Molecular Geometry 1. Draw Lewis structure for molecule. 2. Count number of lone pairs on the central atom and number of atoms bonded to the central atom. 3. Use VSEPR to predict the geometry of the molecule. What are the molecular geometries of SO 2 and SF 4? O S O F AX 4 E AX 2 E bent F S F F See- saw 54

55 Ex:7.5 Predict the geometry of (a) NH 4 (b) GeF 2 (c) PF 3 55

56 Multiple Bonds Insofar as molecular geometry is concerned, a multiple bond behaves like a single bond. the means that the electron pair in a multiple bond must occupy the same region of space as those in a single bond. the extra electron pairs in a multiple bond have no effect on geometry. 56

57 EX:7.6 Predict the geometry of the ClO 3- ion, the NO 3- ion 及 N 2 O molecule, Which have the Lewis structures? Solution: (a) The central atom, Cl, is bonded to three oxygen atoms, it has one unshared pair. The ClO 3- ion is of the type AX 3 E. It is a triangular pyramid (b) AX 3 It has The geometry of an equilateral triangle,the bond angle is The ion is triangular planar. (c) Type AX 2, is linear, with a bond angle of

58 7-3 Polarity of Molecules Polar and Nonpolar covalent Bonds nonpolar bond A symmetrical distribution of electrons leads to a bond or molecule with no positive or negative poles. polar bond An unsymmetrical distribution of electrons, the bond or molecule contains a positive and a negaative pole and is therefore a dipole. 共價鍵極性的強弱與鍵結原子間陰電性差異有關 58

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61 Dipole Moments The extent to which molecules tend to orient themselves in an electrical field is a measure. electron poor region electron rich region H F 箭頭代表電子密度高 µ = Q x r δ+ δ Q is the charge r is the distance between charges 1 D = 3.36 x C m 正號代表電子密度低從電負度較低者移向電負度較高 61

62 Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms electron poor region electron rich region e - poor e - rich H F H F δ + δ - 62

63 Polar and nonpolar Molecules Bond polarity Molecular geometry If the polar A-X bonds in a molecule AX m E n are arranged symmetrically around the central atom A, the molecule is nonpolar. Molecules of the type AX 2 (linear),ax 3 (triangular planar),and AX 4 (tetrahedral)are nonpolar.ex CO 2, BF 3 Molecules of the type AX 2 E(bent),AX 2 E 2 (bent),and AX 3 E (triangular pyramid) are polar. Ex SO 2,H 2 O 63

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65 Which of the following molecules have a dipole moment? H 2 O, CO 2, SO 2, and CH 4 H O H O S O dipole moment polar molecule dipole moment polar molecule H O C O H C H no dipole moment nonpolar molecule H no dipole moment nonpolar molecule 65

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67 BF 3 AX 3 It is triangle;it is nonpolar. Ex:7.7 Determine whether each of the following is polar or nonpolar? SO 2 : AX 2 E It is bent, so it is Ploar CO 2 AX 2 It is a linear,it is nonpolar 67

68 Does CH 2 Cl 2 have a dipole moment? 68

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70 7-4 Atomic orbitals; Hybridization 1. Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from original atomic orbitals. 2. Number of hybrid orbitals is equal to number of pure atomic orbitals used in the hybridization process. 3. Covalent bonds are formed by: a. Overlap of hybrid orbitals with atomic orbitals b. Overlap of hybrid orbitals with other hybrid orbitals 70

71 Formation of sp Hybrid Orbitals 71

72 Formation of sp 2 Hybrid Orbitals 72

73 How do I predict the hybridization of the central atom? Count the number of lone pairs AND the number of atoms bonded to the central atom # of Lone Pairs + # of Bonded Atoms Hybridization Examples 2 sp BeCl 2 3 sp 2 BF 3 4 sp 3 CH 4, NH 3, H 2 O 5 sp 3 d PCl 5 6 sp 3 d 2 SF 6 73

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77 N 1s 2 2s 2 2p 3 3 H 1s 1 Valence Bond Theory and NH 3 If the bonds form from overlap of 3 2p orbitals on nitrogen with the 1s orbital on each hydrogen atom, what would the molecular geometry of NH 3 be? If use the 3 2p orbitals predict 90 0 Actual H-N-H bond angle is

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80 Predict correct bond angle 80

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84 multiple bond Insofar as molecular geometry is concerned, a multiple bond behaves like a single bond. The number of terminal atoms,x, bonded to the central atom, Irrespective of whether the bonds are single, double, or triple. ( 幾何結構與末端原子數 X 的多寡有關, 不必考慮鍵結是否為單雙或參鍵 ) The number of unshared pairs, E,around the central atom.( 圍繞中心原子的未鍵結電子數, 亦會影響分子幾合結構 ) The extra electron pairs in a multiple bond ( one pair in a double bond, two pairs in a triple bond) are not located in hybrid orbital. 84

85 Sigma (σ) and Pi Bonds (π) Single bond Double bond Triple bond 1 sigma bond 1 sigma bond and 1 pi bond 1 sigma bond and 2 pi bonds How many σ and π bonds are in the acetic acid (vinegar) molecule CH 3 COOH? H O H C C O H σ bonds = = 7 π bonds = 1 H 85

86 Pi bond (π) electron density above and below plane of nuclei Sigma bond (σ) electron density between the 2 atoms of the bonding atoms 86

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95 Energy levels of bonding and antibonding molecular orbitals in hydrogen (H 2 ). A bonding molecular orbital has lower energy and greater stability than the atomic orbitals from which it was formed. An antibonding molecular orbital has higher energy and lower stability than the atomic orbitals from which it was formed. 95

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99 Molecular Orbital (MO) Configurations 1. The number of molecular orbitals (MOs) formed is always equal to the number of atomic orbitals combined. 2. The more stable the bonding MO, the less stable the corresponding antibonding MO. 3. The filling of MOs proceeds from low to high energies. 4. Each MO can accommodate up to two electrons. 5. Use Hund s rule when adding electrons to MOs of the same energy. 6. The number of electrons in the MOs is equal to the sum of all the electrons on the bonding atoms. 99

100 bond order = 1 2 Number of electrons in bonding MOs Number of electrons in antibonding MOs ( - ) bond order ½ 1 ½ 0 100

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102 Delocalized molecular orbitals are not confined between two adjacent bonding atoms, but actually extend over three or more atoms. 102

103 Electron density above and below the plane of the benzene molecule. 103

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6 樣本 Microscopic World II 微觀世界 II 6.1 Attraction Attraction Between molecules Intermolecular forces van der Waals forces Hydrogen bonding Intermolecul

6 樣本 Microscopic World II 微觀世界 II 6.1 Attraction Attraction Between molecules Intermolecular forces van der Waals forces Hydrogen bonding Intermolecul 6 Microscopic World II 微觀世界 II 6.1 ttraction ttraction etween molecules Intermolecular forces van der Waals forces ydrogen bonding Intermolecular hydrogen bond (Weaker ) Intramolecular hydrogen bond (Stronger