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WS 8: Molecular Shape and Molecular
Polarity
A bond angle is the angle made by three connected nuclei in a molecule. Electron domains have characteristic angles and structures. For example, a central atom with two regions of electron density is considered linear with a bond angle of 180°.
- Structures that are more complex have multiple angles. We start by examining the bond angles and regions of electron density of simple molecules.
- These orientations lead us to basic molecular shapes. Now we will examine the bond angles and regions of electron density of simple molecules and look for similarities that will lead to insight in molecular polarity.
Part 1: Bond angles and electron domains
Table 1 : Bond angles and bonding domains for some selected molecules Molecular formula Lewis structure Bond angle Bond angle value CAChe
Regions of electron density
Bonding domains
Nonbonding domains CO 2 ÐOCO 180° 2 2 0 HCCH (^) ÐHCC 180° 2 2 0 ClNNCl (^) ÐClNN 117.4° 3 2 1 NO 3 — (^) ÐONO 120° 3 3 0 H 2 CCH 2 ÐHCH 121.1° 3 3 0
Molecular
formula
Lewis
Structure
VSEPR RED, EDG VSEPR
bond
angles
Molecular formula Lewis structure Bond angle Bond angle value CAChe
Regions of electron density
Bonding domains
Nonbonding domains CH 4 ÐCHC 109.45° 4 4 0 CH 3 Cl (^) ÐHCCl ÐHCH
CCl 4 ÐClCCl 109.45° 4 4 0 NH 3 ÐHNH 107° 4 3 1 PF 3 ÐFPF 96.3° 4 3 1 H 2 O (^) ÐHOH 104.5° 4 2 2 SbF 5 aÐFSbF eÐFSbF
BrF 5 ÐFBrF 84.8° 6 5 1
- Look at the information for ammonia and phosphorus tri-fluoride. They have the same molecular geometry. What is it? Are the bond angles the same? If they are not the same, explain why based on lone pairs and sizes of atoms. Nitrogen trifluoride has the same molecular geometry and a bond angle of 102°. Does this information fit into your theory about the differences in the bond angles?
- Antimony penta-fluoride and bromine penta fluoride have the same number of fluorine’s. Are their molecular geometries the same? How does this difference affect their bond angles?
- Nitrate and nitrite have the same electron geometry but different molecular geometries. Thinking about electron pair repulsions, what can you conclude about bond angles based on these two molecules (polyatomic ions). You can draw a similar conclusion by examining the structures of ethane (C 2 H 4 ) and dinitrogen dichloride. ÐClNN = 117° (close to 120°) in ClNNCl.
- Each of the following molecules has more than one “central atom”. For each molecule, draw its Lewis structure. Determine the molecular shape and the electron pair geometry. When possible, give approximate bond angles. Which molecules are polar? a) XeF 2 b) OF 2 c) CH 3 OH d) CH 3 CO 2 H
Dipole moments of selected molecules
Molecule
AB
Moment
(μ, D)
Geometry Molecule
AB 2
Moment
(μ, D)
Geometry Bond
angle
HF 1.78 Linear H 2 O 1.85 bent 104. HCl 1.07 Linear H 2 S 0.95 Bent HBr 0.79 Linear SO 2 1.62 Bent HI 0.38 Linear CO 2 0 linear H 2 0 Linear Linear NH 3 1.47 Trigonal- pyramidal CH 4 0 Tetrahedral 109. NF 3 0.23 Trigonal- pyramidal CHCl 3 1.04 Tetrahedral 110. ClCCl BF 3 0 Trigonal- planar CH 2 Cl 2 1.60 Tetrahedral 112.0, CHC
ClCCl CCl 4 0 Tetrahedral 109. CH 3 Cl 1.89 Tetrahedral HCH
CH 3 Br 1.82 Tetrahedral HCH
CH 3 I 1.62 Tetrahedral HCH
Ammonia and nitrogen tri-fluoride have the same geometry. Thinking about the different bonds in the two molecules, what is the effect of the lone pair and these polar bonds on the overall dipole moment of the two molecules.
Looking at the dipole moment for boron tri-fluoride and nitrogen tri-fluoride, the number of atoms is the same, yet the dipole is very different. Explain why.
Explain the trend of the dipole moment for the molecules H—X.
Looking at the methane series for AB 4 , what molecule has the largest dipole moment? Which one has the smallest, non-zero dipole moment? What is different about their structures? What is the effect of the differences on the dipole? Can you think of a reason why the HCH bond angles are similar in CH 3 X??
- Based on the bond polarities for the methane series of CH 3 X, predict the dipole moment of CH 3 F.
