Lewis Theory Formal Charge Resonance Roy Kennedy Massachusetts

Lewis Theory Formal Charge Resonance Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA Copyright 2011 Pearson Education, Inc.

HIV-Protease • HIV-protease is a protein synthesized by the • • • human immunodeficiency virus (HIV). This particular protein is crucial to the virus’s ability to multiply and cause AIDS Pharmaceutical companies designed molecules that would disable HIV-protease by sticking to the molecule’s active site – protease inhibitors To design such a molecule, researchers used bonding theories to simulate the shape of potential drug molecules and how they would interact with the protease molecule Tro: Chemistry: A Molecular Approach, 2/e 2 Copyright 2011 Pearson Education, Inc.

Covalent Bonds • Nonmetal atoms have relatively high • ionization energies, so it is difficult to remove electrons from them When nonmetals bond together, they share valence electrons ü potential energy lowest when the electrons are between the nuclei • Shared electrons overcomes repulsion Tro: Chemistry: A Molecular Approach, 2/e 3 Copyright 2011 Pearson Education, Inc.

Covalent Bonding: Bonding and Lone Pair Electrons • Electrons that are shared by atoms are called • bonding pairs Electrons that are not shared by atoms but belong to a particular atom are called lone pairs ü aka nonbonding pairs Bonding pairs . . . . O. . S. . O. . Tro: Chemistry: A Molecular Approach, 2/e 4 Lone pairs Copyright 2011 Pearson Education, Inc.

Single, Double, Triple Bonds • • F • • H O H • • F • • F • • • O O • • F • • Tro: Chemistry: A Molecular Approach, 2/e • • N • • N 5 Copyright 2011 Pearson Education, Inc.

Polar Covalent Bonding • Covalent bonding between atoms A and B results in unequal sharing of the electrons ü one atom pulls the electrons in the bond closer to its side ü one end of the bond has larger electron density than the other • The result is a polar covalent bond ü bond polarity ü the end with the larger electron density gets a partial negative charge ü the end that is electron deficient gets a partial positive charge Tro: Chemistry: A Molecular Approach, 2/e 6 Copyright 2011 Pearson Education, Inc.

HF EN 2. 1 EN 4. 0 d+ H • • F d- Tro: Chemistry: A Molecular Approach, 2/e 7 Copyright 2011 Pearson Education, Inc.

Bond Polarity • Most bonds have some degree of sharing and • • some degree of ion formation to them Bonds are classified as covalent if the amount of electron transfer is insufficient for the material to display the classic properties of ionic compounds If the sharing is unequal enough to produce a dipole in the bond, the bond is classified as polar covalent Tro: Chemistry: A Molecular Approach, 2/e 8 Copyright 2011 Pearson Education, Inc.

Electronegativity • The ability of an atom to attract bonding • • electrons to itself is called electronegativity Increases across period (left to right) and Decreases down group (top to bottom) ü fluorine is the most electronegative element ü francium is the least electronegative element ü noble gas atoms are not assigned values ü opposite of atomic size trend • The larger the difference in electronegativity, the more polar the bond ü negative end toward more electronegative atom Tro: Chemistry: A Molecular Approach, 2/e 9 Copyright 2011 Pearson Education, Inc.

Electronegativity Scale Tro: Chemistry: A Molecular Approach, 2/e 10 Copyright 2011 Pearson Education, Inc.

Electronegativity Difference (DEN) and Bond Type DEN = 0, pure covalent ü equal sharing DEN = 0. 1 ~ 0. 4, nonpolar covalent DEN = 0. 5 ~ 1. 9, polar covalent DEN 2. 0, ionic 4% 0 0. 4 Percent Ionic Character 51% 2. 0 Electronegativity Difference Tro: Chemistry: A Molecular Approach, 2/e 11 “ 100%” 4. 0 Copyright 2011 Pearson Education, Inc.

Bond Polarity ENCl = 3. 0 − 3. 0 = 0 Pure Covalent Tro: Chemistry: A Molecular Approach, 2/e ENCl = 3. 0 ENH = 2. 1 3. 0 – 2. 1 = 0. 9 Polar Covalent 12 ENCl = 3. 0 ENNa = 0. 9 3. 0 – 0. 9 = 2. 1 Ionic Copyright 2011 Pearson Education, Inc.

Water – a Polar Molecule stream of water attracted to a charged glass rod Tro: Chemistry: A Molecular Approach, 2/e stream of hexane not attracted to a charged glass rod 13 Copyright 2011 Pearson Education, Inc.

