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第八章 分子结构与晶体结构 (1)

第八章 分子结构与晶体结构 (1). 序言 一、化学键 1 离子键 ( 本质、特征、强弱、离子半径 ) 2 共价键 ( 本质、特征、强弱、共价半径 ) 3 分子构型 ( 价电子对互斥理论 ) 4 杂化轨道理论 5 分子轨道理论 二、 分子间力和氢键 1 分子间力 2 氢键 三、晶体结构 1 晶体的特征 2 晶格和晶胞 3 晶体的基本类型. Questions?.

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第八章 分子结构与晶体结构 (1)

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  1. 第八章 分子结构与晶体结构 (1) • 序言 • 一、化学键 • 1 离子键 (本质、特征、强弱、离子半径) • 2 共价键 (本质、特征、强弱、共价半径) • 3 分子构型 (价电子对互斥理论) • 4 杂化轨道理论 • 5 分子轨道理论 • 二、分子间力和氢键 • 1 分子间力 • 2 氢键 • 三、晶体结构 • 1 晶体的特征 • 2 晶格和晶胞 • 3 晶体的基本类型

  2. Questions? • Why is calcium phosphate so rigid that nature has adopted it for the formation of bones? Can we make better bones? (强离子键) • Why is it so difficult to make compounds from the nitrogen in air? Can we find an easy way? (强共价键) • How can we explain the ability of hemoglobin to form a loosely bonded compound with oxygen, transfer it to another part of the body, and then release it in response to a metabolic need? (配位键)

  3. 序言： Link • 原子怎样结合成为分子？ － 化学键 • 离子键 • 共价键 • 金属键 • 分子的形状？ － 分子构型 • 价电子对互斥理论 • 分子怎样组成物质材料？－分子间作用力 • 固体材料的结构？ －晶体结构 －无定型结构

  4. 一、化学键（Chemical Bonds） 1、离子键 (Ionic Bonds) • Lewis结构式和八隅体规则 • 离子键及其特点 • 离子键强度与晶格能 • 离子化合物的性质

  5. 为什么惰性气体稳定？ns2np6 八电子层结构 Lewis 结构式, 价电子（Valence electrons） · ·· ·· H· He: :N· ·O· :Cl· K· Mg: :Ne: · ·· ·· • • • • · ·· ·· K· + :Cl· → K+[:Cl:]－ ·· ·· 失或得电子 → 八隅体规则(主族) Loss or gain electrons → octet rule

  6. 离子键及其特点： · ·· ·· K· + :Cl· → K+[:Cl:]－ ·· ·· 定义：正负离子间的静电吸引力叫做离子键。 特点：既没有方向性，也不具饱和性。 NaCl 晶体

  7. 离子键强度与晶格能： 晶格能（U） (Lattice Enthalpies) • 定义：晶格能表示相互远离的气态正离子和负离子结合成 1 mol 离子晶体时所释放的能量，或1 mol 离子晶体解离成自由气态离子时所吸收的能量。（A measure of the attraction between ions is lattice enthalpy, the enthalpy change per mol of formula units when a solid is broken up to a gas of widely separated ions.） （取其绝对值）如： Ca2+(g) + 2Cl- (g) CaCl2(s) – H = U = 2260.kJ/mol

  8. 离子键强度与晶格能： Born-Lande 公式 U = -Ve ∝ Z1Z2/r 其中： Ve 为正负离子间吸引力和排斥力达平衡时，体系的位能；Z1、Z2分别为正负离子的电荷数； r为正负离子间距。 the closer the center of charge and the greater the charges, the stronger will be the interaction。 • Born-Haber 循环计算U（自学，第11章，p区元素）

  9. 解离能 电子亲核能 升华热 + 电离能 生成焓 晶格能 晶格能 生成焓 离子型卤化钠的生成焓和晶格能

  10. 离子化合物的性质 (The Properties of Ionic Compounds) • 高熔点 High melting points • 高沸点 High boiling points • 易脆性 Brittleness • 溶解性 Some are soluble, some not. [例] Ca3(PO4)2：骨头的主要成分 The doubly charged small Ca2+ ions, and the triply charged PO43-ions attract one another very strongly and clamp together to form a rigid, insoluble solid. (not completely insoluble, osteoporosis，骨质疏松症)

