1. Chapter 6
The Shape of Molecules

2. 6.1 The Importance of Molecular Shape
Even though composition matters in determining the properties of a compound…
Shape matters just as much!
Determined by composition
Determined by arrangement of atoms
These shapes define the chemical properties of the molecule, so we need to know exaclty what the shapes are!

3. Are all Covalent Bonds Created Equal?
NO! Electronegativity (E.N.) comes to play
High EN= greater ability to take electrons
Nonmetals tend to be HIGH  F is the highest!
Metals tend to be LOW Big alkali metals are the lowest!
Metals can lose electrons to nonmetals to form ionic bonds…but what happens with two nonmetals?

4. Covalent Bond Classification
Electronegativity and Covalent Bond Types
Two atoms with identical E.N. result in pure covalent bonding…
EQUAL sharing of electrons
Different E.N. causes polar covalent bonding…
unequal electron sharing
- shows a partial negative charge

5. Covalent Bond Classification
Electronegativity difference (EN)
The larger the EN, the greater the polarity of a bond…
Polarity means “unequal sharing”

6. Classifying Bonds based on EN
Greater EN means more polarity
EN difference below 0.5 = covalent
EN Difference = polar covalent
EN difference greater than 1.9 = ionic (usually only for metal and nonmetal)

7. Classifying Bonds based on EN
Classify the bond type of the following compounds based on EN difference
H2O (between one H and the O)
NH3 (between one H and the N)
Cl2
RbCl
Which has the greatest ionic character?
Which is most polar?
Which has the greatest covalent character?
(answers on next slide)

8. Classifying Bonds based on EN
Classify the bond type of the following compounds based on EN difference
H2O Polar covalent
NH3 Polar covalent
Cl2 Covalent
RbCl Ionic
Which has the greatest ionic character?
Which is most polar?
Which has the greatest covalent character?

9. 6.2 Valence Shell Electron Pair Repulsion (VSEPR) Theory
Valence shell electron pair repulsion theory
Bond type does not fully determine shape!
VSEPR Model is used to predict three-dimensional shapes of molecules.
Based on how negative charges on electrons repel one another…
the electrons move as far apart as possible!

10. Steps in using VSEPR theory:
Draw the electron dot diagram
Find the steric number for the atom
Predict the molecular shape…
SN of 4 = tetrahedral
SN of 3= trigonal planar
SN of 2 = linear
4. Account for lone (non-bonding) pairs
SN 4, one lone pair = PYRAMIDAL
SN 4, two lone pairs= BENT
SN 3, one lone pair= BENT

11. 6.2 VSEPR Theory

12. 6.2 VSEPR Theory
Study these rules: p 226 in text!

13. 6.2 VSEPR Theory Methane (CH4)
Four bonding pairs around the central C atom
Maximum space between electron pairs is 109.5 in three-dimensional space.
Steric number =4, no lone pairs
Called the tetrahedral shape
Electrons repel each other into this shape

14. 1. Draw the electron dot diagram.
6.2 VSEPR Theory
Predict the shape of NH3
1. Draw the electron dot diagram.
2. Find the steric number for the atom.
SN = 3 bonding pairs + 1 lone pair
SN = 4
3. Predict the maximum angle.
With SN of 4, it SHOULD be tetrahedral

15. BUT…Lone Pairs Affect Shape!
6.2 VSEPR Theory
BUT…Lone Pairs Affect Shape!
Ignore them in determining molecular shape
Electron arrangement is tetrahedral, but molecular shape is pyramidal!

16. Because they do affect the bond angles!
6.2 VSEPR Theory
Because they do affect the bond angles!
Lone pairs repel bonding pairs
They will slightly shrink other bond angles.

17. Team Work Problems 1

18. Molecular Polarity= distribution of electrons
6.3 Polarity of Molecules
Molecular Polarity= distribution of electrons
Related to bond polarity but not the same
Related to the molecule shape
Even electron distribution= symmetrical shape= nonpolar
Uneven electron distribution = asymmetrical shape =polar
Polar molecules have a charged end
Nonpolar molecules are even or neutral
See examples that follow…

19. 6.2 VSEPR Theory and Polarity

20. Consider bonding in H2 and HF
6.3 Polarity of Molecules
Consider bonding in H2 and HF
H2 is covalent and HF is polar covalent
Electrons are shared equally on H-H
Electrons are drawn closer to F in H-F
Partial charges on the molecule…
Partial positive on the H (fewer electrons)
Partial negative on the F (more electrons)

21. 6.3 Polarity of Molecules Molecular polarity
H2 is a nonpolar molecule (see previous slide)
Electrons are equally distributed, even charge
HF and HCl are polar molecules
Electrons are not equally distributed
The large difference in electronegativity draws electrons toward the F atom making it negative
Which one is more polar? Why?

22. Showing molecular polarity
6.3 Polarity of Molecules
Showing molecular polarity
Arrow with the tail at the + and head at the – end. This is a “dipole moment” which tells the amount of polarity
The longer the arrow is, the greater the polarity (predicted by greater EN Difference)

23. Dipole moment and molecular polarity
6.3 Polarity of Molecules
Dipole moment and molecular polarity
Dipoles in an asymmetrical molecule don’t cancel
Water shows this!
Water is polar, unequal charge distribution, etc…
Dipoles in linear or symmetrical molecules cancel out because they go in opposite directions
C2Cl2 shows this!
It is nonpolar, equal charge distribution, etc…

24. 6.3 Polarity of Molecules
Dipole vectors of water don’t cancel
Therefore water is a polar molecule (opposite charged ends)

25. 6.3 Polarity of Molecules
Dipole vectors of C2Cl2 cancel out
Therefore it is a nonpolar molecule (no overall charge)

26. 6.3 Polarity of Molecules
Determine if each of these is polar or nonpolar…
Phosphorus trichloride
Ammonia (nitrogen trihydride)
Ethene C2H4 (don’t forget the C=C)

27. 6.3 Polarity of Molecules
Is phosphorus trichloride, PCl3, a polar molecule?
Yes, it is…why?
Its dipole vectors do not cancel
It has an asymmetrical shape
It has a charge

28. End of Chapter 6 Chapter 6 HW Explain the cartoon on 215
Practice Problems , ,
Complete Lab 8 by drawing dipole moment vectors on the molecular diagram and determine the polarity of the molecule.

29. 6.2 Valence Shell Electron Pair Repulsion (VSEPR) Theory (Continued)
PBr3
Draw the Lewis Structure with electron groups
Find the SN
Determine the electron arrangement shape
Determine the true molecular shape
Estimate the size of the bond angles present

30. 6.2 Valence Shell Electron Pair Repulsion (VSEPR) Theory (Continued)
For PBr3
Draw the electron-group geometry…SN= 4
Electron arrangement is tetrahedral
Molecular shape is Pyramidal
Bond angles will be less then 109.5