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Chemistry: A Molecular Approach , 1 st Ed. Nivaldo Tro. Chapter 9 Chemical Bonding I: Lewis Theory. Question. Complete the following sentence… Properties of substances can be explained in terms of differences in chemical __________ e.g. -salt dissolves in water better than oil

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Chapter 9 Chemical Bonding I: Lewis Theory


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    1. Chemistry: A Molecular Approach, 1st Ed.Nivaldo Tro Chapter 9Chemical Bonding I:Lewis Theory

    2. Question Complete the following sentence… Properties of substances can be explained in terms of differences in chemical __________ e.g. -salt dissolves in water better than oil -certain substances are electrolytes -alcohol evaporates quicker than water -wax melts at a lower temperature than salt

    3. Bonding Theories • explain how and why atoms attach together • one of the simplest bonding theories is called Lewis Theory • Lewis Theory uses valence electrons to explain bonding • explains why some combinations of atoms are stable and others are not • using Lewis Theory, we can draw models – called Lewis structures – that allow us to predict many properties of molecules • such as molecular shape, size, polarity Tro, Chemistry: A Molecular Approach

    4. Types of Bonds Tro, Chemistry: A Molecular Approach

    5. Types of Bonding

    6. 1A 2A 3A 4A 5A 6A 7A 8A Li Be B C N O F Ne 1 e- 2 e- 3 e- 4 e- 5 e- 6 e- 7 e- 8 e- Determining the Number of Valence Electrons in an Atom • the column number on the Periodic Table tells us the no. valence e- Tro, Chemistry: A Molecular Approach

    7. Lewis Symbols of Atoms • use symbol of element to represent nucleus and inner electrons • use dots around the symbol to represent valence electrons • pair first two electrons for the s orbital • put one electron on each open side for p electrons • then pair rest of the p electrons Tro, Chemistry: A Molecular Approach

    8. Lewis Symbols of Ions • Cations have Lewis symbols without valence e- e.g. lithium • Anions have Lewis symbols with 8 valence electrons e.g. flourine Li• Li+ e- loss e- gain Tro, Chemistry: A Molecular Approach

    9. Question Draw Lewis dot structures of elemental magnesium and magneisum ion Draw Lewis dot structures of elemental nitrogen and the nitride ion

    10. Stable Electron ArrangementsAnd Ion Charge • Metals form cations by losing e- to become isoelectric to the previous noble gas • Nonmetals form anions by gaining enough e- to become isoelectric to the previous noble gas [Ne] = 1s22s22p6 Tro, Chemistry: A Molecular Approach

    11. Lewis Theory • the basis of Lewis Theory is that there are certain electron arrangements in the atom that are more stable • octet rule • bonding occurs so atoms attain a more stable electron configuration Tro, Chemistry: A Molecular Approach

    12. Octet Rule • when atoms bond, they tend to gain, lose, or share e- to result in 8 valence e- • ns2np6 • noble gas configuration • many exceptions • H, Li, Be, B attain an electron configuration like He • He = 2 valence e- • Li loses its one valence e- • H shares or gains one e- • though it commonly loses its one electron to become H+ • Be loses 2 electrons to become Be2+ • though it commonly shares its two electrons in covalent bonds, resulting in 4 valence electrons • B loses 3 electrons to become B3+ • though it commonly shares its three electrons in covalent bonds, resulting in 6 valence electrons • expanded octets for elements in Period 3 or below • using empty valence d orbitals Tro, Chemistry: A Molecular Approach

    13. Na + Lewis Theory and Ionic Bonding • Transfer of e- from metal atom to nonmetal atom, resulting in ions that are attracted to each other and therefore bond, e.g. NaCl NaCl + Tro, Chemistry: A Molecular Approach

    14. Tro, Chemistry: A Molecular Approach

    15. 2 Li + Predicting Ionic FormulasUsing Lewis Symbols • e- are transferred until the metal loses all its valence e- and the nonmetal obtains an octet Li2O Tro, Chemistry: A Molecular Approach

    16. Crystal Lattice • Ionic substances exist as crystal lattices of repeating unit cells Model of NaCl Tro, Chemistry: A Molecular Approach