  11. 2、共价键 (Covalent Bonds) • 从原子到分子 (from atoms to molecules) • 八隅率与Lewis结构 (The octet rule and Lewis structure) 双原子分子 多原子分子 共振杂化 形式电荷 例外 • 广义八隅率 (Expended valence shells) • Lewis 酸碱 (Lewis acids and bases)

  12. 共享电子对 · · ● ● Nonmetals form covalent bonds to one another by sharing pairs of electrons

  13. 八隅率与Lewis结构The octet rule and Lewis structure • The octet rule: In covalent bond formation, atoms go as far as possible toward completing their octet by sharing electron pairs. • The valence of an element is the number of covalent bonds of the element forms. (饱和性) • Lone pairs of electrons, pairs of valence electrons not involved in bondng. ( 双原子分子中，仅H2无孤对) • A Lewis structure shows the arrangement of valence electrons as shared pairs (line) and lone pairs (dots)

  14. 多原子分子片的结构The structures of polyatomic species • Choose the atom with the lowest ionization energy (electronegativity) to be the central atom. • Arrange the atoms symmetrically around the central atom. For example, SO2 is OSO. [例1] HCN Valence electrons: 1+4+5 = 10, five pairs. C, lower ionization energy than N, C is central atom ·· H:C:N:  ·· H:C:::N: 或 H-CN:

  15. Suggested steps: a. Determine the number of valence electrons. * Each atom provide all its valence electrons   (only outshell ns and np electrons are considered for main group elements)   * a negative charge – * a positive charge + b. Write the chemical symbols of the atoms in the arrangement that shows which bonds are formed. The less electronegative element is usually the center atom. c.Distribute the electronin pairs to achieve octet rule. Elements in period 3 and below could have more than 8 electrons in the valence shell.

  16. p1, 失去p电子，易 p轨道半满，p3, 稳定 电离能数据 趋势(与原子半径相反) 增 减

  17. Suggested procedure to distribute electrons: a. Put one pair between each pair of atom. b. Put enough pair on the central atom so that it can achieve octet. c. Distribute the remaining electron pair as evenly as possible to the surrounding atoms to achieve octet. * If octet is not achieved change the electron lone pairs on the central atom to bonding pairs. * If there are some electron pairs left after octet is achieved for every atom, then put the extra electron pair at the central atom. * Elements in period 3 or below may not follow the octet rule. Additional Lewis structures could also be drawn by change the electron lone pairs on the terminal atoms to bonding pairs.

  18. 共振结构(Resonance structure) For some molecules or ions, more than one Lewis structure can be drawn. e.g. The blending of structures with the same arrangements of atoms but different arrangements of electrons. It spreads multiple bond character over a molecular and also lower its energy. In this case, neither one of Lewis structures can adequately represent the structure of the molecules. The actual structure is taken to be a blend of all the feasible structures. The concept of blend is called resonance and is indicated by a double-headed arrow.

  19. 形式电荷 (formal charge, FC) FC = V – (L+1/2S) = 自由原子价电子数 - 孤对电子数 – ½(成键电子数) = = # of valence electron – 2  # of lone pairs – # of bonds An indication of the extent to which atoms have gained or lose electrons in the process of covalent bond formation. Lowest FC, lowest energy. [例] CO2 N2O NCO-

  20. 八隅率例外, Radicals · ·· [例] NO :N=O It is one of neurotransmitters. ·· (神经传递者)

  21. 广义八隅率 (Expended valence shells) • 空的d轨道 • 足够的原子尺寸 第3周期及以后的元素 Lewis 酸碱 (Lewis acids and bases) A Lewis acid is an electron pair acceptor; A Lewis base is an electron pair donor. They react to form a Lewis acid-base complex.

  22. 共价键的形成过程： 键（头碰头） The overlap of orbitals to form covalent bonds. (a) The bond in H2 results from the overlap of two 1s orbitals from two H atoms. (b) The bond in HCl results from the overlap of a 1s orbital of H and one of the lobes of a 3p orbital of Cl. (c) The bond in Cl2 results from the overlap of two 3p orbitals from two Cl atoms.

