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Chemical Bonding 化學鍵結

7. Chemical Bonding 化學鍵結. Chapter Goals. 1. Lewis Dot Formulas of Atoms 路易士電子點結構式 Ionic Bonding 離子鍵結 2. Formation of Ionic Compounds 形成離子化合物 Covalent Bonding 共價鍵結 3. Formation of Covalent Bonds 形成共價鍵 4. Bond Lengths and Bond Energies 鍵長及鍵能

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Chemical Bonding 化學鍵結

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  1. 7 Chemical Bonding 化學鍵結

  2. Chapter Goals 1. Lewis Dot Formulas of Atoms 路易士電子點結構式 Ionic Bonding 離子鍵結 2. Formation of Ionic Compounds 形成離子化合物 Covalent Bonding 共價鍵結 3. Formation of Covalent Bonds 形成共價鍵 4. Bond Lengths and Bond Energies 鍵長及鍵能 5. Lewis Formulas for Molecules and Polyatomic Ions 分子極多元子離子之路易士結構式 6. Writing Lewis Formulas: The Octet Rule 八隅體法則 7. Formal Charges 形式電荷 8. Writing Lewis Formulas: Limitations of the Octet Rule 八隅體法則之限制 9. Resonance 共振 10. Polar and Nonpolar Covalent Bonds 極性及非極性共價鍵結 11. Dipole Moments 偶極距 12. The Continuous Range of Bonding Types 鍵結形式之連續範圍

  3. Introduction • Attractive forces that hold atoms together in compounds are called chemical bonds (化合物中將原子拉進的吸引力稱之為化學鍵結) • The electrons involved in bonding are usually those in the outermost (valence) shell(與鍵結有關的電子通常是指最外層的電子)

  4. Introduction • Chemical bonds are classified into two types: • Ionic bonding離子鍵結 results from electrostatic attractions among ions, which are formed by the transfer of one or more electrons from one atom to another. • 離子鍵為陽離子與陰離子間之庫倫靜電力，陽離子由金屬失去電子形成，陰離子由非金屬得到電子形成。兩正負電荷之離子吸引力，稱為離子鍵。 • Ionic compound 離子化合物 NaCl • Covalent bonding共價鍵結 results from sharing one or more electron pairs between two atoms. • 價鍵為兩原子共用電子對之化學鍵，其結合原子皆為非金屬原子 • Covalent compound H2, Cl2

  5. Lewis Dot Formulas of Atoms原子的路易士電子點結構式 • Lewis dot formulas or Lewis dot representations are a convenient bookkeeping method for tracking valence electrons價電子. • The electrons in the outermost occupied shells(最外層的電子數) • s and p orbitals • Valence electrons are those electrons that are transferred or involved in chemical bonding(價電子是指與化學鍵結有關或轉移的電子) • They are chemically important

  6. Lewis Dot Formulas路易士結構 of Atoms Elements that are in the same periodic group have the same Lewis dot structures (同族元素具相同路易士結構) 1 electron in valence shell 5 electrons in valence shell Not as useful for the transition and inner transition elements (不適用於過渡元素)

  7. Ionic Bonding 離子鍵結 Formation of Ionic Compounds 形成離子化合物 • An ion is an atom or a group of atoms possessing a net electrical charge. • Ions come in two basic types: • positive (+) ions or cations • These atoms have lost 1 or more electrons. • negative (-) ions or anions • These atoms have gained 1 or more electrons.

  8. Formation of Ionic Compounds • Monatomic ions consist of one atom. • Examples: • Na+ sodium ion, Ca2+, Al3+ -- cations • Cl- chloride ion, O2-, N3- -- anions • Polyatomic ions contain more than one atom. • Examples: • NH4+ ammonium ion -- cation • NO2-, CO32-, SO42- sulfate ion – anions • The atoms of a polyatomic ion are held together by covalent bonds(多原子離子之原子間以共價鍵結)

  9. Ionic Bonding is the attraction of oppositely charged ions (cations and anions) in large numbers to form a solid. Such a solid compound is called an ionic solid. • Elements that have low electronegativities and low ionization enengy — metals (oxidozed; lose electrons to form cations) • Elements that have high electronegativities and very negative electron affinities — nonmetals (reduced; gain electrons to form anions) React Ionic compound

  10. Formation of Ionic Compounds • Reaction of Group IA Metals with Group VIIA Nonmetals 1A metal 7A nonmetal 2Li(s) + F2(g) 2LiF(s) Silveryellow white solid Solid gas with an 842oC melting point

  11. Formation of Ionic Compounds • The underlying reason for the formation of LiF lies in the electron configurations of Li and F. 1s2s2p Li  F   These atoms form ions with these configurations. Li+same configuration as [He] loss one electron F- same configuration as [Ne] gained one electron Li+ isoelectronic with He F+ isoelectronic with Ne 等電子離子

  12. Formation of Ionic Compounds • We can also use Lewis dot formulas to represent the neutral atoms and the ions they form.