    17. Ionic BondingModel vs. Reality • ionic compounds have high melting points and boiling points • MP generally > 300°C • all ionic compounds are solids at room temperature • because the attractions between ions are strong, breaking down the crystal requires a lot of energy • the stronger the attraction (larger the lattice energy), the higher the melting point Tro, Chemistry: A Molecular Approach

    18. Properties • Describe the general properties of ionic compounds • Metals react with non-metals: • Crystalline solids • 3-D units extended • high mp/bp (all solids) • brittle • aqueous solutions conduct electricity

    19. Ionic Bonding • When ionic compounds are dissolved in water, they dissociate to form aqueous ions: NaCl(s) → Na+(aq) + Cl-(aq) • The resulting solution conducts electricity and is called an electrolyte Tro, Chemistry: A Molecular Approach

    20. in NaCl(s), the ions are stuck in position and not allowed to move to the charged rods in NaCl(aq), the ions are separated and allowed to move to the charged rods Conductivity of NaCl Tro, Chemistry: A Molecular Approach

    21. Question Use Lewis dot structures to represent the formation of aluminum bromide Use Lewis dot structures to represent the formation of lithium hydride

    22. Tro, Chemistry: A Molecular Approach

    23. Types of Bonds Tro, Chemistry: A Molecular Approach

    24. Types of Bonding

    25. F F •• •• • • •• F F •• •• •• •• •• •• •• •• F F •• •• Single Covalent Bonds e.g. fluorine • two atoms share a pair of electrons Tro, Chemistry: A Molecular Approach

    26. •• • • • • H H O •• •• •• H H •• O •• Single Covalent Bonds e.g. water duet duet octet 2 bonding pairs 2 lone pairs Tro, Chemistry: A Molecular Approach

    27. •• •• • • • • O O •• •• •• •• O •• •• •• •• O Double Covalent Bond • two atoms sharing two pairs of electrons e.g. oxygen Tro, Chemistry: A Molecular Approach

    28. •• •• • • • • N N • • N N •• •• •• •• •• Triple Covalent Bond • two atoms sharing 3 pairs of electrons e.g. nitrogen Tro, Chemistry: A Molecular Approach

    29. Covalent BondingPredictions from Lewis Theory • Lewis theory allows us to predict the formulas of molecules • Lewis theory predicts that some combinations should be stable, while others should not • because the stable combinations result in “octets” • Lewis theory also shows that covalent bonds are highly directional • the shared electrons are most stable between the bonding atoms • resulting in molecules rather than an array Tro, Chemistry: A Molecular Approach

    30. Ionic BondingModel vs. Reality • molecular compounds do not conduct electricity in the liquid state • molecular acids conduct electricity when dissolved in water, but not in the solid state • in molecular solids, there are no charged particles around to allow the material to conduct • when dissolved in water, molecular acids are ionized, and have the ability to move through the structure and therefore conduct electricity Tro, Chemistry: A Molecular Approach

    31. Bond Polarity • covalent bonding between unlike atoms results in unequal sharing of the e- • one atom pulls the electrons in the bond closer to its side • one end of the bond has larger electron density than the other • the result is a polar covalent bond • bond polarity • the end with the larger electron density gets a partial negative charge • the end that is electron deficient gets a partial positive charge Tro, Chemistry: A Molecular Approach

    32. HF d- d+ EN 2.1 EN 4.0 EN 2.1 Tro, Chemistry: A Molecular Approach

    33. Electronegativity • Ability of an atom to attract e- to itself in a chemical bond • increases across period (left to right) and • decreases down group (top to bottom) Tro, Chemistry: A Molecular Approach

    34. Percent Ionic Character 4% 51% 0 0.4 2.0 4.0 Electronegativity Difference Electronegativity and Bond Polarity • If ΔE.N. between bonded atoms is 0, the bond is pure covalent • equal sharing • If ΔE.N. between bonded atoms is 0.1 - 0.4, the bond is nonpolar covalent • If ΔE.N. between bonded atoms 0.5 - 1.9, the bond is polar covalent • If ΔE.N. between bonded atoms ≥ 2.0, the bond is ionic “100%” NP PC IONIC

    35. ENCl = 3.0 ENH = 2.1 ΔEN = 3.0 – 2.1 = 0.9 Polar Covalent ENCl = 3.0 ENNa = 1.0 ΔEN = 3.0 – 0.9 = 2.1 Ionic ENCl = 3.0 ΔEN = 3.0 - 3.0 = 0 Pure Covalent Bond Polarity Tro, Chemistry: A Molecular Approach