  23. HF的生成

  24. 键：p轨道（肩并肩）

  25. 共价键的本质和特点 共价键的本质是由于原子相互接近时轨道重叠（即波函数叠加），原子间通过共用自旋相反的电子对使能量降低而成键。 共价键的主要特点是具有饱和性和方向性。

  26. 离子键与共价键 • 共价模型的修正 Electronegativity (EN) is a measure of the electron-pulling power of an atom on an electron pair in a molecule. Compounds composed of elements with large difference in EN (≥2) tend to have significant ionic character in their bonding. • 离子模型的修正 Compounds composed of high polarizing (使极化) cations and highly polarizable (被极化) anions have a significant covalent character in their bonding.

  27. 3、分子和离子的形状 (The shapes of molecules and ions) 价层电子对互斥理论 The VSEPR model (Valence-shell electron-pair repulsion )

  28. VSEPR (Valence-shell electron-pair repulsion )价层电子对互斥理论可以定性判断和预见分子的几何构型 • 分子的共价键中的价电子对以及孤对电子由于相互排斥作用而趋向尽可能彼此远离，分子尽可能采取对称的结构。 • 若一个中心原子和几个配位原子形成分子时，分子的几何构型取决于中心原子周围地价电子数目。价电子包括价层轨道中成键电子对(bp)和孤电子对(lp). • 不同价电子对间排斥作用的顺序为： lp－lp  lp－bp  bp－bp • 分子中的多重键按单键处理（忽略键！）。 价层电子对数确定方法： 价层电子对数 ＝ ½(中心原子价电子总数＋配位原子提供电子数 – 离子电荷数) 配位原子提供电子数：H, Cl, 1; O, S, 0; N, -1.

  29. VSEPR (Valence-shell electron-pair repulsion ) • According to the VSEPR model, bonding pairs and lone pairs, to reduce repulsions, take up positions around an atom that maximize their separations. The shape of the molecule is determined by the locations of the atoms attached to the central atom. • Electron pairs in multiple bonds are treated as a single unit equivalent to one region of high electron concentration. • Lone pairs on the central atom contribute to the shape of the molecule but are ignored when we name the shape. The molecule adjusts its shape to reduce lone pair-lone pair and lone pair-bonding pair repulsions.

  30. Rules of VSEPR Theory • 1) Draw the best Lewis dot structure of the molecule • 2) Assign a steric number (SN) to the structure SN = (# of bonded atoms) + (# of lone pairs) • 3) Place the atoms and lone pairs as far apart as possible (while still keeping them connected to the central atom) • 4) Deduce the molecular geometry by ignoring the positions of the lone pairs • 5) Remember, lone pairs are FAT

  31. 价电子对数目与分子构型

  32. 电子对数目：2，3，4

  33. 电子对数目：5，6

  34. 价层电子对数 = ½（中心原子价电子数 + 配位原子提供电子数 – 离子电荷数代数值） = 键数 + 孤电子对数 配位原子提供电子数： 氢和卤素原子：各提供1个电子(形成一个键)； 氧和硫原子：提供0个电子(中心原子提供2个电子形 成一个键)； 氮原子：提供-1个电子(中心原子提供3个电子形 成一个键)；

  35. 甲烷 Methane (CH4) Lewis structure: Central atom carbon Valence electrons on central atom 4 4 H each contribute 1 electron: 4 Total 8 Divide by 2 to give electron pairs 4 4 electron pairs: tetrahedral for the four shape-determining electron pairs 分子构型：正四面体

  36. Ammonia, NH3 Lewis structure: Central atom nitrogen Valence electrons on central atom 5 3 H each contribute 1 electron: 3 Total 8 Divide by 2 to give electron pairs 4 4 electron pairs: tetrahedral geometry for the four shape-determining electron pairs The H-N-H bond angles are slightly less (106.6°) than the ideal tetrahedral angle of 109.5°. 氨 分子构型：角锥型 电子构型与分子构型不一致

