How does the number of bonds and nonbonded pairs of electrons affect the shape of a molecule?

Transcription

1 Reading: Chapter 9, sections As you read these sections ask yourself: ow does the number of bonds and nonbonded pairs of electrons affect the shape of a molecule? Why is the repulsion between two domains of nonbonded pairs of electrons greater than between two domains of bonded pairs of electrons? ow can a molecule with polar bonds be nonpolar? Why do we need theories of bonding that differ from VSEPR? ow does Valence Bond theory differ from the Lewis concept of chemical bonding? ow does molecular orbital theory differ from valence bond theory? ow does a hybrid orbital differ from a pure atomic orbital? ow are hybrid orbitals related to the VSEPR shapes you learned earlier? Chem ow do sigma and pi bonds differ from each other? Does the Lewis structure of this AB 3 molecule follow the octet rule? ow many electron domains are there around the atom A? A. No; 5 domains. B. No; 4 domains. C. No; 3 domains. D. Yes; 4 domains. E. Yes; 3 domains. 1

2 Does the Lewis structure of this AB 3 molecule follow the octet rule? ow many electron domains are there around the atom A? A. No; 5 domains. B. No (2 electrons from lone pair + 4( 2 electron from each single bond)+ 4 electrons from double bond=10 electrons; 4 domains (2 single bonds+1 double bond +1 lone pair=4 domains). C. No; 3 domains. D. Yes; 4 domains. E. Yes; 3 domains. One of the resonance structures of the nitrate ion, NO 3, is shown. The bond angles are exactly 120. Is this observation consistent with the effect of multiple bonds on bond angles? A. No, the double bond should squeeze the bond angle between single bonds to less than 120. B. Yes, resonance equalizes repulsions between bonds, making all angles 120. C. Sorry; all future questions will have to be addressed to my lawyer. 2

3 One of the resonance structures of the nitrate ion, NO 3, is shown. The bond angles are exactly 120. Is this observation consistent with the effect of multiple bonds on bond angles? A. No, the double bond should squeeze the bond angle between single bonds to less than 120. B. Yes, resonance equalizes repulsions between bonds, making all angles 120. C. Sorry; all future questions will have to be addressed to my lawyer. Shapes of larger molecules The interior atoms of more complicated molecules can be dealt with in turn using the VSEPR model. tetrahedral 4 3 trigonal planar 4 bent 3

4 In what situation is the name of the molecular geometry the same as the name of the electronic geometry? a) When all of the electron pairs are involved in bonding. b) When all but one of the electron pairs are involved in bonding. c) When the central atom does not obey the octet rule. d) When the central atom does obey the octet rule. e) More than one of the above is correct. In what situation is the name of the molecular geometry the same as the name of the electronic geometry? a) When all of the electron pairs are involved in bonding. When the number of electron domains equals the number of bonding domains, the electronic geometry and molecular geometry are the same. If there are no invisible non bonding electrons changing the shape, the bonding domains determine the shape. 4

5 Which of the following molecular shapes has a tetrahedral electron domain arrangement? A) trigonal planar B) trigonal pyramid C) seesaw D) square planar E) linear Which of the following molecular shapes has a tetrahedral electron domain arrangement? B) trigonal pyramid

6 The molecule O=C=S has a Lewis structure analogous to that of CO 2 and is a linear molecule. Will it have a zero dipole moment like CO 2? A. No B. Yes C. It depends on whether it is O=C=S or S=C=O The molecule O=C=S has a Lewis structure analogous to that of CO 2 and is a linear molecule. Will it have a zero dipole moment like CO 2? A. No : the C-S and C-O dipoles are not equal. Therefore the O end will be slightly more negative. B. Yes C. It depends on whether it is O=C=S or S=C=O 6

7 In the organic molecule diagrammed below which carbon uses trigonal planar geometry in the bonding? (B) C C C C C (A) (C) In the organic molecule diagrammed below which carbon uses trigonal planar geometry in the bonding? B: 3 electron domains is triganol planar (also sp 2 hybridization) (B) C C C C C (A) (C) 7

8 In an sp 2 hybridized atom, what is the orientation of the unhybridized p orbital relative to the three sp 2 hybrid orbitals? A. The unhybridized p-orbital is 180 from the plane of the sp 2 orbitals. B. The unhybridized p-orbital is coplanar with the plane of the sp 2 orbitals. C. The unhybridized p-orbital is from the plane of the sp 2 orbitals. D. The unhybridized p-orbital is perpendicular to the plane of the sp 2 orbitals. E. The unhybridized p-orbital is reeling drunkenly about the atom. In an sp 2 hybridized atom, what is the orientation of the unhybridized p orbital relative to the three sp 2 hybrid orbitals? D. The unhybridized p-orbital is perpendicular to the plane of the sp 2 orbitals. one s orbital hybridize two p orbitals three sp 2 hybrid orbitals 8

