Chapter , Exam 2

Transcription

1 Chapter , Exam 2

2 Covalent Bonding Covalent bonds are formed when two nonmetals combine The atoms share electrons. If the atoms share 2 electrons a single covalent bond is formed. If the atoms share 4 electrons a double covalent bond is formed. If the atoms share 6 electrons a triple covalent bond is formed. 2

3 Formation of Covalent Bonds Representation of the formation of an 2 molecule from atoms. 3

4 Formation of Covalent Bonds We can use Electron dot formulas to show covalent bond formation. Electron Dot formulas are a way to represent the number of valence electrons graphically. Each element in a molecule must have eight dots representing electrons when full shell is made Exceptions: (2 dots); e (2 dots); Group 3A (B, Al ) 6 dots; some transition metals and some elements at row 3 and below can have 10 or more dots. 4

5 Writing Electron Dot Formulas:The Octet Rule The octet rule states that representative elements usually attain stable noble gas electron configurations in most of their compounds. Electron dot formulas are based on the octet rule. 5

6 Writing Electron Dot Formulas:The Octet Rule Write the element symbol Write in a counter clockwise fashion, the valence electrons of the elements 6

7 Writing Electron Dot Formulas Determine the central atom Place the other atoms near the central atom Count the total number of valence electrons Make a single bond between the two atoms For a single bond, 2 dots 1 single bond Represent by Place the remaining electrons around each atom to give the atoms an octet (8 electrons, remember the exceptions) 7

8 Writing Electron Dot Formulas:The Octet Rule If the atoms do not get an octet before the electrons run out, use multiple bonds. For double bond, 4 dots double bond Represent by = For triple bond, 6 dots triple bond Represent by 8

9 Formation of Covalent Bonds 1.) molecule formation representation or 2 2. Cl molecule formation Cl..... Cl or Cl.... 9

10 Electron Dot Formulas for Molecules and Polyatomic Ions hydrogen fluoride, F. F or F hydrogen bromide, Br. Br or Br 10

11 Formation of Covalent Bonds 11

12 Writing Electron Dot Formulas:The Octet Rule Example 7-1: Write Lewis dot and dash formulas for silicon tetrafluoride, SiF 4. Total = 4 (Si) + 7x4 (F) = 32 Si is central atom, bonded to each of 4 F F F Si F F 8 electrons used for bonds (32-8=24) 24 electrons placed around the 4 Fs. 12

13 Writing Electron Dot Formulas:The Octet Rule F F Si F F Recount to make sure that all elements have 8 electrons 13

14 Writing Electron Dot Formulas:The Octet Rule Example 7-2: Write Lewis dot and dash formulas for hydrogen cyanide, CN. N = 1 () + 4 (C) + 5 (N) = 10 Carbon is central atom One bond to and one to N (4 electrons) 6 electrons left, but neither C or N have an octet Must share more electrons Triple bond give C 8 electrons and N 6 electrons. 2 left over sit on N to make 8 electrons C N or C N 14

15 Writing Electron Dot Formulas:Limitations of the Octet Rule Example 7-4: Write dot formulas for BBr 3. You do it! 15 Br B Br. B Br Br. or Br B Br Br

16 Naming Some Inorganic Compounds Covalent molecular compounds composed of two nonmetals other than hydrogen Nomenclature must include prefixes that specify the number of atoms of each element in the compound. Use the minimum number of prefixes necessary to specify the compound. Frequently drop the prefix mono-. 16

17 Naming Some Inorganic Compounds 17

18 Naming Some Inorganic Compounds Formula CO CO 2 SO 3 OF 2 P 4 O 6 Name carbon monoxide carbon dioxide sulfur trioxide oxygen difluoride You do it! 18

19 Naming Some Inorganic Compounds Formula CO CO 2 SO 3 OF 2 P 4 O 6 hexoxide P 4 O 10 Name carbon monoxide carbon dioxide sulfur trioxide oxygen difluoride tetraphosphorus You do it! 19

20 Naming Some Inorganic Compounds Formula CO?? SO 3?? P 4 O 6?? Name carbon monoxide carbon dioxide sulfur trioxide oxygen difluoride tetraphosphorus hexoxide tetraphosphorus decoxide 20

21 Naming Some Inorganic Compounds Formula N 2 O NO N 2 O 3 NO 2???? Modern Name dinitrogen monoxide nitrogen monoxide dinitrogen trioxide nitrogen dioxide dinitrogen tetroxide dinitrogen pentoxide 21