  37. 水 Water, OH2 Lewis structure: Central atom oxygen Valence electrons on central atom 6 2 H each contribute 1 electron: 2 Total 8 Divide by 2 to give electron pairs 4 4 electron pairs: tetrahedral for the four shape-determining electron pairs 分子构型：角型 电子构型与分子构型不一致

  38. BF3 Boron trifluoride, BF3 Lewis structure: Central atom boron Valence electrons on central atom 3 3 F each contribute 1 electron: 3 Total 6 Divide by 2 to give electron pairs 3 3 electron pairs: trigonal geometry for the three shape-determining electron pairs 分子构型：平面三角

  39. [PF6]- hexafluorophosphate, [PF6]- Lewis structure: Central atom phosphorus Valence electrons on central atom 5 6 F each contribute 1 electron: 6 Add one for the negative charge on P 1 Total 12 Divide by 2 to give electron pairs 6 6 electron pairs: octahedral geometry for the six shape-determining electron pairs 分子构型：正八面体

  40. ClF3 Chlorine trifluoride, ClF3 Lewis structure: Central atom chlorine Valence electrons on central atom 7 3 F atoms each contribute 1 electron: 3 Total 10 Divide by 2 to give electron pairs 5 5 electron pairs: trigonal bipyramidal geometry for the five shape-determining electron pairs 电子构型与分子构型不一致 电子数为5时，孤对总是尽先处于三角双锥的腰部位置 分子构型：T字型

  41. Isomers The case of ClF3 is interesting. The calculation shows that the shape is based upon five electron pairs and the favoured geometry is therefore trigonal bipyramidal. There are three bonded groups and so two lone pairs. This is indeed the case, but the point of interest here is the location of the lone pairs. There are three possible ways of placing two electron pairs in a trigonal bipyramidal geometry. These three structures have respectively zero, one, and two lone pairs in the axial sites. For the VSEPR method to be worth much, it has to successfully predict the correct geometry. To approach this problem it is necessary to know the relative magnitude of the various kinds of electron pair-electron pair interactions. There are three possible interactions: 三种构型 选择 lp－lp  lp－bp  bp－bp

  42. VSEPR calculation for perchlorate, [ClO4]- Perchlorate, [ClO4]- Lewis structure: Central atom chlorine Valence electrons on central atom 7 4 terminal oxygens each contribute 1 electron in the four  bonds 4 Add one for the negative charge located on Cl 1 Subtract four for the four electrons contributed by Cl to the four  bonds (one for each): -4 (双键因素） Total 8 Divide by 4 to give electron pairs 4 4 electron pairs: tetrahedral geometry for the four shape-determining electron pairs [ClO4]– 分子构型：正四面体 含氧原子的情况：净结果是O原子不提供价电子。

  43. Nitrogen dioxide, NO2 Lewis structure: Central atom nitrogen Valence electrons on central atom 5 2 terminal oxygens each contribute 1 electron in the two s bonds: 2 Subtract two for the two electrons contributed by N to the two pi bonds: -2 (双键因素） Total 5 Divide by 2 to give electron pairs 2.5 3 3 electron pairs: trigonal geometry for the 3 shape-determining s-framework orbitals NO2 分子构型：角型 含氧原子的情况：净结果是O原子不提供价电子。 电子构型与分子构型不一致

  44. 适用性与局限性 价层电子对互斥理论 • Predicts the shapes of molecules • Works very well for octets and for “expanded octets” (2nd and 3rd row elements) • Doesn't work at all for transition metal complexes--too many groups and electrons to allow the use of sterics alone 不能说明成键原理和键的相对稳定性 Ronald J. Gillespie Department of Chemistry, McMaster Uni6ersity, 1280 Main Street West, Hamilton, Ont., Canada Coordination Chemistry Reviews,197 (2000), 3-19; 51–69.

  45. 4 、杂化轨道 (Hybrid Obital) 实验测得CCl4、 CH4等的立体构型为正四面体（tetrahedral） 在同一个原子中能量相近的不同类型（s, p, d, ）的几个原子轨道波函数可以相互叠加而组成同等数目的能量完全相同的杂化轨道。

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