Dr. Lori Stepan Van Der Sluys 2 Chapter 8 Part 3

Transcription

1 ovalent Bonding Describe covalent bonding and how it differs from ionic bonding Know how to tell if a substance is covalently bonded Know how to draw Lewis structures Know the two types of multiple bonds discussed in lecture, and how the bond length and bond strength vary. Bond Polarity Be able to describe the difference in location of electron density between non-polar covalent, polar covalent, and ionic bonding. Know the definition of electronegativity. Which is the most electronegative element? Why? What value do we assign to the most electronegative element? Bond Polarity ontinued Which is the least electronegative element? What does electropositive mean? ow is electronegativity related to electron affinity? Remember the relative order of electronegativity (or our new clambakes...) Be able to calculate the electronegativity difference between two atoms, and decide if the bond is ionic, polar covalent, or non-polar covalent. Know the definition of dipole moment, and what it tells you about a molecule. Be familiar with 2 ways to illustrate polarity (!+ or -, arrow with positive sign at base) Know how to name ionic and molecular compounds keeping in mind electronegativity Dr. Lori Stepan Van Der Sluys 1 hapter 8 Part 3 Dr. Lori Stepan Van Der Sluys 2 hapter 8 Part 3

2 hapter 8 part 3: Lewis Structures Read: BLB W: BLB 8: 45, 47, 49, 53, 61, 63, 64, 71; Sup 8:2-10 Know: Lewis structures valence formal charge resonance structures octet rule exceptions bond energy and length What Bonus Deadlines are oming Up?? When is Exam 1?? Where is my section taking the Exam? o Data sheet in packet will be provided o Bring:! pencils,! student ID! and a calculator N text-programmable calculators or wireless devices Dr. Lori Stepan Van Der Sluys 3 hapter 8 Part 3 Drawing Structural ormulas 1. number of valence electrons Use the periodic table (# valence e - = for main group elements) 2. valence of an atom a. ow many bonds does it form? b. ow many nonbonding e -? Rules for Drawing Lewis Structures 1. ount up the number of valence electrons. 2. Write the atom symbols and connect bonded atoms with single bonds. (irst atom in formula is usually central; most electropositive) 3. Distribute electrons (in pairs) to complete octets of atoms. 4. Not enough electrons? Make multiple bonds to complete octets if necessary. 5. Extra electrons? Put them on the central atom. Dr. Lori Stepan Van Der Sluys 4 hapter 8 Part 3

3 Example: sulfite ion S 3 2" 1. # valence electrons = Example: N 1. valence electrons = 2. connect atoms 2. connect atoms 3. distribute electrons - complete octets 3. distribute electrons - complete octets 4. not enough electrons? Multiple bonds heck to make sure all atoms have a complete octet! heck to make sure all atoms have a complete octet! Dr. Lori Stepan Van Der Sluys 5 hapter 8 Part 3 Dr. Lori Stepan Van Der Sluys 6 hapter 8 Part 3

4 arbonyl chloride: l 2 1. # of valence electrons = 2. onnect atoms Deciding between alternate Lewis Structures Use RMAL ARGE (): = VE " LSE VE = # valence electrons in an isolated atom 3. distribute electrons - complete octets? LSE = # electrons on atom in Lewis Structure LSE = lone pair electrons + 1/2 shared electrons #The most stable structure is the one in which the atoms bear the smallest formal charge. 4. not enough electrons? Multiple bonds l l l l Dr. Lori Stepan Van Der Sluys 7 hapter 8 Part 3 Dr. Lori Stepan Van Der Sluys 8 hapter 8 Part 3

5 NITRATE IN: N 3 " 1. Valence electrons: RESNANE STRUTURES Some molecules can not be represented by a single Lewis structure, such as N 3 " 2 & 3. onnect atoms, distribute electron pairs, check for octets N N N 4. Multiple bonds? 3 resonance structures All 3 are proper and equal Lewis structures. Together, they represent the best picture of bonding. Molecule does not flip-flop: the TRUE structure is a MIXTURE of the resonance structures. What does the Lewis structure indicate about bond lengths and strengths in N 3 "? Dr. Lori Stepan Van Der Sluys 9 hapter 8 Part 3 Dr. Lori Stepan Van Der Sluys 10 hapter 8 Part 3

6 Resonance: A molecule has RESNANE when more than 1 Lewis structure can be drawn for a fixed nuclear arrangement. Different Lewis structures are usually equally good. Review Lewis structures What is the Lewis structure for 2 4 Molecules with resonance exhibit. Properties (e.g. bond length, bond strength) are over resonance structures. Aromatic hydrocarbons have resonance. Si 4 Xe 4 Te 4 Dr. Lori Stepan Van Der Sluys 11 hapter 8 Part 3 Dr. Lori Stepan Van Der Sluys 12 hapter 8 Part 3

7 EXEPTINS T TE TET RULE: 1. dd number of electrons 2. Less than an octet on central atom 3. More than an octet on central atom 1. dd # of valence electrons (rare) # of valence e N N 2, EXEPTINS T TE TET RULE: 2. Incomplete ctet (Less than an octet on the central atom) Rare: Be, B Example: B 3 B ormal charges is too = zero electronegative B 3 reacts readily with electron pair donors (like N 3 ). B l 2 Dr. Lori Stepan Van Der Sluys 13 hapter 8 Part 3 Dr. Lori Stepan Van Der Sluys 14 hapter 8 Part 3

8 EXEPTINS T TE TET RULE: 3. Expanded octet (More than an octet on the central atom) oncept of Valence of an Atom fairly common never occurs with period atoms occurs with atoms in period WY?? These atoms are: onditions for expanded octets: a. large central atom b. attached atoms small c. attached atoms highly electronegative, such as Examples N 5 doesn't exist P 5 stable P 5 stable Pl 5 less stable As 5 stable PBr 5 unstable PI 5 doesn't exist Definition; or the first row main group elements, the pattern of valence is usually followed. or the other rows, not so much. Dr. Lori Stepan Van Der Sluys 15 hapter 8 Part 3 Dr. Lori Stepan Van Der Sluys 16 hapter 8 Part 3

9 Bond properties VALENT BND LENGTS and ENERGIES Bond length: distance between nuclei BND ENERGY bond (dissociation) energy: D enthalpy of bond breaking reaction in the gas phase; requires D > 0 ($ > 0) bond Bond energy Bond length pm kj/mol % = ' more electrons shared, shorter bond length bond Bond energy kj/mol Bond length pm % %l %Br Shorter bond length, stronger the bond for diatomics, D is $ of one reaction: %(g) & 2(g) D - = $ rxn = 436kJ/mol for polyatomics, D is an averaged quantity %%(g) & (g) + (g) +494kJ/mol %(g) & (g) + (g) +424kJ/mol D % = 463 kj/mol * value obtained from averaging over many molecules not exact for any one case Table 8.4 and 8.5 Dr. Lori Stepan Van Der Sluys 17 hapter 8 Part 3 Dr. Lori Stepan Van Der Sluys 18 hapter 8 Part 3