Introductory Chemistry , 2 nd Edition Nivaldo Tro

1. Introductory Chemistry, 2nd EditionNivaldo Tro Chapter 10 Chemical Bonding

2. Bonding Theories • Chemical bonding describes way atoms attach to make compounds • Understanding of bonding allows chemists to: • predict shapes of molecules and properties of substances • design and build molecules with particular sets of chemical and physical properties Tro's Introductory Chemistry, Chapter 10

3. • •• •• •• •• Li• Be• •B• •C• •N• •O::F: :Ne: • • • • • • •• Lewis Symbols of Atoms • Also known as electron dot symbols • Symbol of element represents nucleus and inner electrons • Valence electrons are dots in imaginary 4 sides around symbol • put one electron on each side first, then pair • Elements in same group have same number of valence electrons; therefore their Lewis dot symbols will look alike Tro's Introductory Chemistry, Chapter 10

4. Lewis Bonding Theory • Atoms bond because it results in a more stable electron configuration • Atoms bond together by • transferring electrons (ionic bond) or • sharing electrons (covalent bond) so that all atoms obtain an outer shell with 8 electrons = Octet Rule • there are exceptions to this rule – the key to remember is to try to get a valence electron configuration like a noble gas • Which noble gas is an exception? He, why? Tro's Introductory Chemistry, Chapter 10

5. •• •• Li•  Li+1:F: → [:F:]-1 • •• 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's Introductory Chemistry, Chapter 10

6. What determines which type of bond is formed? • Electronegativity = attraction for electrons • ionic bond: two atoms have a large difference in electronegativity. • covalent bond: atoms have similar electronegativities Tro's Introductory Chemistry, Chapter 10

7. I would do anything for an e- !!! I really don’t want this electron… Ionic bond: very different electronegativities Metals = give up electrons easily; become + Non-metals = like to gain electrons become - Metal(+) and non-metal(-) = ionic bond will be formed (not covalent).

8. We seem to both want to keep our electrons… How can we get octets? We could share the electrons we have! Covalent bond: similar electronegativities 2 non-metals = covalent bond will be formed (not ionic).

9. Ionic Bonds • Metal transfers electron(s) to nonmetal • Metal loses electrons to form cation • Nonmetal gains electrons to form anion • Ionic bond results from + to - attraction • larger charge = stronger attraction • smaller ion = stronger attraction • Lewis Theory allows us to predict the correct formulas of ionic compounds Tro's Introductory Chemistry, Chapter 10

10. Na Cl Example: ionic bond

11. Cl - + Na Cl Example: ionic bond Na

12. Charges of ions: • If an atom:Its charge is now: • loses an e- +1 • gains an e- -1 • loses 2 e- +2 • gains 2 e- -2

13. Draw Lewis structure for compound formed between Mg and O. +2 -2 x Mg O x magnesium oxide

14. Draw Lewis structures for the compounds formed by: • sodium & sulfur • calcium & bromine

15. -2 + + x x Na S Na - - +2 x Ca x Br Br Name these! sodium & sulfur Na2S calcium & bromine CaBr2 sodium sulfide calcium bromide

16. H H x x H H Covalent Bonds = sharing of electrons • Type of bond for two nonmetal atoms • Attraction for electrons (electronegativity) is similar for both atoms involved H2

17. Covalent Bonds • Formed between two nonmetals • Atoms bonded together to form molecules • strong attraction • Sharing pairs of electrons to attain octets • Molecules generally weakly attracted to each other • observed physical properties of molecular substance due to these attractions Tro's Introductory Chemistry, Chapter 10

18. Cl Cl Cl Cl Cl2 Cl - Cl Shared electrons are always in pairs. single bond = one shared pair of electrons

19. •• •• • • •• F F •• •• •• •• •• •• •• •• F F F F •• •• Single Covalent Bonds • Two atoms share one pair of electrons • 2 electrons • One atom may have more than one single bond •• • • • • H H O •• •• •• H H •• O •• Tro's Introductory Chemistry, Chapter 10

20. bond line formula H H H H C H H C H H Write the Lewis structure for CH4 How many single bonds does it have? 4

21. •• •• • • • • O O •• •• •• •• O •• •• •• •• O O O Double Covalent Bond • Two atoms sharing two pairs of electrons • 4 electrons • Shorter and stronger than single bond Tro's Introductory Chemistry, Chapter 10

22. •• •• • • • • N N • • N •• N •• •• •• •• N N Triple Covalent Bond • Two atoms sharing 3 pairs of electrons • 6 electrons • Shorter and stronger than single or double bond Tro's Introductory Chemistry, Chapter 10

23. Ionic or Covalent? and Name? ionic sodium chloride • NaCl • MgBr • PH3 • NO • KNO3 • Mg(OH)2 • NH4Br • H2O ionic magnesium bromide covalent phosphorus trihydride covalent nitrogen monoxide ionic potassium nitrate ionic magnesium hydroxide ionic ammonium bromide covalent dihydrogen monoxide