Bond Dipole Moments • Dipole moment, m, is a measure of bond polarity ü a dipole is a material with a + and − end ü it is directly proportional to the size of the partial charges and directly proportional to the distance between them Øm = (q)(r) Ønot Coulomb’s Law Ømeasured in Debyes, D • Generally, the more electrons two atoms share and the larger the atoms are, the larger the dipole moment Tro: Chemistry: A Molecular Approach, 2/e 14 Copyright 2011 Pearson Education, Inc.

Dipole Moments Tro: Chemistry: A Molecular Approach, 2/e 15 Copyright 2011 Pearson Education, Inc.

Percent Ionic Character • The percent ionic character is the • percentage of a bond’s measured dipole moment compared to what it would be if the electrons were completely transferred The percent ionic character indicates the degree to which the electron is transferred Tro: Chemistry: A Molecular Approach, 2/e 16 Copyright 2011 Pearson Education, Inc.

Tro: Chemistry: A Molecular Approach, 2/e 17 Copyright 2011 Pearson Education, Inc.

Lewis Structures of Molecules • Lewis theory allows us to predict the distribution • • • of valence electrons in a molecule Useful for understanding the bonding in many compounds Allows us to predict shapes of molecules Allows us to predict properties of molecules and how they will interact together Tro: Chemistry: A Molecular Approach, 2/e 18 Copyright 2011 Pearson Education, Inc.

Beware!! • Lewis Theory predicts that atoms will be most stable • when they have their octet of valence electrons It does not require that atoms have the same number of lone pair electrons they had before bonding ü first use the octet rule • Some atoms commonly violate the octet rule ü Be generally has two bonds and no lone pairs in its compounds ü B generally has three bonds and no lone pairs in its compounds ü many elements may end up with more than eight valence electrons in their structure if they can use their empty d orbitals for bonding Ø expanded octet Tro: Chemistry: A Molecular Approach, Approach 2/e 19 Copyright 2011 Pearson Education, Inc.

Lewis Structures • Generally try to follow the common bonding patterns ü C = 4 bonds & 0 lone pairs, N = 3 bonds & 1 lone pair, O= 2 bonds & 2 lone pairs, H and halogen = 1 bond, Be = 2 bonds & 0 lone pairs, B = 3 bonds & 0 lone pairs ü often Lewis structures with line bonds have the lone pairs left off Ø their presence is assumed from common bonding patterns • Structures that result in bonding patterns different from the common may have formal charges B C Tro: Chemistry: A Molecular Approach, 2/e N 20 O F Copyright 2011 Pearson Education, Inc.

Formal Charge • During bonding, atoms may end with more or fewer electrons than the valence electrons they brought in order to fulfill octets • This results in atoms having a formal charge FC = valence e− − nonbonding e− − ½ bonding e− left OFC = 6 − 4 − ½ (4) = 0 S FC = 6 − 2 − ½ (6) = +1 right O FC = 6 − ½ (2) = − 1 • Sum of all the formal charges in a molecule = 0 ü in an ion, total equals the charge Tro: Chemistry: A Molecular Approach, 2/e 21 Copyright 2011 Pearson Education, Inc.

Writing Lewis Formulas of Molecules (cont’d) 7. Assign formal charges to the atoms a) fc = valence e− − lone pair e− − ½ bonding e− b) or follow the common bonding patterns − 1 0 +1 − 1 0 0 +1 Tro: Chemistry: A Molecular Approach, 2/e 22 0 Copyright 2011 Pearson Education, Inc.

Common Bonding Patterns B B − C N O + C + N + O − − C Tro: Chemistry: A Molecular Approach, 2/e N 23 O F − F F + − Copyright 2011 Pearson Education, Inc.

Exceptions to the Octet Rule • Expanded octets ü elements with empty d orbitals can have more than eight electrons • Odd number electron species e. g. , NO ü will have one unpaired electron ü free-radical ü very reactive • Incomplete octets ü B, Al Tro: Chemistry: A Molecular Approach, 2/e 24 Copyright 2011 Pearson Education, Inc.

Practice – Assign formal charges CO 2 H 3 PO 4 Se. OF 2 SO 32− NO 2− P 2 H 4 Tro: Chemistry: A Molecular Approach, 2/e 25 Copyright 2011 Pearson Education, Inc.

Practice - Assign formal charges CO 2 H 3 PO 4 all 0 P = +1 rest 0 SO 32− Se. OF 2 S = +1 Se = +1 P 2 H 4 NO 2− all 0 Tro: Chemistry: A Molecular Approach, 2/e 26 Copyright 2011 Pearson Education, Inc.

