Chapter 8 Basic Concepts of Chemical Bonding

Transcription

1 Sec$on 8.1 Types of Chemical Bonds Chapter 8 Basic Concepts of Chemical Bonding

2 Chapter 8 Ques$ons to Consider What is meant by the term chemical bond? Why do atoms bond with each other to form compounds? How do atoms bond with each other to form compounds? Copyright Cengage Learning. All rights reserved 2

3 Sec$on 8.1 Types of Chemical Bonds A Chemical Bond No simple, and yet complete, way to define this. Forces that hold groups of atoms together and make them func$on as a unit. A bond will form if the energy of the aggregate is lower than that of the separated atoms. Copyright Cengage Learning. All rights reserved 3

4 Sec$on 8.1 Types of Chemical Bonds Types of Bonds Ionic Bonding electrons are transferred Covalent Bonding electrons are shared equally by nuclei What about intermediate cases? Copyright Cengage Learning. All rights reserved 4

5 Sec$on 8.1 Types of Chemical Bonds Polar Covalent Bond Unequal sharing of electrons between atoms in a molecule. Results in a charge separa$on in the bond (par$al posi$ve and par$al nega$ve charge). Copyright Cengage Learning. All rights reserved 5

6 Sec$on 8.2 Electronega7vity Electronega$vity The ability of an atom in a molecule to awract shared electrons to itself. For a molecule HX, the rela$ve electronega$vi$es of the H and X atoms are determined by comparing the measured H X bond energy with the expected H X bond energy. Copyright Cengage Learning. All rights reserved 6

7 Sec$on 8.2 Electronega7vity Electronega$vity On the periodic table, electronega$vity generally increases across a period and decreases down a group. The range of electronega$vity values is from 4.0 for fluorine (the most electronega$ve) to 0.7 for cesium (the least electronega$ve). Copyright Cengage Learning. All rights reserved 7

8 Sec$on 8.2 Electronega7vity The Pauling Electronega$vity Values Copyright Cengage Learning. All rights reserved 8

9 Sec$on 8.2 Electronega7vity Electronega$vity and Bond Type Copyright Cengage Learning. All rights reserved

10 Sec$on 8.3 Bond Polarity and Dipole Moments Polarity - Dipole Moment Property of a molecule whose charge distribu$on can be represented by a center of posi$ve charge and a center of nega$ve charge. Use an arrow to represent a dipole moment. Point to the nega$ve charge center with the tail of the arrow indica$ng the posi$ve center of charge. Copyright Cengage Learning. All rights reserved 10

11 Sec$on 8.3 Bond Polarity and Dipole Moments Polarity - Dipole Moment 11

12 Sec$on 8.3 Bond Polarity and Dipole Moments No Net Dipole Moment (Dipoles Cancel) Copyright Cengage Learning. All rights reserved 12

13 Sec$on 8.3 Bond Polarity and Dipole Moments Dipole Moment - Calcula$on A dipole moment ( µ ) produced by two equal but opposite charges separated by a distance, r, is calculated: µ = Qr It is measured in debyes (D).

14 Sec$on 8.4 Ions: Electron Configura7ons and Sizes Electron Configura$ons in Stable Compounds Atoms in stable compounds usually have a noble gas electron configura$on. Copyright Cengage Learning. All rights reserved 14

15 Sec$on 8.4 Ions: Electron Configura7ons and Sizes Electron Configura$ons in Stable Compounds When two nonmetals react to form a covalent bond, they share electrons in a way that completes the valence electron configura$ons of both atoms. When a nonmetal and a representa7ve-group metal react to form a binary ionic compound, the ions form so that the valence electron configura$on of the nonmetal achieves the electron configura$on of the next noble gas atom. The valence orbitals of the metal are emp$ed. Copyright Cengage Learning. All rights reserved 15

16 Sec$on 8.4 Ions: Electron Configura7ons and Sizes Isoelectronic Series A series of ions/atoms containing the same number of electrons. O 2-, F -, Ne, Na +, Mg 2+, and Al 3+ Ca#on is always smaller than atom from which it is formed. Anion is always larger than atom from which it is formed. Copyright Cengage Learning. All rights reserved 16

