1. 2008, Prentice Hall Chemistry: A Molecular Approach, 1 st Ed. Nivaldo Tro Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA

2. 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 Transition Elements all have 2 valence electrons; Why? Tro, Chemistry: A Molecular Approach2 1A2A3A4A5A6A7A8A LiBeBCNOFNe 1 e -1 2 e -1 3 e -1 4 e -1 5 e -1 6 e -1 7 e -1 8 e -1

3. Lewis Symbols of Atoms aka electron dot symbols 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 Approach3

4. Lewis Symbols of Ions Cations have Lewis symbols without valence electrons Lost in the cation formation Anions have Lewis symbols with 8 valence electrons Electrons gained in the formation of the anion Tro, Chemistry: A Molecular Approach4 Li Li +1

5. Stable Electron Arrangements And Ion Charge Metals form cations by losing enough electrons to get the same electron configuration as the previous noble gas Nonmetals form anions by gaining enough electrons to get the same electron configuration as the next noble gas The noble gas electron configuration must be very stable Tro, Chemistry: A Molecular Approach5

6. Octet Rule when atoms bond, they tend to gain, lose, or share electrons to result in 8 valence electrons ns 2 np 6 noble gas configuration many exceptions H, Li, Be, B attain an electron configuration like He He = 2 valence electrons Li loses its one valence electron H shares or gains one electron though it commonly loses its one electron to become H + Be loses 2 electrons to become Be 2+ though it commonly shares its two electrons in covalent bonds, resulting in 4 valence electrons B loses 3 electrons to become B 3+ 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 Approach6

7. 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 more stable = lower potential energy no attempt to quantify the energy as the calculation is extremely complex Tro, Chemistry: A Molecular Approach7

8. Single Covalent Bonds two atoms share a pair of electrons 2 electrons one atom may have more than one single bond Tro, Chemistry: A Molecular Approach8 F F F F H H O H H O F F

9. Double Covalent Bond two atoms sharing two pairs of electrons 4 electrons Tro, Chemistry: A Molecular Approach9 O O O O

10. Triple Covalent Bond two atoms sharing 3 pairs of electrons 6 electrons Tro, Chemistry: A Molecular Approach10 N N N N

11. Covalent Bonding Predictions 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 predicts in covalent bonding that the attractions between atoms are directional the shared electrons are most stable between the bonding atoms resulting in molecules rather than an array Tro, Chemistry: A Molecular Approach11

12. Bond Polarity covalent bonding between unlike atoms results in unequal sharing of the electrons 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 Approach12

13. HF Tro, Chemistry: A Molecular Approach13 H F  EN 2.1 EN 4.0

14. Electronegativity measure of the pull an atom has on bonding electrons increases across period (left to right) and decreases down group (top to bottom) fluorine is the most electronegative element francium is the least electronegative element the larger the difference in electronegativity, the more polar the bond negative end toward more electronegative atom Tro, Chemistry: A Molecular Approach14

15. Electronegativity Scale Tro, Chemistry: A Molecular Approach15

16. Electronegativity and Bond Polarity If difference in electronegativity between bonded atoms is 0, the bond is pure covalent equal sharing If difference in electronegativity between bonded atoms is 0.1 to 0.4, the bond is nonpolar covalent If difference in electronegativity between bonded atoms 0.5 to 1.9, the bond is polar covalent If difference in electronegativity between bonded atoms larger than or equal to 2.0, the bond is ionic 16 “100%” 00.42.04.0 4%51% Percent Ionic Character Electronegativity Difference

17. Bond Polarity Tro, Chemistry: A Molecular Approach17 EN Cl = 3.0 3.0 - 3.0 = 0 Pure Covalent EN Cl = 3.0 EN H = 2.1 3.0 – 2.1 = 0.9 Polar Covalent EN Cl = 3.0 EN Na = 1.0 3.0 – 0.9 = 2.1 Ionic

18. Bond Dipole Moments the dipole moment is a quantitative way of describing the polarity of a bond a dipole is a material with positively and negatively charged ends measured dipole moment, , is a measure of bond polarity it is directly proportional to the size of the partial charges and directly proportional to the distance between them  = (q)(r) not Coulomb’s Law measured in Debyes, D the percent ionic character is the percentage of a bond’s measured dipole moment to what it would be if full ions Tro, Chemistry: A Molecular Approach18

19. Dipole Moments Tro, Chemistry: A Molecular Approach19

20. Water – a Polar Molecule Tro, Chemistry: A Molecular Approach20 stream of water attracted to a charged glass rod stream of hexane not attracted to a charged glass rod

21. Example Determine whether bond formed between the following pair is ionic, covalent, or polar covalent N and O Sr and F N and Cl

22. 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 Approach22

23. Lewis Structures use common bonding patterns 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 often Lewis structures with line bonds have the lone pairs left off their presence is assumed from common bonding patterns structures which result in bonding patterns different from common have formal charges Tro, Chemistry: A Molecular Approach23 B C NOF

24. Example Write a Lewis structures for the following compounds CO 2 SiH 4 HNO 3

25. Examples Write a Lewis structure for the following polyatomic ions NH 4 + SO 3 2- NO 2 -

26. Formal Charge during bonding, atoms may wind up with more or less electrons 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 SFC = 6 - 2 - ½ (6) = +1 right OFC = 6 - 6 - ½ (2) = -1 sum of all the formal charges in a molecule = 0 in an ion, total equals the charge Tro, Chemistry: A Molecular Approach26 O S O

27. Common Bonding Patterns Tro, Chemistry: A Molecular Approach27 B C N O C + N + O + C - N - O - B - F F + - F

28. Examples Assign formal charge for the following polyatomic ions NH 4 + SO 3 2- NO 2 - - OH

29. 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 Approach29 O S O

30. Resonance Tro, Chemistry: A Molecular Approach30

31. 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 Approach31

32. Drawing Resonance Structures Tro, Chemistry: A Molecular Approach32 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. +1 +1

33. 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 Approach33

34. Drawing Resonance Structures Tro, Chemistry: A Molecular Approach34 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. +2 0 0 0

35. Examples Identify Structures with Better or Equal Resonance Forms and Draw Them NO 3 - PO 4 3-