Resonance • Lewis theory localizes the electrons between • • the atoms that are bonding together Extensions of Lewis theory suggest that there is some degree of delocalization of the electrons – we call this concept resonance Delocalization of charge helps to stabilize the molecule Tro: Chemistry: A Molecular Approach, 2/e 27 Copyright 2011 Pearson Education, Inc.

Resonance Structures • More than one Lewis structure for a molecule • that differ only in the position of the electrons, they are called resonance structures The actual molecule is a combination of the resonance forms – a resonance hybrid ü the molecule does not resonate between the two forms, though we often draw it that way • Look for multiple bonds or lone pairs . . . . O. . S. . O. . Tro: Chemistry: A Molecular Approach, 2/e 28 . . . . O. . S. . O. . Copyright 2011 Pearson Education, Inc.

Resonance of Ozone Tro: Chemistry: A Molecular Approach, 2/e 29 Copyright 2011 Pearson Education, Inc.

O 3 in Ozone Layer Tro: Chemistry: A Molecular Approach, 2/e 30 Copyright 2011 Pearson Education, Inc.

Rules of Resonance Structures • Resonance structures must have the same connectivity ü only electron positions can change • Resonance structures must have the same number • of electrons Second row elements have a maximum of eight electrons ü bonding and nonbonding ü third row can have expanded octet • Formal charges must total same Tro: Chemistry: A Molecular Approach, 2/e 31 Copyright 2011 Pearson Education, Inc.

Drawing Resonance Structures 1. Draw first Lewis structure that maximizes octets 2. Assign formal charges 3. Move electron pairs from atoms with (−) formal charge toward atoms with (+) formal charge 4. If (+) fc atom 2 nd row, only move in electrons if you can move out electron pairs from multiple bond 5. If (+) fc atom 3 rd row or below, keep bringing in electron pairs to reduce the formal charge, even if get expanded octet Tro: Chemistry: A Molecular Approach, 2/e 32 − 1 − 1 +1 Copyright 2011 Pearson Education, Inc.

Drawing Resonance Structures 1. Draw first Lewis structure that maximizes octets 2. Assign formal charges 3. Move electron pairs from atoms with (−) formal charge toward atoms with (+) formal charge 4. If (+) fc atom 2 nd row, only move in electrons if you can move out electron pairs from multiple bond 5. If (+) fc atom 3 rd row or below, keep bringing in electron pairs to reduce the formal charge, even if get expanded octet Tro: Chemistry: A Molecular Approach, 2/e 33 − 1 +2 − 1 Copyright 2011 Pearson Education, Inc.

Evaluating Resonance Structures • Better structures have fewer formal • • charges Better structures have smaller formal charges Better structures have the negative formal charge on the more electronegative atom Tro: Chemistry: A Molecular Approach, 2/e 34 Copyright 2011 Pearson Education, Inc.

Practice – Identify Structures with Better or Equal Resonance Forms and Draw Them CO 2 H 3 PO 4 all 0 P = +1 rest 0 SO 32− Se. OF 2 S = +1 Se = +1 P 2 H 4 NO 2− all 0 Tro: Chemistry: A Molecular Approach, 2/e 35 Copyright 2011 Pearson Education, Inc.

Practice – Identify Structures with Better or Equal Resonance Forms and Draw Them H 3 PO 4 CO 2 none SO 32− Se. OF 2 − 1 +1 NO 2 P 2 H 4 − none Tro: Chemistry: A Molecular Approach, 2/e 36 Copyright 2011 Pearson Education, Inc.

Bond Energies • Chemical reactions involve breaking bonds in • • reactant molecules and making new bonds to create the products The DH°reaction can be estimated by comparing the cost of breaking old bonds to the income from making new bonds The amount of energy it takes to break one mole of a bond in a compound is called the bond energy ü in the gas state ü homolytically – each atom gets ½ bonding electrons Tro: Chemistry: A Molecular Approach, 2/e 37 Copyright 2011 Pearson Education, Inc.

Trends in Bond Energies • In general, the more electrons two atoms share, the stronger the covalent bond ü must be comparing bonds between like atoms ü C≡C (837 k. J) > C=C (611 k. J) > C−C (347 k. J) ü C≡N (891 k. J) > C=N (615 k. J) > C−N (305 k. J) • In general, the shorter the covalent bond, the stronger the bond ü must be comparing similar types of bonds ü Br−F (237 k. J) > Br−Cl (218 k. J) > Br−Br (193 k. J) ü bonds get weaker down the column ü bonds get stronger across the period Tro: Chemistry: A Molecular Approach, 2/e 38 Copyright 2011 Pearson Education, Inc.