17 Sec$on 8.4 Ions: Electron Configura7ons and Sizes Ionic Radii (in ppm) Copyright Cengage Learning. All rights reserved 17

18 Sec$on 8.4 Ions: Electron Configura7ons and Sizes Periodic Table Allows Us to Predict Many Proper$es Trends for: Atomic size, ion radius, ioniza$on energy, electronega$vity Electron configura$ons Formula predic$on for ionic compounds Covalent bond polarity ranking Copyright Cengage Learning. All rights reserved 18

19 Sec$on 8.5 Energy Effects in Binary Ionic Compounds Energy Effects in Binary Ionic Compounds What are the factors that influence the stability and the structures of solid binary ionic compounds? How strongly the ions awract each other in the solid state is indicated by the lahce energy. Copyright Cengage Learning. All rights reserved 19

20 Sec$on 8.5 Energy Effects in Binary Ionic Compounds Lahce Energy The change in energy that takes place when separated gaseous ions are packed together to form an ionic solid. Lattice energy = k = propor$onality constant Q 1 and Q 2 = charges on the ions QQ k r 1 2 r = shortest distance between the centers of the ca$ons and anions Copyright Cengage Learning. All rights reserved 20

21 Sec$on 8.5 Born-Haber Cycle for determining Lahce Energy of LiF Energy Effects in Binary Ionic Compounds 21 ΔH overall = ΔH 1 + ΔH 2 + ΔH 3 + ΔH 4 + ΔH 5

22 Sec$on 8.5 Energy Effects in Binary Ionic Compounds Forma$on of an Ionic Solid 1.Sublima$on of the solid metal. M(s) M(g) [endothermic] 2. Ioniza$on of the metal atoms. M(g) M + (g) + e - 3. Dissocia$on of the nonmetal. 1 /2X 2 (g) X(g) [endothermic] [endothermic] Copyright Cengage Learning. All rights reserved 22

23 Sec$on 8.5 Energy Effects in Binary Ionic Compounds Forma$on of an Ionic Solid (con$nued) 4 Forma$on of nonmetal ions in the gas phase. X(g) + e - X - (g) [exothermic] 5.Forma$on of the solid ionic compound. M + (g) + X - (g) MX(s) [quite exothermic] Copyright Cengage Learning. All rights reserved 23

24 Sec$on Comparing 8.5 Energy Changes Energy Effects in Binary Ionic Compounds

25 Sec$on 8.6 Par7al Ionic Character of Covalent Bonds Par$al Ionic Character of Covalent Bonds No bonds reach 100% ionic character even with compounds that have the maximum possible electronega$vity difference. measured dipole moment of X Y % ionic character of a bond = 100% + calculated dipole moment of X Y Copyright Cengage Learning. All rights reserved 25

26 Sec$on 8.6 Par7al The rela$onship Ionic Character between of the Covalent ionic character Bonds of a covalent bond and the electronega$vity difference of the bonded atoms Copyright Cengage Learning. All rights reserved 26

27 Sec$on 8.6 Par7al Ionic Character of Covalent Bonds Opera$onal Defini$on of Ionic Compound Any compound that conducts an electric current when melted will be classified as ionic. Copyright Cengage Learning. All rights reserved 27

28 Sec$on 8.7 The Covalent Chemical Bond: A Model The Covalent Chemical Bond : A Model Models are awempts to explain how nature operates on the microscopic level based on experiences in the macroscopic world. Copyright Cengage Learning. All rights reserved 28

29 Sec$on 8.7 The Covalent Chemical Bond: A Model Fundamental Proper$es of Models 1. A model does not equal reality. 2. Models are oversimplifica$ons, and are therefore open wrong. 3. Models become more complicated and are modified as they age. 4. We must understand the underlying assump$ons in a model so that we don t misuse it. 5. When a model is wrong, we open learn much more than when it is right. Copyright Cengage Learning. All rights reserved 29