24. Ionic or Covalent? Write the formula: • ammonium sulfide • dihydrogen dioxide • potassium sulfate • copper II nitrate ionic (NH4)2S covalent H2O2 ionic K2SO4 ionic Cu(NO3)2 Tro's Introductory Chemistry, Chapter 10

25. Exceptions to the Octet Rule • H & Li, lose one electron to form cation • Li now has electron configuration like He • H can also share or gain one electron to have configuration like He • Be shares 2 electrons to form two single bonds • B shares 3 electrons to form three single bonds Tro's Introductory Chemistry, Chapter 10

26. Exceptions to the Octet Rule • Expanded octets for elements in Period 3 or below • using empty valence d orbitals • Some molecules have odd numbers of electrons • NO Tro's Introductory Chemistry, Chapter 10

27. Molecular Geometry • Molecules are 3-dimensional objects • Describe shape of a molecule with terms that relate to geometric figures • These geometric figures have characteristic “corners” that indicate positions of surrounding atoms with central atom in center of the figure • The geometric figures also have characteristic angles that we call bond angles Tro's Introductory Chemistry, Chapter 10

28. 180° 120° 109.5° Some Geometric Figures • Linear • 2 atoms on opposite sides of central atom • 180° bond angles • Trigonal Planar • 3 atoms form a triangle around the central atom • Planar, 120° bond angles • Tetrahedral • 4 surrounding atoms form a tetrahedron around central atom • 109.5° bond angles Tro's Introductory Chemistry, Chapter 10

29. H How will the electrons distribute themselves around C in order to be as far as possible away from each other? H H C H Predicting Molecular Shape: VSEPR Valence Shell Electron Pair Repulsion Theory = the shape of a molecule can be predicted by assuming that the electron pairs repel each other.

30. Tetrahedral Distribution of Four Electron Pairs

31. Central Atom with 4 electron pairs bonded to 4 atoms in corners CH4 =molecule is “tetrahedral”

32. N H H H H H O What would be the shape of these molecules?

33. N H H H H H O • “pyramidal” • “bent”

34. -1 H “linear” O When there are only two atoms, the only molecular shape possible is:

35. Linear Shapes • Linear • 2 areas of electrons around the central atom, both bonding • Or two atom molecule • 180° Bond Angles Tro's Introductory Chemistry, Chapter 10

36. • • d+ H Cl d- Bond Polarity • Unequal sharing of electrons between unlike atoms • one atom pulls electrons in the bond closer to its side • one end of the bond has larger electron density than the other • The end with the larger electron density gets a partial negative charge and the end that is electron deficient gets a partial positive charge Tro's Introductory Chemistry, Chapter 10

37. d+ H — F d- Electronegativity • Attraction an atom has for bonding electrons in covalent bond • Increases across period (left to right) • Decreases down group (top to bottom) • Larger difference in electronegativity, more polar the bond • negative end toward more electronegative atom Tro's Introductory Chemistry, Chapter 10

38. 2.1 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.9 1.2 1.5 1.8 2.1 2.5 3.0 0.8 1.0 1.3 1.5 1.6 1.6 1.5 1.8 1.8 1.8 1.9 1.6 1.6 1.8 2.0 2.4 2.8 0.8 1.0 1.2 1.4 1.8 1.9 2.2 2.2 2.2 1.9 1.7 1.7 1.8 1.9 2.1 2.5 1.6 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.2 2.2 2.2 2.4 1.9 1.8 1.8 1.9 2.0 2.2 0.7 0.9 1.1 Electronegativity Tro's Introductory Chemistry, Chapter 10

39. Electronegativity Tro's Introductory Chemistry, Chapter 10

40. Electronegativity & Bond Polarity • Nonpolar covalent = difference in electronegativity between bonded atoms is 0 to 0.3 • Polar covalent=difference in electronegativity between bonded atoms is 0.4 to 1.9 • Ionic =difference in electronegativity between bonded atoms larger than or equal to 2.0 Tro's Introductory Chemistry, Chapter 10

41. Bond Polarity 3.0-3.0 = 0.0 4.0-2.1 = 1.9 3.0-0.9 = 2.1 covalent ionic non polar polar 4.0 0 0.4 2.0 Electronegativity Difference Tro's Introductory Chemistry, Chapter 10

42. Polarity of Molecules • For a molecule to be polar it must • have polar bonds • electronegativity difference of 0.4 – 1.9 • bond dipole moments - measured • have an unsymmetrical shape • Polarity affects the intermolecular forces of attraction Tro's Introductory Chemistry, Chapter 10

43. Polarity of Molecules polar bonds, but nonpolar molecule because pulls cancel polar bonds, and unsymmetrical shape causes molecule to be polar Tro's Introductory Chemistry, Chapter 10

44. Polar or Nonpolar Molecule? CCl4 m = 0.0 D CH2Cl2 m = 2.0 D Tro's Introductory Chemistry, Chapter 10