Princ of Chemistry I
Princ of Chemistry I CHEM 1211K
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Chapter 5 Covalent Bonds and Molecular Structure Molecules and the Covalent Bond Covalent Bond A bond that results from the sharing of electrons between atoms Molecule The unit of matter held together by covalent bonds Similarly two atoms are joined together when 39 i 39 39 t 39 H A both nuclei tug on The two teams are joined together because both are tugging on the same rope the same electrons dots amp ii 4 Copyright 2010 Pearson Prentice Hall Inc Molecules and the Covalent Bond Attractive Electron cloud Repulswe Nucleus The nucleus electron attractions are greater than the nucleus nucleus and electron electron repulsions resulting in a net attractive force that binds the atoms together Copyright 2010 Pearson Prentice Hall Inc Chapter 53 Molecules and the Covalent Bond If the atoms are too close strong repulsions occur HH gt too Close If the atoms are too far apart attract Jons are weak and no bonding occurs 0 VH3 H too far When the atoms are optimally separated energy is at a minimum l Bond length Lnternuclear gt 74 pm distance Copyright 2010 Pearson Prentice Hall Inc Strengths of Covalent Bonds TABLE 51 Average Bond Dissociation Energies D kJmol H H 436a C H 410 N H 390 O H 460 F F H C 410 C C 350 N C 300 O C 350 Cl Cl H F 570a C F 450 N F 270 O F 180 Br Br H Cl 432a C Cl 330 N C1 200 0 0 200 1 1 H Br 366a C Br 270 N Br 240 O Br 210 S F H I 298a C I 240 N I 0 1 220 S Cl H N 390 C N 300 N N 240 O N 200 S Br H O 460 C 0 350 N O 200 0 0 180 5 5 H s 340 C 5 260 N S O S Dduhq ecovdentbondsb CC 728 39 CEC 965 CO 732 00 4983 NEN 159a 243a 193a 151a 310 250 210 225 945a aExact value bWe ll discuss multiple covalent bonds in Section 56 Copyright 2010 Pearson Prentice Hall Inc A Comparison of Ionic and Covalent Bonds TABLE 52 Some Physical Properties of NaCl and HCI Property NaCl HCI Formula mass 5844 amu 3646 amu Physical appearance White solid Colorless gas Type of bond Melting point 801 C 115 C Boiling point 1465 C 849 C Copyright 2010 Pearson Prentice Hall Inc Chapter 56 7 I A Chapter 57 Copyright 2010 Pearson Prentice Hall Inc Polar Covalent Bonds Electronegativity Electronegativity The ability of an atom in a molecule to attract the shared electrons in a covalent bond Ionic Polar covalent Nonpolar covalent full charges partial charges electronically symmetrical Polar covalent bonds have an unsymmetrical electron distribution in which the bonding electrons are attracted more strongly by one atom than the other Copyright 2010 Pearson Prentice Hall Inc Copyright 2010 Pearson Prentice Hall Inc Chapter 58 Polar Covalent Bonds Electronegativity Electronegativity increases from left to right Electronegativity decreases from top to bottom 08 10 12 14 16 18 19 22 22 22 l 1 11 17 22 22 Copyright 2010 Pearson Prentice Hall Inc Chapter 59 Polar Covalent Bonds Electronegativity 5H 15 H 2C1 A polar covalent bond The bonding electrons are attracted more strongly by C1 than by H C1 3 C1 A nonpolar covalent bond Copyright 2010 Pearson Prentice Hall Inc Which of the bonds in chlorofluoromethanol is most polar EDH cl CI C H Br H 21 He Li Be 13 C N O F Ne 10 15 20 25 30 35 40 Na Mg A1 Si P S C1 Ar 09 12 15 18 21 25 30 K Ca Ga Ge As Se Br Kr 08 10 16 18 20 24 28 Rb Sr 111 Sn Sb Te I Xe 08 10 17 18 19 21 25 Cs Ba T1 Pb Bi Po At Rn 07 09 18 19 19 20 21 Classify the O H bond in CH3OH as ionic polar 1A 2B 3C 4D covalent or nonpolar covalent Ionic Polar covalent Nonpolar covalent None of the above Naming Molecular Compounds TABLE 53 Numerical Prefixes