  13. Formation of Ionic Compounds • The Li+ ion contains two electrons, same as the helium atom. • Li+ ions are isoelectronic 等電子離子 with helium. • The F- ion contains ten electrons, same as the neon atom. • F- ions are isoelectronic with neon. • Isoelectronic species contain the same number of electrons. • Most ionic compounds formed by reactions between representative metals 典型金屬and representative nonmetals典型非金屬

  14. Formation of Ionic Compounds • The reaction of potassium with bromine is a second example of a group IA metal with a Group VIIA non metal. • Write the reaction equation. 1A metal 7A nonmetal 2K(s) + Br2(g) 2KBr(s) ionic solid

  15. Formation of Ionic Compounds • We look at the electronic structures of K and Br. 4s4p K [Ar]  Br [Ar] and the d electrons The atoms form ions with these electronic structures. 4s4p K+ same configuration as[Ar] Br- same configuration as[Kr]

  16. Formation of Ionic Compounds • Write the Lewis dot formula representation for the reaction of K and Br.

  17. Formation of Ionic Compounds • There is a general trend evident in the formation of these ions. • Cations become isoelectronicwith the preceding noble gas. 之前的鈍氣 • Anions become isoelectronicwith the followingnoble gas.之後的鈍氣

  18. Formation of Ionic Compounds • In general for the reaction of IA metals and VIIA nonmetals, the reaction equation is: 2 M(s) + X2 2 M+ X-(s) • where M is the metals Li to Cs • and X is the nonmetals F to I. Electronically this is occurring. nsnp nsnp M   M+ X   X-

  19. Formation of Ionic Compounds • Next we examine the reaction of IIA metals with VIIA nonmetals. • This reaction forms mostly ionic compounds. • Notable exceptions are BeCl2, BeBr2, and BeI2 whichare covalent compounds. • One example is the reaction of Be and F2. Be(s)+ F2(g)BeF2(g)

  20. Formation of Ionic Compounds • The valence electrons in these two elements are reacting in this fashion. 2s2p2s2p Be [He]   Be2+ F [He]   F-  Next, draw the Lewis dot formula representation of this reaction.

  21. Formation of Ionic Compounds • The remainder of the IIA metals and VIIA nonmetals react similarly. • Symbolically this can be represented as: M(s) + X2 M2+ X2- M can be any of the metals Be to Ba. X can be any of the nonmetals F to Cl.

  22. Formation of Ionic Compounds • For the reaction of IA metals with VIA nonmetals, a good example is the reaction of lithium with oxygen. • The reaction equation is:

  23. Formation of Ionic Compounds • Draw the electronic configurations for Li, O, and their appropriate ions. 2s2p2s2p Li [He]   Li1+ O [He]   O2-  Draw the Lewis dot formula representation of this reaction.

  24. Formation of Ionic Compounds • The remainder of the IA metals and VIA nonmetals behave similarly. • Symbolically this can be represented as: 2 M (s) + X  M21+ X- M can be any of the metals Li to Cs. X can be any of the nonmetals O to Te.

  25. Formation of Ionic Compounds • The reaction of IIA metals and VA nonmetals also follows the trends that we have established in this chapter. • The reaction of calcium with nitrogen is a good example. • The reaction equation is: 3Ca(s)+ N2(g) Ca3N2 (s)

  26. Formation of Ionic Compounds • Draw the electronic representation of Ca, N, and their ions. 4s4p4s4p Ca [Ar]   Ca2+ 2s2p2s2p N [He]    N3-  • Draw the Lewis dot representation of this reaction.

  27. Formation of Ionic Compounds • Other IIA and VA elements behave similarly. • Symbolically, this reaction can be represented as: 3 M(s) + 2 X(g) M32+ X23- M can be the IIA elements Be to Ba. X can be the VA elements N to As.