30 Sec$on 8.8 Covalent Bond Energies and Chemical Reac7ons Covalent Bond Energies To break bonds, energy must be added to the system (endothermic, energy term carries a posi$ve sign). To form bonds, energy is released (exothermic, energy term carries a nega$ve sign). Copyright Cengage Learning. All rights reserved 30

31 Sec$on 8.8 Covalent Bond Energies and Chemical Reac7ons Covalent Bond Energies ΔH = Σn D(bonds broken) Σn D(bonds formed) D represents the bond energy per mole of bonds (always has a posi$ve sign). Copyright Cengage Learning. All rights reserved 31

32 Sec$on 8.8 Covalent Bond Energies and Chemical Reac7ons Example From the figure on the last slide CH 4 (g) + Cl 2 (g) CH 3 Cl(g) + HCl(g) In this example, one C H bond and one Cl Cl bond are broken; one C Cl and one H Cl bond are formed.

33 Sec$on 8.8 Covalent Bond Energies and Chemical Reac7ons Answer ΔH = [D(C H) + D(Cl Cl)] [D(C Cl) + D(H Cl)] = [(413 kj) + (242 kj)] [(328 kj) + (431 kj)] = (655 kj) (759 kj) = 104 kj

34 Sec$on 8.8 Covalent Bond Energies and Chemical Reac7ons Bond Enthalpy and Bond Length We can also measure an average bond length for different bond types. As the number of bonds between two atoms increases, the bond length decreases.

35 Sec$on 8.9 The Localized Electron Bonding Model The Localized Electron Bonding Model (LE) A molecule is composed of atoms that are bound together by sharing pairs of electrons using the atomic orbitals of the bound atoms. Copyright Cengage Learning. All rights reserved 35

36 Sec$on 8.9 The Localized Electron Bonding Model The Localized Electron Bonding Model (LE) Electron pairs are assumed to be localized on a par$cular atom or in the space between two atoms: Lone pairs pairs of electrons localized on an atom Bonding pairs pairs of electrons found in the space between the atoms Copyright Cengage Learning. All rights reserved 36

37 Sec$on 8.9 The Localized Electron Bonding Model The Localized Electron Bonding Model (LE) 1. Descrip$on of valence electron arrangement (Lewis structure). 2. Predic$on of geometry (VSEPR model). 3. Descrip$on of atomic orbital types used by atoms to share electrons or hold lone pairs. Copyright Cengage Learning. All rights reserved 37

38 Sec$on 8.10 Lewis Structures Lewis Structure Shows how valence electrons are arranged among atoms in a molecule. Reflects central idea that stability of a compound relates to noble gas electron configura$on. Copyright Cengage Learning. All rights reserved 38

39 Sec$on 8.10 Lewis Duet Structures Rule Hydrogen forms stable molecules where it shares two electrons.

40 Sec$on 8.10 Lewis Structures Octet Rule Elements form stable molecules when surrounded by eight electrons. Copyright Cengage Learning. All rights reserved

41 Sec$on 8.10 Lewis Structures Single Covalent Bond A covalent bond in which two atoms share one pair of electrons. H H Copyright Cengage Learning. All rights reserved 41

42 Sec$on 8.10 Lewis Structures Double Covalent Bond A covalent bond in which two atoms share two pairs of electrons. O=C=O Copyright Cengage Learning. All rights reserved 42

43 Sec$on 8.10 Lewis Structures Triple Covalent Bond A covalent bond in which two atoms share three pairs of electrons. N N Copyright Cengage Learning. All rights reserved 43

44 Sec$on 8.10 Lewis Structures Steps for Wri$ng Lewis Structures 1. Sum the valence electrons from all the atoms. 2. Use a pair of electrons to form a bond between each pair of bound atoms. 3. Atoms usually have noble gas configura$ons. Arrange the remaining electrons to sa$sfy the octet rule (or duet rule for hydrogen). Copyright Cengage Learning. All rights reserved 44