22 Covalent Compound Naming Review 22

23 Covalent Compound Naming Review 23

24 Polar and Nonpolar Covalent Bonds Covalent bonds in which the electrons are shared equally are designated as nonpolar covalent bonds. Nonpolar covalent bonds have a symmetrical charge distribution. To be nonpolar the two atoms involved in the bond must be the same element or have the similar electronegativity to share equally. 24

25 Polar and Nonpolar Covalent Bonds Some examples of nonpolar covalent bonds.. or 2 N 2 N N or N N 25

26 Polar and Nonpolar Covalent Bonds Covalent bonds in which the electrons are not shared equally are designated as polar covalent bonds Polar covalent bonds have an asymmetrical charge distribution To be a polar covalent bond the two atoms involved in the bond must have different electronegativities. 26

27 Polar and Nonpolar Covalent Bonds Some examples of polar covalent bonds. F Electronegativities F Difference = 1.9 very polar bond 27

28 Polar and Nonpolar Covalent Bonds 28

29 Polar and Nonpolar Covalent Bonds 29

30 Dipole Moments Molecules whose centers of positive and negative charge do not coincide, have an asymmetric charge distribution, and are polar. These molecules have a dipole moment. The dipole moment has the symbol µ. 30 δ Fδ 1.91 Debye units δ I δ 0.38 Debye units

31 Dipole Moments There are some nonpolar molecules that have polar bonds. There are two conditions that must be true for a molecule to be polar. 1. There must be at least one polar bond present or one lone pair of electrons. 2. The polar bonds, if there are more than one, and lone pairs must be arranged so that their dipole moments do not cancel one another. 31

32 Polar and Nonpolar Covalent Bonds 32

33 VSEPR TEORY Valence Shell Electron Pair Repulsion Theory Commonly designated as VSEPR Principal originator R. J. Gillespie in the 1950 s Arrange electrons as far apart as possible to minimize repulsion Give the shape of the molecule 33

34 Predicting Shapes The same basic approach will be used in every example of molecular structure prediction: 1. Draw the correct Electron dot structure. Identify the central atom. Designate the bonding pairs and lone pairs of electrons on central atom. 2. Count the regions of high electron density on the central atom. Include both bonding and lone pairs in the counting. 34

35 Predicting Shapes 3. Determine the electronic geometry around the central atom. VSEPR is a guide to the geometry. 4. Determine the molecular geometry around the central atom. Ignore the lone pairs of electrons. 5. Determine polarity from the molecular geometry using electronegativity differences. 35

36 VSEPR Theory Regions of high electron density around the central atom are arranged as far apart as possible to minimize repulsions. There are 5 basic molecular shapes that we will use. 36

37 VSEPR Theory 1 Two regions of high electron density around the central atom. 37

38 VSEPR Theory 2 Three regions of high electron density around the central atom. 38

39 VSEPR Theory 3 Four regions of high electron density around the central atom. 39

40 40

41 VSEPR Theory Frequently, we describe two geometries for each molecule. 1. Electronic geometry is determined by the locations of regions of high electron density around the central atom(s). 2. Molecular geometry determined by the arrangement of atoms around the central atom(s). Electron pairs are not used in the molecular geometry determination just the positions of the atoms in the molecule are used. 41

42 VSEPR Theory An example of a molecule that has the same electronic and molecular geometries is methane - C 4. Electronic and molecular geometries are tetrahedral. 42 C

43 VSEPR Theory An example of a molecule that has different electronic and molecular geometries is water - 2 O. Electronic geometry is tetrahedral. Molecular geometry is bent or angular. C 43

44 Polar Molecules: The Influence of Molecular Geometry Molecular geometry affects molecular polarity. Due to the effect of the bond dipoles and how they either cancel or reinforce each other. 44 A B A linear molecule nonpolar A B A angular molecule polar

45 Polar Molecules: The Influence of Molecular Geometry Polar Molecules must meet two requirements: 1. One polar bond or one lone pair of electrons on central atom. 2. Neither bonds nor lone pairs can be symmetrically arranged that their polarities cancel. 45

46 Molecular Shapes and Bonding A = central atom B = bonding pairs around central atom U = lone pairs around central atom For example: AB 3 U designates that there are 3 bonding pairs and 1 lone pair around the central atom. 46

47 Linear Electronic Geometry:AB 2 Species Some examples of molecules with this geometry are: BeCl 2, BeBr 2,BeI 2, gcl 2, CdCl 2 All of these examples are linear, nonpolar molecules. Important exceptions occur when the two substituents are not the same! BeClBr or BeIBr will be linear and polar! 47