for Naming Compounds Prefix Meaning mono 1 di 2 tri 3 tetra 4 penta 5 hexa 6 gt hepta 7 octa 8 nona 9 deca 10 Copyright 2010 Pearson Prentice Hall Inc Copyright Z lr i Pearson Prenii39tiice Haiti Enact Because nonmetals often combine with one another in different proportions to form different compounds numerical prefixes are usually included in the names of binary molecular compounds Chapter 513 Naming Molecular Compounds N204 The first element listed has The second element listed lower electronegativity has higher electronegativity The prefix is added to the front of each to indicate the number of each atom l dinitrogen tetraoxide Cmimirrigi nt Milli earsm i Premite Haii EE DCv Chapter 514 ElectronDot Structures ElectronDot Structures Lewis Structures A representation of an atom s valence electrons by using dots and indicates by the placement of dots how the valence electrons are distributed in the molecule An electronpalr bond H39 39H gt HiH Two hydrogen A hydrogen atoms molecule This hydrogen shares and this hydrogen an electron pair shares an electron pair O Copyright 2010 Pearson Prentice Hall Inc ElectronDot Structures A bonding pair F F H FF2lt Alonepair Two F atoms An F2 molecule seven valence electrons each F is surrounded per atom by eight valence electrons Copyright 2010 Pearson Prentice Hall Inc Octet Rule Maingroup elements tend to undergo reactions that leave them with eight outershell electrons Exceptions Be and B Chapter 516 ElectronDot Structures Two electron pairs R a double bond gt 339 gt NN Three electron pairs a triple bond Copyright 2010 Pearson Prentice Hall Inc Chapter 51 7 Which of the following electron dot diagrams is correct E ElectronDot Structures of Polyatomic Molecules Step 1 Valence Electrons Count the total number of valence electrons for all atoms in the molecule Add one additional electron for each negative charge in an anion or subtract one for each positive charge in a cation TABLE 54 Covalent Bonding for Second Row Elements Number of Valence Number of Group Electrons Bonds Example 3A 3 3 BH3 4A 4 4 CH4 5A 5 3 NH3 6A 6 2 H20 7A 7 1 HF 8A 8 0 Ne Chapter 519 Copyright 2010 Pearson Prentice Hall Inc ElectronDot Structures of Polyatomic Molecules Step 2 Connect Atoms Draw lines to represent bonds between atoms For hydrogen and second row atoms use the number of bonds listed below For third row and greater atoms they may have more bonds than predicted by the octet rule The least electronegative atom is usually the central atom TABLE 54 Number of Valence Group Electrons Number of Bonds Covalent Bonding for Second Row Elements Example 3A 4A 5A 6A 7A 8A 0 g OO IOU39IHgtUJ lt a 3 m 5 OHNUJ BH3 CH4 NH3 Chapter SIZU ElectronDot Structures of Polyatomic Molecules Step 3 Assign Electrons to the Terminal Atoms Subtract the number of electrons used for bonding in the previous step from the total number determined in step 1 Complete each terminal atom s octet except for hydrogen Step 4 Assign Electrons to the Central Atom If unassigned electrons remain after step 3 place them on the central atom Eopwrigi nt 2 Ztlillr Pearson s Prentice Hall lino Chapter 521 ElectronDot Structures of Polyatomic Molecules Step 5 Multiple Bonds If no unassigned electrons remain after step 3 but the central atom does not yet have an octet use one or more lone pairs of electrons from a neighboring atom to form a multiple bond either a double or a triple Copyright 2am earson Prentice Hall lino Chapter 522 ElectronDot Structures of Polyatomic Molecules Draw an electrondot structure for H20 Step 1 21 6 8 valence electrons H T I 5 Step 