  28. Formation of Ionic Compounds • d-transition Metal Ions • The outermost s electrons and energy level lower d electrons are always the first ones lost when transition metals form simple ions. 鈧 3d 4s 3d 4s Sc[Ar]    Sc3+[Ar] 3e- lost Zn[Ar]   Zn2+[Ar]   2e- lost • Most other 3d-transition metals can form at least two cations in their compounds. 3d 4s 3d 4s Co[Ar]      Co2+[Ar]    2e- lost Co[Ar]      Co3+[Ar]         3e- lost

  29. Formation of Ionic Compounds 3A • H, a nonmetal, forms ionic compounds with IA and IIA metals for example, LiH, KH, CaH2, and BaH2. Other hydrogen compounds are covalent. Group IA and IIA can form peroxide (contain O22- ion ) or superoxide (contain O2- ion). The peroxide and superoxide ions contain atoms that are covalently bonded to one another

  30. Group IA and IIA can form peroxide (contain O22- ion ) or superoxide (contain O2- ion). The peroxide and superoxide ions contain atoms that are covalently bonded to one another 31

  31. Formation of Ionic Compounds • Ionic compounds form extended three dimensional arrays of oppositely charged ions. • Ionic compounds have high melting points because the coulomb force庫侖力, which holds ionic compounds together, is strong.

  32. Formation of Ionic Compounds • Coulomb’s Law describes the attraction of positive ions for negative ions due to the opposite charges.

  33. Formation of Ionic Compounds • Small ions with high ionic charges havelarge Coulombic forces of attraction. • Large ions with small ionic charges have small Coulombic forces of attraction. • Use this information, plus the periodicity rules from Chapter 6, to arrange these compounds in order of increasing attractions among ions KCl, Al2O3, CaO

  34. Covalent Bonding 共價鍵結 • Covalent bonds are formed when atomsshare electrons. ItOccurs when the electronegativity difference between elements (atoms) is zero or relativity small(電負度幾乎沒差) • The bonds between atoms within a molecule (intramolecular bonds 分子內鍵結) are relatively strong, but the force of attraction between molecules (intermolecular forces 分子間鍵結) are relatively weak lower melting and boiling points than ionic compound(較離子化合物的熔點及沸點低) • If the atoms share 2 electrons a single covalent bond is formed(若原子共享2個電子則形成單一共價鍵) • If the atoms share 4 electrons a double covalent bond is formed(若原子共享4個電子則形成二個共價鍵) • If the atoms share 6 electrons a triple covalent bond is formed(若原子共享6個電子則形成三個共價鍵) • The attraction between the electrons is electrostatic in nature • The atoms have a lower potential energy when bound.

  35. Formation of Covalent Bonds • Representation of the formation of an H2 molecule from H atoms. The electron of each atom is attracted by the positively charged nucleus of the other atom, so the electron density begins to shift (blue arrows)(電子受到帶正電的原子核的吸引, 電子團開始變化 ) The electron clouds of the two atoms repel one another, and so do the nuclei of the two atoms (Red arrows)(兩個原子亦會有排斥力) The two 1s orbitals overlap

  36. Formation of Covalent Bonds • This figure shows the potential energy of an H2 molecule as a function of the distance between the two H atoms. 太靠近,產生斥力 太遠,引力太小,無鍵結 斥力與引力達成平衡,穩定的排列 For any covalent bond there is an internuclear distance where the attractive and repulsive forces balance. This distance is the bond length(彼此間的距離稱鍵長). The energy difference is thebond energy(能量的差異稱為鍵能)

  37. Bond dissociation energy 鍵離解能 Bond energy 鍵能

  38.   H +F: H:F: or HF       :F +F: :F:F: or F2     Formation of Covalent Bonds We can use Lewis dot formulas to show covalent bond formation. • H molecule formation representation. H +H H:H or H-H 2. HF molecule formation 3. F2 molecule formation

  39.      H O  H H or Dot formula Dash formula線結構式 • H2O H O • CO2 • NH4+ N: 5 electrons H: 1 electrons Total 9 electrons

  40. Writing Lewis Formulas:The Octet Rule八隅體法則 • N - A = S rule Simple mathematical relationship to help us write Lewis dot formulas. • N = number of electrons needed to achieve a noble gas configuration. (要達到鈍氣組態的電子數目,通常為8) • N usually has a value of 8 for representative elements. • N has a value of 2 for H atoms. • A = number of electrons available in valence shells of the atoms.(原子的價電子數) • A is equal to the periodic group number for each element. • A is equal to 8 for the noble gases. • S = number of electrons shared in bonds.(形成鍵結可共享的電子數) • A-S = number of electrons in unshared, lone, pairs.(不共享的電子數,又稱孤電子對)

