Lecture 8 Chemical bonding 1 Read HW BLB 2 6 2 8 8 1 8 4 BLB 2 51 53 57 59 65 67 8 16 22 25 29 37 39 Sup 2 4 Know chemical bonding ionic bonding covalent bonding metallic bonding Lewis symbols lattice energy Exam 1 Monday Feb 9 6 30 start preparing now only non text programmable calculators are allowed no PDAs blackberries cell phones etc will be permitted Bring pencils student ID and a calculator Absolutely NO text programmable calculators or wireless devices will be checked Form a study group use the CRC take advantage of SI info on web use the online resources and work those problems practice practice practice Bonus deadline for Skill check test 5 is Thursday 2 5 Skill check test 6 is SUNDAY 2 8 Sheets s office hours Mondays 12 30 2 Tuesdays 10 30 12 in 324 Chem or 326 Chem Sheets Page 1 Lecture 8 Chemical bonding what is a chemical bond anyway force holding atoms or ions together it is potential energy that can be exploited Sheets Page 2 Lecture 8 3 types of chemical bonds ionic valence electrons traded to form separate ions Na Cl attraction between positive negative ions each ion has noble gas e configuration covalent valence electrons between a few nuclei distinct molecules molecules molecular compounds metallic will see in Chem 112 valence electrons shared among all nuclei electron sea conduction found in metals Sheets Page 3 valence e core e valence e core e Lecture 8 Dots octets valence electrons are involved in bonding e C 1s22s22p2 C 4 valence electrons a dot a S Ne 3s23p4 S 6 valence electrons octet rule elements tend to gain lose or share electrons to achieve an inert gas configuration compounds form to achieve octet for all atoms ns2np6 valence octet inert gases have 8 valence electrons duet for H and He Sheets Page 4 Lecture 8 knowing the valence electrons you should be able to easily draw Lewis structures DO NOT memorize this table You can figure it out more about Lewis structures in next lecture talk about a cliffhanger Sheets Page 5 Lecture 8 Transition metals talked about last time in Lecture 7 TM are exceptions to the octet rule lose s electrons first lose d electrons only after valence s gone transition metal ions can have variable charges examples Fe Fe2 Fe3 Ar 4s23d6 Ar 3d6 Ar 3d5 Ag Ag Kr 5s14d10 Kr 4d10 Sheets Page 6 Lecture 8 Ionic bonding electrostatic attraction between ions of opposite charge Coulomb s law Q1 Q2 d E Q1Q2 d charge on 1st atom charge on 2nd atom distance between charges e s exchanged to form separate ions with complete octets Na 1s22s22p6 Ne Sheets Cl 1s22s22p63s23p6 Ar Page 7 Lecture 8 This should be a review for you we won t discuss in class Look at placement in periodic table and relate to charge of cation or anion We will refer to these by name throughout the semester Know these Common cations Common anions 1 1 hydrogen ion H hydride sodium F fluoride K potassium Cl chloride Ag silver Br bromide NH4 ammonium H Na I 2 3 iodide OH hydroxide NO3 nitrate CN cyanide Mg2 magnesium Ca2 calcium Ba2 barium Co2 cobalt II O2 oxide Cu2 copper II S2 sulfide Fe2 iron II CO32 carbonate Pb2 lead II SO42 sulfate Mn2 manganese Hg2 mercury II Zn2 zinc PO43 phosphate Al3 aluminum Cr3 chromium III Fe3 iron III Sheets 2 3 Page 8 Lecture 8 Properties of ionic compounds distinct ions crystal lattice strong omnidirectional bonds characteristics low electrical conductivity as solids high as liquids very high melting boiling points hard but brittle soluble only in polar solvents water electrolytes more Lecture 28 formed from what metal non metal Sheets Page 9 Lecture 8 Strength of ionic bonds lattice energy lattice energy the change in energy when an ionic solid is separated into isolated ions in the gas phase NaCl s Na g Cl g E 788 kJ lattice energy lattice energy cannot be directly determined experimentally therefore use Hess s Law Chap 5 and known atomic properties to get the value of the lattice energy more on Hess s Law Lectures 38 39 Born Haber cycle a thermochemical cycle used to analyze factors contributing to stability of ionic compounds BLB Fig 8 4 Sheets Page 10 Lecture 8 More on lattice energy NaCl MgO E 786 kJ E 3934 kJ factor of 4 for MgO why the big difference Q1Q 2 E d different salts have different Q s and d NaCl radii MgO Na Cl 0 97 1 81 d 2 78 Mg2 O2 0 66 1 40 2 06 d is smaller for thus lattice energy for MgO is 4 that of NaCl Sheets Page 11 Lecture 8 Lattice energy of salts melting points are indicative of lattice energy higher M P L E MP LE relative magnitudes of lattice energy predictable based on charges Q1 Q2 and separation d recall ion size predictions from periodic trends anion size NaF NaCl NaBr NaI MgO Sheets MP 993 C 801 C 747 C 661 C 2800 C d L E higher charge Q1Q2 L E Page 12 Lecture 8 Example Which one of the following expresses the correct relationship between the melting points of the 3 ionic solids NaCl MgS and BaS A NaCl MgS BaS B NaCl BaS MgS C MgS BaS NaCl D MgS NaCl BaS E BaS MgS NaCl Sheets Page 13 Lecture 8 Covalent bonding when neither atom is willing to give up electrons completely atoms electrons each atom has a noble gas configuration H H Sheets H H Page 14 Lecture 8 Covalent bonding and Lewis structures covalent bond build up of electron density between 2 nuclei or sharing of electron pair Lewis structures show how electrons are shared atom connectivity shared e pairs BOND also display unshared e pairs or pairs example NH3 Sheets Page 15 Lecture 8 More on covalent bonding more electrons shared the bond bond strengths 100 1000 kJ mole more electrons shared the bond single double triple bonds allowed these multiple bonds have distinctive properties l N N N N N N 1 47 1 24 1 10 418 941 kJ mole E 163 bond length l bond strength E kJ mol we ll talk more about multiple bonds in the coming lectures Sheets Page 16 Lecture 8 Covalent bonding cont to understand covalent bonding at molecular atomic level we will explore the concept at three levels Lewis structures tells where e are analogous to a census telling where people are VSEPR theory Chap 9 shows how to use Lewis structures to predict shapes of molecules valence bond theory Chap 9 explains how covalent bonds are the result of orbital overlap and bonds Sheets Page 17 Lecture 8 Before next class Read HW BLB 2 6 2 8 8 1 8 4 BLB 2 51 53 57 59 65 67 8 16 22 25 29 37 39 Sup 2 4 Know electronegativity Lewis structures formal charges Answer p 13 C Sheets Page 18 Lecture 8
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