2 Q H Step 4 O H bonding pair of electrons lone pair of electrons COEMJ HQE W 2am earson Premium Hall inc Chapter 523 ElectronDot Structures of Polyatomic Molecules Draw an electrondot structure for CCI4 Step 1 4 47 32 valence electrons O 0 CI 0039 o o I o o Step2 CI C CI Step 3 CI C CI I O O I O O 039 91 Cmmrr gm mm earem n Pu39emfate Harlin Emu Chapter 524 ElectronDot Structures of Polyatomic Molecules Draw an electrondot structure for H3O Step 1 31 6 1 8 valence electrons 1 H H Step2 H O H Step4 H O H O Emplrr g nt lt1 mm Peareom Pl em tlce Hall Zinc Chapter 525 ElectronDot Structures of Polyatomic Molecules Draw an electrondot structure for CHZO Step 1 4 21 6 12 valence electrons C 5 Step2 H C H Step5H C H O O Step3 H C H HCH Cmmrr g nt mm earepm PE EW EGEE Harlin Erna Chapter 526 ElectronDot Structures of Polyatomic Molecules Draw an electrondot structure for SF6 Step 1 6 47 34 valence electrons F quotFquot F F 1 g I Step 2 8 Step 339 S C F I F F I F F o O O Cmmrr g nt mm Fearem l PE EW EEEE Hall Erna Chapter 527 ElectronDot Structures of Polyatomic Molecules Draw an electrondot structure for ICI3 Step 1 7 37 28 valence electrons O Cquot O 0 0 CI CI CI CI Step2 I Step4 I 0 O JCT 390 00 10 0 Step 3 39 I 39 Cmmrr g nt mm earem l PE EW EEEE Hall Erna Chapter 528 ElectronDot Structures and Resonance Draw an electrondot structure for 03 Step 1 36 18 valence electrons 0 O 0 Step2 O O O Step4O O O C 0 0 O 0 Step3 O O O Step5 020 0 0 C O C Cmmrrr g nt Milli earsmn Marriage Harlin inc Chapter 529 ElectronDot Structures and Resonance Move a lone pair from this oxygen 0 O C C O 0 Step 4 O O O Or move a lone pair from this oxygen C 0 0 O OZO O O OZO C O C C Resonance Cmpyr g tt 20m Pearsm n PE QW ECE Ha line Chapter 530 Formal Charges F I of of of C rma valence e39 bonding nonbonding arge in free atom e39 e39 Calculate the formal charge on each atom in 03 O O o o 39oO39 oo 6440 6621 6261 Comrr gm 2am eareom Pu39emface Harlin Emu Chapter 531 Formal charge guidelines A Lewis structure with no formal char es is generally better than one WIth formal c arges Small formal charges are generally better than large formal charges Negative formal charges should be on the more electronegatIve atoms Copyright Z 39 earson Prentice Hall lino Chapter 532 Identify the best structure for the isocyanate ion below mLEN6 2 mLCEN 1 1 1 0 1 L NEOJ 3 1 1 Copyright McGraw Hill 2009 Copyright MCGa39awiltll 2069 Below are three resonance structures for Cdeprotonated acetamide ie an anion c c c HZC NHZ H2C qu Hzc39 QNH2 A B C Which of the following is the correct order of best to worst resonance structures based on formal charges aAgtBgtC bAgtCgtB cBgtAgtC dBgtCgtA eCgtBgtA Molecular Shapes The VSEPR Model VSEPR ValenceShell ElectronPair Repulsion model Electrons in bonds and in lone pairs can be thought of as charge clouds that repel one another and stay as far apart as possible this causes molecules to assume specific shapes Working from an electrondot structure count the number of charge clouds on the central atom and then determine the molecular shape Copyright mm earson Prentice Ha i lino Chapter 535 Molecular Shapes The VSEPR Model Two Charge Clouds 1800 A C02 molecule is linear with a bond angle of 180 C An HCN molecule 1800 is linear with a bond A angle of 180 H CENi Copyright 2010 Pearson Prentice Hall Inc Chapter 536 Molecular Shapes The VSEPR Model Three Charge Clouds 122quot i H 117 C O A formaldehyde molecule is H 39 trigonal planar with bond Top Vlew Rf angles of roughly 120 Ha x C O 7 H Side View 0 OS 9 A11 302 molecule is bent Top View with a bond