Using Bond Energies to Estimate DH°rxn • The actual bond energy depends on the • surrounding atoms and other factors We often use average bond energies to estimate the DHrxn ü works best when all reactants and products in gas state • Bond breaking is endothermic, DH(breaking) = + • Bond making is exothermic, DH(making) = − DHrxn = ∑ (DH(bonds broken)) + ∑ (DH(bonds formed)) Tro: Chemistry: A Molecular Approach, 2/e 39 Copyright 2011 Pearson Education, Inc.

Example: Estimate the enthalpy of the following reaction DHrxn = ∑ (DH(bonds broken)) + ∑ (DH(bonds made)) Bond breaking 1 mole C─H +414 k. J 1 mole Cl─Cl +243 k. J total +657 k. J Bond making 1 mole C─Cl 1 mole Cl─H total DHrxn = (+657 k. J) + (− 770 k. J) DHrxn = − 113 k. J − 339 k. J − 431 k. J − 770 k. J Tro: Chemistry: A Molecular Approach, 2/e 40 Copyright 2011 Pearson Education, Inc.

Break 1 mol C─H +414 k. J 1 mol Cl─Cl +243 k. J Tro: Chemistry: A Molecular Approach, 2/e 1 mol C─Cl 1 mol H─Cl 41 Make − 339 k. J − 431 k. J Copyright 2011 Pearson Education, Inc.

Practice – Estimate the enthalpy of the following reaction H H + O Tro: Chemistry: A Molecular Approach, 2/e O H 42 O O H Copyright 2011 Pearson Education, Inc.

Practice – Estimate the enthalpy of the following reaction H 2(g) + O 2(g) ® H 2 O 2(g) Reaction involves breaking 1 mol H–H and 1 mol O=O and making 2 mol H–O and 1 mol O–O bonds broken (energy cost) (+436 k. J) + (+498 k. J) = +934 k. J bonds made (energy release) 2(− 464 k. J) + (− 142 k. J) = − 1070. k. J DHrxn = (+934 k. J) + (− 1070. k. J) = − 136 k. J (Appendix DH°f = − 136. 3 k. J/mol) Tro: Chemistry: A Molecular Approach, 2/e 43 Copyright 2011 Pearson Education, Inc.

Bond Lengths • The distance between the nuclei • of bonded atoms is called the bond length Because the actual bond length depends on the other atoms around the bond we often use the average bond length ü averaged for similar bonds from many compounds Tro: Chemistry: A Molecular Approach, 2/e 44 Copyright 2011 Pearson Education, Inc.

Trends in Bond Lengths • In general, the more electrons two atoms share, the shorter the covalent bond ü must be comparing bonds between like atoms ü C≡C (120 pm) < C=C (134 pm) < C−C (154 pm) ü C≡N (116 pm) < C=N (128 pm) < C−N (147 pm) • Generally bond length decreases from left to right across period ü C−C (154 pm) > C−N (147 pm) > C−O (143 pm) • Generally bond length increases down the column ü F−F (144 pm) > Cl−Cl (198 pm) > Br−Br (228 pm) • In general, as bonds get longer, they also get weaker Tro: Chemistry: A Molecular Approach, 2/e 45 Copyright 2011 Pearson Education, Inc.

Bond Lengths Tro: Chemistry: A Molecular Approach, 2/e 46 Copyright 2011 Pearson Education, Inc.

Additional Comments: Covalent Bonding • Lewis theory implies that some combinations should be stable, whereas others should not ü because the stable combinations result in “octets” • using these ideas of Lewis theory allows us to • • predict the formulas of molecules of covalently bonded substances Hydrogen and the halogens are all diatomic molecular elements, as predicted by Lewis theory Oxygen generally forms either two single bonds or a double bond in its molecular compounds, as predicted by Lewis theory ü though, as we’ll see, there are some stable compounds in which oxygen has one single bond another where it has a triple bond, but it still has an octet Tro: Chemistry: A Molecular Approach, 2/e 47 Copyright 2011 Pearson Education, Inc.

Determining the Number of Valence Electrons in an Atom • The column number on the Periodic Table will tell you how many valence electrons a main group atom has ü Transition Elements all have two valence electrons. Why? Tro: Chemistry: A Molecular Approach, 2/e 48 Copyright 2011 Pearson Education, Inc.

Example: Writing Lewis structures of molecules, HNO 3 1. Write skeletal structure ü H always terminal Ø in oxyacid, H outside attached to O’s ü make least electronegative atom central O H O N O Ø N is central Ø not H 2. Count valence electrons ü sum the valence electrons for each atom ü add one electron for each − charge ü subtract one electron for each + charge Tro: Chemistry: A Molecular Approach, 2/e 49 N=5 H=1 O 3 = 3 6 = 18 Total = 24 e− Copyright 2011 Pearson Education, Inc.