9 Which types of atomic orbitals of the central atom mix to form hybrid orbitals in NF 3? A) one s and one p B) one s and 2 p C)one s and 3 p D)one s, 3 p, and one d E) one s, 3 p, and 2 d Which types of atomic orbitals of the central atom mix to form hybrid orbitals in NF 3? C) one s and 3 p Recall: ybridization gives the electronic geometry 9

10 Which of the following statements is true? A) Two sigma bonds comprise a double bond. B) A triple bond consists of one pi bond and two sigma bonds. C)A pi bond consists of two pairs of electrons. D)End-to-end overlap results in a sigma bond with electron density asymmetrically above and below the bond axis. E) Bonds formed only from atomic s orbitals are always sigma bonds. Which of the following statements is true? A) Two sigma bonds comprise a double bond. No-a double bond is a sigma and one pi B) A triple bond consists of one pi bond and two sigma bonds. No-A triple bond is one sigma and two pi C) A pi bond consists of two pairs of electrons. No-a single pi bond has 2 electrons (one pair) D) End-to-end overlap results in a sigma bond with electron density asymetrically above and below the bond axis. No- a sigma bond is symmetric E) Bonds formed only from atomic s orbitals are always sigma bonds. Yes-s orbitals can only form sigma bond (p orbital can form either sigma or pi depending on orientation) 10

11 Give it Some Thought: Suppose light is used to excite one of the electrons in the 2 molecule from the σ 1s MO and completely remove it from the molecule. Would you expect the atoms to remain bonded together, or would the molecule fall apart? (Recall, this is the question I asked verbally, as I misread the next question) 1. The atoms would remain bonded to one another. 2. The molecule would fall apart. Give it Some Thought: Suppose light is used to excite one of the electrons in the 2 molecule from the σ 1s MO and completely remove it from the molecule. Would you expect the atoms to remain bonded together, or would the molecule fall apart? 1. The atoms would remain bonded to one another. 2. The molecule would fall apart. + 11

12 1. Bond order is Bond order is 1/2. 3. Bond order is Bond order is 3/2. 1. Bond order is Bond order is ½ (One electron in a bonding orbital. None in an antibonding). 3. Bond order is Bond order is 3/2. 12

13 1. The atoms would remain bonded to one another. 2. The molecule would fall apart. 1. The atoms would remain bonded to one another. 2. The molecule would fall apart. * 13

14 1. Bond order is Bond order is 1/2. 3. Bond order is Bond order is 3/2. 1. Bond order is 0 (One electron in a bonding orbital. One in an antibonding).. 2. Bond order is ½ 3. Bond order is Bond order is 3/2. 14

15 Bonus: iclicker questions not shown in class The following questions were not answered in class and generally do not have explanations. They are just available if you want some extra practise. If you have any queries about the answer key (last slide), let me know. 1) One of the common shapes for AB 4 molecules is square planar: all five atoms lie in the same plane, the atoms B lie at the corners of a square, and the atom A is at the centre. Which of the shapes below could lead to a square-planar geometry upon the removal of one or more atoms? A. linear B. trigonal planar C. tetrahedral D. trigonal bipyramidal E. octahedral 15

16 2)Can orbital overlap explain why the bond length in Cl 2 is longer than the bond length in F 2? A. Yes B. No 3. The molecule diazine has the formula N 2 2 and the Lewis structure shown below. Would you expect diazine to be a linear molecule (all four atoms on the same line)? Would you expect the molecule to be planar (all four atoms in the same plane)? A. The molecule is both linear and planar. B. The molecule is not linear but is planar. C. The molecule is linear but not planar. D. The molecule is neither linear nor planar. E. The molecule is orange with green polka dots. 16

17 4. Which of the following numbers of electron groups can give rise to a bent (V-shaped) molecule? A) 2 B) 3 C) 4 D) 3 or 4 E) 4 or 5 5. Using VSEPR, predict the molecular geometry of O 3. A. Bent B. Trigonal planar C. Trigonal pyramidal D. Linear 17

18 6.What is the shape of the molecule CN? C N A) Bent B) Trigonal planar C) See saw D) linear 7.What is the molecular geometry of formaldehyde? A)trigonal pyramidal B) trigonal planar C) angular (or bent) D)t-shape O C 18

19 8.What is the electron pair geometry and molecular geometry around the oxygen in the structure? A)linear, linear B) linear, bent C) tetrahedral, linear D)tetrahedral, bent E) tetrahedral, tetrahedral C O 9. A molecule with the formula AB 2 E 0 uses orbitals to form its bonds. A) sp hybrid orbitals B) sp 2 hybrid orbitals C) sp 3 hybrid orbitals D) sp 3 d hybrid orbitals E) sp 3 d 2 hybrid orbitals 19

20 10.Valence bond theory predicts that carbon will use hybrid orbitals in the carbonate anion, CO 2-3. C A) sp B) sp 2 C) sp 3 D) sp 3 d O E) sp 3 d 2 O O 1. E 2. A 3. B 4. D 5. A 6. D 7. B 8. D 9. B 10.B Answer Key for Bonus Questions 20