    36. Lewis Structures of Molecules • shows pattern of valence electron distribution in the 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

    37. Writing Lewis Structures of Molecules HNO3 • Write skeletal structure • H always terminal • in oxyacid, H outside attached to O’s • make least electronegative atom central • N is central • Count valence e- • sum the valence electrons for each atom • add 1 e- for each −ve charge • subtract 1 e- for each +ve charge N = 5 H = 1 O3 = 3(6) = 18 Total = 24 e- Tro, Chemistry: A Molecular Approach

    38. Writing Lewis Structures of Molecules HNO3 • Attach central atom to the surrounding atoms with pairs of e- and subtract from the total e- Start 24 Used 8 Left 16 Tro, Chemistry: A Molecular Approach

    39. Writing Lewis Structures of Molecules HNO3 • Complete octets, outside-in • H is already complete with 2 • 1 bond and re-count e- N = 5 H = 1 O3 = 3(6) = 18 Total = 24 e- e- Start 24 Used 8 Left 16 e- Start 16 Used 16 (8 pairs) Left 0 Tro, Chemistry: A Molecular Approach

    40. Writing Lewis Structures of Molecules HNO3 • If all octets complete, give extra electrons to central atom. • elements with d orbitals can have more than 8 electrons • Period 3 and below • If central atom does not have octet, bring in electrons from outside atoms to share • follow common bonding patterns if possible Tro, Chemistry: A Molecular Approach

    41. CO2 NO2- NH3 Practice - Lewis Structures Draw Lewis structures for the following: Tro, Chemistry: A Molecular Approach

    42. Writing Lewis Formulas of Molecules (cont’d) • Assign formal charges to the atoms • formal charge = valence e- - lone pair e- - ½ bonding e- • follow the common bonding patterns 0 +1 -1 all 0 sum of all the formal charges in a molecule = 0 in an ion, total equals the charge Tro, Chemistry: A Molecular ApproacH

    43. CO2 NO2- NH3 Practice - Assign Formal Charges - Tro, Chemistry: A Molecular Approach

    44. CO2 NO2- NH3 Practice - Assign Formal Charges all 0 all 0 - -1 Tro, Chemistry: A Molecular Approach

    45. •• •• •• •• O S O •• •• O S O •• •• • • •• •• • • • • • • •• •• • • • • Resonance • when there is 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 • it does not resonate between the two forms, though we often draw it that way • look for multiple bonds or lone pairs Tro, Chemistry: A Molecular Approach

    46. Rules of Resonance Structures • Resonance structures must have the same connectivity • only electron positions can change • Resonance structures must have the same number of electrons • Second row elements have a maximum of 8 electrons • bonding and nonbonding • third row can have expanded octet • Formal charges must total same • Better structures have fewer formal charges • Better structures have smaller formal charges • Better structures have − formal charge on more electronegative atom Tro, Chemistry: A Molecular Approach

    47. -1 -1 +1 Drawing Resonance Structures • draw first Lewis structure that maximizes octets • assign formal charges • move electron pairs from atoms with (-) formal charge toward atoms with (+) formal charge • if (+) fc atom 2nd row, only move in electrons if you can move out electron pairs from multiple bond • if (+) fc atom 3rd row or below, keep bringing in electron pairs to reduce the formal charge, even if get expanded octet. - -1 0 -1 +1 - Tro, Chemistry: A Molecular Approach

    48. Exceptions to the Octet Rule • expanded octets • elements with empty d orbitals can have more than 8 electrons • odd number electron species e.g., NO • will have 1 unpaired electron • free-radical • very reactive • incomplete octets • B, Al Tro, Chemistry: A Molecular Approach

    49. 0 0 0 Drawing Resonance Structures • draw first Lewis structure that maximizes octets • assign formal charges • move electron pairs from atoms with (-) formal charge toward atoms with (+) formal charge • if (+) fc atom 2nd row, only move in electrons if you can move out electron pairs from multiple bond • if (+) fc atom 3rd row or below, keep bringing in electron pairs to reduce the formal charge, even if get expanded octet. -1 +2 -1 Tro, Chemistry: A Molecular Approach

    50. Question Draw Lewis structures with assigned formal charges of HCl, H2O2 and SF6