Example: Writing Lewis structures of molecules, HNO 3 3. Attach atom together with pairs of electrons, and subtract from the total ü don’t forget, a line represents 2 electrons Electrons Start 24 Used 8 Left 16 Tro: Chemistry: A Molecular Approach, 2/e 50 Copyright 2011 Pearson Education, Inc.

Example: Writing Lewis structures of molecules, HNO 3 4. Complete octets, outside-in ü H is already complete with 2 Ø 1 bond and re-count electrons N=5 H=1 O 3 = 3 6 = 18 Total = 24 e− Electrons Start 24 Used 8 Left 16 Tro: Chemistry: A Molecular Approach, 2/e 51 Electrons Start 16 Used 16 Left 0 Copyright 2011 Pearson Education, Inc.

Example: Writing Lewis structures of molecules, HNO 3 5. If all octets complete, give extra electrons to the central atom ü elements with d orbitals can have more than eight electrons Ø Period 3 and below 6. If central atom does not have octet, bring in electrons from outside atoms to share ü follow common bonding patterns if possible Tro: Chemistry: A Molecular Approach, 2/e 52 Copyright 2011 Pearson Education, Inc.

Practice – Draw Lewis Structures of the Following CO 2 H 3 PO 4 Se. OF 2 SO 32− NO 2− P 2 H 4 Tro: Chemistry: A Molecular Approach, 2/e 53 Copyright 2011 Pearson Education, Inc.

Practice – Lewis Structures CO 2 H 3 PO 4 16 e− 32 e− Se. OF 2 SO 32− NO 2− P 2 H 4 26 e− 18 e− Tro: Chemistry: A Molecular Approach, 2/e 14 e− 54 Copyright 2011 Pearson Education, Inc.

Covalent Bonding Model vs. Reality • Lewis theory of covalent bonding implies that the attractions between atoms are directional ü the shared electrons are most stable between the bonding atoms • Therefore Lewis theory predicts covalently bonded compounds will be found as individual molecules ü rather than an array like ionic compounds • Compounds of nonmetals are made of individual molecule units Tro: Chemistry: A Molecular Approach, 2/e 55 Copyright 2011 Pearson Education, Inc.

Covalent Bonding Model vs. Reality • Lewis theory predicts the melting and boiling points of molecular compounds should be relatively low ü involves breaking the attractions between the molecules, but not the bonds between the atoms ü the covalent bonds are strong, but the attractions between the molecules are generally weak • Molecular compounds have low melting points and boiling points ü MP generally < 300 °C ü molecular compounds are found in all three states at room temperature Tro: Chemistry: A Molecular Approach, 2/e 56 Copyright 2011 Pearson Education, Inc.

Intermolecular Attractions vs. Bonding Tro: Chemistry: A Molecular Approach, 2/e 57 Copyright 2011 Pearson Education, Inc.

Covalent Bonding Model vs. Reality • Lewis theory predicts that the hardness and brittleness of molecular compounds should vary depending on the strength of intermolecular attractive forces ü the kind and strength of the intermolecular attractions varies based on many factors • Some molecular solids are brittle and hard, but many are soft and waxy Tro: Chemistry: A Molecular Approach, 2/e 58 Copyright 2011 Pearson Education, Inc.

Covalent Bonding Model vs. Reality • Lewis theory predicts that neither molecular solids nor liquids should conduct electricity ü there are no charged particles around to allow the material to conduct • Molecular compounds do not conduct electricity in • the solid or liquid state Molecular acids conduct electricity when dissolved in water, but not in the solid or liquid state, due to them being ionized by the water Tro: Chemistry: A Molecular Approach, 2/e 59 Copyright 2011 Pearson Education, Inc.

Covalent Bonding Model vs. Reality • Lewis theory predicts that the more electrons two • • atoms share, the stronger the bond should be Bond strength is measured by how much energy must be added into the bond to break it in half In general, triple bonds are stronger than double bonds, and double bonds are stronger than single bonds ü however, Lewis theory would predict double bonds are twice as strong as single bonds, but the reality is they are less than twice as strong Tro: Chemistry: A Molecular Approach, 2/e 60 Copyright 2011 Pearson Education, Inc.

Covalent Bonding Model vs. Reality • Lewis theory predicts that the more electrons two atoms share, the shorter the bond should be ü when comparing bonds to like atoms • Bond length is determined by measuring the • distance between the nuclei of bonded atoms In general, triple bonds are shorter than double bonds, and double bonds are shorter than single bonds Tro: Chemistry: A Molecular Approach, 2/e 61 Copyright 2011 Pearson Education, Inc.