48 Linear Electronic Geometry:AB 2 Species Dot Formula Cl Be Cl Electronic Geometry Cl Be Cl 180 o - linear 48

49 Linear Electronic Geometry:AB 2 Species A) Molecular Geometry Cl Be Cl 180 o -linear Electronegativities Cl Polarity - Be Cl bond dipolesare 2.0 symmetric 2.0 very polar bonds - Cl Be- - - Cl nonpolar molecule 49 C

50 Trigonal Planar Electronic Geometry: AB 3 Species Some examples of molecules with this geometry are: BF 3, BCl 3 All of these examples are trigonal planar, nonpolar molecules. Important exceptions occur when the three substituents are not the same! BF 2 Cl or BCI 2 Br will be trigonal planar and polar! 50

51 Trigonal Planar Electronic Geometry: AB 3 Species Dot Formula Cl B Cl Cl Electronic Geometry B 120-trigonal planar 51

52 Trigonal Planar Electronic Geometry: AB 3 Species Molecular Geometry Polarity Cl B Cl Cl 120 o -trigonal planar Cl B Electronegativities Cl Cl B - Cl very polar bonds bond dipoles are symmetric nonpolar molecule 52 C

53 Tetrahedral Electronic Geometry: AB 4 Species Some examples of molecules with this geometry are: C 4, CF 4, CCl 4, Si 4, SiF 4 All of these examples are tetrahedral, nonpolar molecules. Important exceptions occur when the four substituents are not the same! CF 3 Cl or C 2 CI 2 will be tetrahedral and polar! 53

54 Tetrahedral Electronic Geometry: AB 4 Species 54

55 Tetrahedral Electronic Geometry: AB 4 Species Dot Formula.... C C.. Electronic Geometry..... tetrahedral o bond angles. 55

56 Tetrahedral Electronic Geometry: AB 4 Species Molecular Geometry C tetrahedral Electronegativities Polarity C C slightly polar bonds symmetric dipoles nonpolar molecule 56 C

57 57 Tetrahedral Electronic Geometry: AB 3 U Some examples of molecules with this geometry are: N 3, NF 3, P 3, PCl 3, As 3 These molecules are our first examples of central atoms with lone pairs of electrons. Thus, the electronic and molecular geometries are different. All three substituents are the same but molecule is polar. N 3 and NF 3 are trigonal pyramidal, polar molecules.

58 Tetrahedral Electronic Geometry: AB 3 U Species Dot Formulas Electronic Geometry N F. N. F F... N.. tetrahedral......

59 Tetrahedral Electronic Geometry: AB 3 U Species Molecular Geometry 1 lo n e p a ir p y r a m 1 lo n e p a ir 59 F.. N.. N p y r a m F id a l F id a l.. N Electronegativities very polar bonds N Electronegativities F F Polarity N - asym m etrical dipoles polar m olecule µ = 1.5 D.. C F asym m etrical dipoles polar m olecule µ = 0.2 D bond dipoles reinforce effect of lone pair bond dipoles oppose N - effect F of lone pair very polar bonds

60 Tetrahedral Electronic Geometry: AB 2 U 2 Species Some examples of molecules with this geometry are: 2 O, OF 2, 2 S These molecules are our first examples of central atoms with two lone pairs of electrons. Thus, the electronic and molecular geometries are different. Both substituents are the same but molecule is polar. Molecules are angular, bent, or V-shaped and polar. 60

61 Tetrahedral Electronic Geometry: AB 2 U 2 Species Molecular Geometry O bent, angular or V-shaped 2 lone pairs Electronegativities O Polarity bond dipoles reinforce O - lone pairs very polar bonds C asymetric dipoles very polar molecule µ 1.7 D 61

62 Tetrahedral Electronic Geometry: ABU 3 Species Some examples of molecules with this geometry are: F, Cl, Br, I, FCl, IBr These molecules are examples of central atoms with three lone pairs of electrons. Again, the electronic and molecular geometries are different. Molecules are linear and polar when the two atoms are different. Cl 2, Br 2, I 2 are nonpolar. 62

63 Tetrahedral Electronic Geometry: ABU 3 Species Dot Formula F Electronic Geometry F tetrahedral 63

64 Tetrahedral Electronic Geometry: ABU 3 Species Molecular Geometry Polarity F is a polar molecule. F 3 lone pairs C linear 64