45 Sec$on 8.10 Lewis Structures Steps for Wri$ng Lewis Structures 1. Sum the valence electrons from all the atoms. (Use the periodic table.) Example: H 2 O 2 (1 e ) + 6 e = 8 e total Copyright Cengage Learning. All rights reserved 45

46 Sec$on 8.10 Lewis Structures Steps for Wri$ng Lewis Structures 2. Use a pair of electrons to form a bond between each pair of bound atoms. Example: H 2 O H O H Copyright Cengage Learning. All rights reserved 46

47 Sec$on 8.10 Lewis Structures Steps for Wri$ng Lewis Structures 3. Atoms usually have noble gas configura$ons. Arrange the remaining electrons to sa$sfy the octet rule (or duet rule for hydrogen). Examples: H 2 O, PBr 3, and HCN H O H Br Br P Br H C N Copyright Cengage Learning. All rights reserved 47

48 Sec$on 8.11 Excep7ons to the Octet Rule Boron tends to form compounds in which the boron atom has fewer than eight electrons around it (it does not have a complete octet). BH 3 = 6e H H B H Copyright Cengage Learning. All rights reserved 48

49 Sec$on 8.11 Excep7ons to the Octet Rule When it is necessary to exceed the octet rule for one of several third-row (or higher) elements, place the extra electrons on the central atom. SF 4 = 34e AsBr 5 = 40e F Br Br F S F Br As Br F Br Copyright Cengage Learning. All rights reserved 49

50 Sec$on 8.11 Excep7ons to the Octet Rule Let s Review C, N, O, and F should always be assumed to obey the octet rule. B and Be open have fewer than 8 electrons around them in their compounds. Second-row elements never exceed the octet rule. Third-row and heavier elements open sa$sfy the octet rule but can exceed the octet rule by using their empty valence d orbitals. Copyright Cengage Learning. All rights reserved 50

51 Sec$on 8.11 Excep7ons to the Octet Rule Let s Review When wri$ng the Lewis structure for a molecule, sa$sfy the octet rule for the atoms first. If electrons remain aper the octet rule has been sa$sfied, then place them on the elements having available d orbitals (elements in Period 3 or beyond). Copyright Cengage Learning. All rights reserved 51

52 Sec$on 8.12 Resonance More than one valid Lewis structure can be wriwen for a par$cular molecule. NO 3 = 24e O N O O N O O N O O O O Copyright Cengage Learning. All rights reserved 52

53 Sec$on 8.12 Resonance Actual structure is an average of the resonance structures. Electrons are really delocalized they can move around the en$re molecule. O O O O O O N N N O O O Copyright Cengage Learning. All rights reserved 53

54 Sec$on 8.12 Resonance Formal Charge Used to evaluate nonequivalent Lewis structures. Atoms in molecules try to achieve formal charges as close to zero as possible. Any nega$ve formal charges are expected to reside on the most electronega$ve atoms. Copyright Cengage Learning. All rights reserved 54

55 Sec$on 8.12 Resonance Formal Charge Formal charge = (# valence e on free neutral atom) (# valence e assigned to the atom in the molecule) Assume: Lone pair electrons belong en$rely to the atom in ques$on. Shared electrons are divided equally between the two sharing atoms. Copyright Cengage Learning. All rights reserved 55

56 Sec$on 8.12 Resonance Rules Governing Formal Charge To calculate the formal charge on an atom: 1. Take the sum of the lone pair electrons and one-half the shared electrons. 2. Subtract the number of assigned electrons from the number of valence electrons on the free, neutral atom. Copyright Cengage Learning. All rights reserved 56

57 Sec$on 8.12 Resonance Rules Governing Formal Charge The sum of the formal charges of all atoms in a given molecule or ion must equal the overall charge on that species. Copyright Cengage Learning. All rights reserved 57

58 Sec$on 8.12 Resonance Rules Governing Formal Charge If nonequivalent Lewis structures exist for a species, those with formal charges closest to zero and with any nega$ve formal charges on the most electronega$ve atoms are considered to best describe the bonding in the molecule or ion. O C O O C O Copyright Cengage Learning. All rights reserved 58