  41.       F F −     F F            N N N N Lewis Formulas for Molecules and Polyatomic Ions • First, we explore Lewis dot formulas of homonuclear diatomic molecules. • Two atoms of the same element. • Hydrogen molecule, H2. N=2x2=4 e- needed A=2x1=2 e- available S=N-A=2 e- shared H:H or H−H 2. Fluorine, F2 N=2x8=16 e- needed A=2x7=14 e- available S=N-A=2 e- shared or • Nitrogen, N2 N=2x8=16 e- needed A=2x5=10 e- available S=N-A=6 e- shared 2 molecule with 6 electrons 3 covalent bond or

  42.      F F H H −        Cl Cl H H −        Br Br H H −   Lewis Formulas for Molecules and Polyatomic Ions • Next, look at heteronuclear diatomic molecules. • Two atoms of different elements. • Hydrogen halides 鹵化氫are good examples. • hydrogen fluoride, HF N=1x2+1x8=10 e- needed A=1x1+1x7=8 e- available S=N-A=2 e- shared or 2. hydrogen chloride, HCl or 3. hydrogen bromide, HBr or

  43.      H O  H H H O     H H N H N H  H H Lewis Formulas for Molecules and Polyatomic Ions • Now we will look at a series of slightly more complicated heteronuclear molecules. • Water, H2O N=2x2+1x8=12 e- needed A=2x1+1x6=8 e- available S=N-A=4 e- shared or • Ammonia molecule , NH3 N=3x2+1x8=14 e- needed A=3x1+1x5=8 e- available S=N-A=6 e- shared or

  44. Lewis Formulas for Molecules and Polyatomic Ions • Lewis formulas can also be drawn for molecular ions. • One example is the ammonium ion , NH4+. N=1x8+4x2=16 e- need 1C atom 4H atom A=1x5+4x1-1=8e- available 1C atom 4H atom (+1charge) S=N-A=16-8=8e- share • Notice that the atoms other than H in these molecules have eight electrons around them.

  45. A Guild to Writing Lewis Formulas • Select a reasonable (symmetrical) “skeleton” for the molecule or polyatomic ion 選擇最合理分子當作骨架 • The least electronegative element is usually the central element, except the H電負度最小者通常為中心分子,氫除外 • Carbon bonds to two, three or four atoms, but never more than four碳可與2,3及4個原子鍵結 • Oxygen atoms do not bond to each other except in(氧原子不會鍵結在一起,除非) • O2 and O3 • hydrogen peroxide, H2O2, and the peroxide contain the O22- group • Superoxide, which contain the O2- group • In ternary oxoacids, hydrogen usually bond to an O atom, not to the central atom, HNO2三元含氧酸中,氫通常與氧鍵結 • For those have more than one central atom, the most symmetrical skeletons possible are used, such as C2H4, P2O74-

  46. A Guild to Writing Lewis Formulas 2. Calculate N, the number of valence shell electrons needed by all atoms in the molecule or iron to achieve noble gas configurations PF3 N=1x8(P atom)+3x8(F atoms)= 32e- need CH3OH N=1x8(C atom)+4x2(H atoms)+1x8 (O atom) = 24e- need NO3-N=1x8(N atom)+3x8(O atoms)= 32e- need Calculate A, the number of electrons available in the outer shells of all the atoms PF3 A=1x5(P atom)+3x7(F atoms)= 26e- available CH3OH A=1x4(C atom)+4x1(H atoms)+1x6 (O atom) = 14e- available NO3-A=1x5(N atom)+3x6(O atoms)+1 (for 1-charge) = 24e- available

  47. A Guild to Writing Lewis Formulas Calculate S, total number of electrons shared in the molecule or ion, using the relationship S=N-A PF3 S=N-A= 32-26=6 e- shared (3 pairs of e-shared) CH3OH S=24-14= 10 e- shared (5 pairs of e-shared) NO3-S= 32-24=8 e- hared (4 pairs of e-shared)

  48. 3. Place the S electron in to the skeleton as shared pairs. Use double and triple bonds only when necessary. S=6 (3 pairs of e-shared) S=10 (5 pairs of e-shared) S=8 (4 pairs of e-shared)

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