angle of approximately 120 a S quot Side View Chapter 537 Copyright 2010 Pearson Prentice Hall Inc Molecular Shapes The VSEPR Model Four Charge Clouds H 50 A methane molecule is I 10939 l tetrahedral with bond C 7 I angles of 1095 H H quot H w An ammonia molecule is 39 trigonal pyramidal with H bond angles of 107 HK L1 1070 A water molecule is 39 bent with a bond H I 3 angle of 1045quot KL 1 10450 H My Copyright 2010 Pearson Prentice Hall Inc apter 538 TABLE 55 Mo ecular Geometry Around Atoms mm 2 3 4 5 and 6 Ch ds eClou Nuluher Numbu Numbu of Lane m Barge Muccuhr uf Bands Fain Clouds Genmeh y 2 u 2 7 Lmear Tnzuml 3 u pmw 3 1 399 Bcnl H 4 quot T ha I n em a m 7 H 396 I H a r c o P 39 Hem 339 c heater ssu TABLE 55 Continued N umber N umber Number of Lane of Charge Molecular of Bonds Pairs Clouds Geometry Example r Cr r I 5 0 Tugonal PC bipyramidal C17 Cl r T O F 4 1 Seesaw S 3 F I F 5 F 3 2 T shaped 6 C17 F 2 3 Lmear I I I 1 39 39 O r a lt1 6 0 O t h d I tfo V7 0 F r Square 5 1 6 pyramidal 74 2 Square planar Chapter 540 Copyright 2010 Pearson Prentice Hall Inc Valence Bond Theory Valence Bond Theory A quantum mechanical model which shows how electron pairs are shared in a covalent bond 15 Is H2 molecule Copyright 2010 Pearson Prentice Hall Inc sigma bonds F 2FC FF 2p F2 molecule Copyright 2010 Pearson Prentice Hall Inc Copyright 2010 Pearson Prentice Hall Incl Chapter 541 Valence Bond Theory Valence Bond Theory A quantum mechanical model which shows how electron pairs are shared in a covalent bond Covalent bonds are formed by overlap of atomic orbitals each of which contains one electron of opposite spin Each of the bonded atoms maintains its own atomic orbitals but the electron pair in the overlapping orbitals is shared by both atoms The greater the amount of overlap the stronger the bond Copyright 2pm earsm i Prentice Ha i inc Chapter 542 Hybridization and sp3 Hybrid Orbitals How can the bonding in CH4 be explained 4 valence electrons 2 unpaired electrons 229 w w Carbon groundstate electron configuration Copyrighl Z l Pean eon Wren izice Hall Zinc Chapter 543 Hybridization and sp3 Hybrid Orbitals How can the bonding in CH4 be explained 4 valence electrons 4 unpaired electrons 219 4 219 l 25 25 15 15 Carbon Carbon groundstate electron excitedstate electron configuration configuration Copyright Z l Penman Prentice Hall Zinc Chapter 544 Hybridization and sp3 Hybrid Orbitals How can the bonding in CH4 be explained 4 nonequivalent orbitals Carbon excitedstate electron configuration Copyright Z l Pean gom Pmnt izice Hamil Zinc Chapter 545 Hybridization and sp3 Hybrid Orbitals How can the bonding in CH4 be explained 4 nonequivalent orbitals 4 equivalent orbitals 2p 4 4 quotiquot 439 2s i 15 ls Carbon Carbon excitedstate electron sp3hybridized configuration configuration Copyright 201G Peamon IP WEWHQE Hail Zinc Chapter 546 Hybridization and sp3 Hybrid Orbitals Each 5113 hybrid orbital has two lobes of different phase one of which is larger than the other C An 5P3 orbital Hybridization gt 0 Four tetrahedral sp5 orbitals The four large lobes are oriented toward the corners of a tetrahedron at angles of 1095quot The large lobes of the hybrid orbitals are shown in green and the small lobes are not shown Copyright 2010 Pearson Prentice Hall Inc Hybridization and sp3 Hybrid Orbitals 45 4 Four carbon 5173 Methane Each of the four C H bonds hybrid orbitals results from headon overlap of a singly occupied carbon sp3 hybrid orbital with a singly occupied hydrogen 15 orbital Four hydrogen ls atomic orbitals Copyright 2010 Pearson Prentice Hall Inc