1. Molecular Structure and Shape
Chapter 6 Sections 2 and 5

2. Lewis Structures of Second-Period Elements

3. Vocabulary for Lewis Structures
Unshared pairs (lone pairs): a pair of valence electrons not involved in bonding to another atom
Single bond: a covalent bond in which one pair of electrons is shared between two atoms
Multiple bonds:
double bond shares 2 pairs of electrons
triple bond shares 3 pairs of electrons

4. Steps for Drawing Lewis Structures

5. Drawing Lewis Structures for Molecules
The atom with the lowest electronegativity is usually the central atom
If carbon is present, it is usually the central atom
If hydrogen is present, it is never in the center
can only have 2 e-

6. Drawing Ions
You must consider the absence of an electron or the extra electrons
If the charge is positive, subtract an absent electron from the total
If the charge is negative, add any extra electrons to the total
Place the entire structure in brackets and put the charge in the upper right corner outside the brackets

7. Resonance Structures
Resonance Structures: bonding in molecules or ions that cannot be correctly represented by a single Lewis structure
Draw each possible structure and the hybrid structure

8. VSEPR Theory
Electron groups around the central atom will be most stable when they are as far apart as possible. We call this Valence Shell Electron Pair Repulsion theory.
Because electrons are negatively charged, they should be most stable when they are separated as much as possible.
The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule.

10. Electron Groups
The Lewis structure predicts the number of valence electron pairs around the central atom(s).
Each lone pair of electrons constitutes one electron group on a central atom.
Each bond constitutes one electron group on a central atom, regardless of whether it is single, double, or triple. O N •
There are three electron groups on N:
One lone pair
One single bond
One double bond

11. Molecular Shapes
Three-dimensional shape helps determine physical and chemical properties
Valence Shell Electron Pair Repulsion (VSEPR) theory predicts molecular shapes based on the idea that electrons repel one another

12. Molecular Shapes
Lewis structures show which atoms are connected, where they are connected, and by how many bonds, but they don’t properly show the 3-D shapes of molecules
To find the geometric shape:
draw the Lewis structure
then use VSEPR theory

13. VSEPR Rules To apply VSEPR theory:
Draw the Lewis structure of the molecule and identify the central atom
Count the number of electron charge clouds (lone and bonding pairs) surrounding the central atom.
Predict molecular shape by assuming that clouds orient so they are as far away from one another as possible

14. (Could include 180º)
(Could include 180º)

15. Polarity of Molecules Molecules of:
Two atoms: bond polarity is the molecular polarity
More than 2 atoms: the geometry of the molecule must be considered
If the bonds are identically non-polar, the molecular is non-polar
Some molecules with polar bonds can be non-polar

16. More
Sometimes the partial charges cancel each other out because they are directly opposite each other
Consider CO2 and CCl4
The symmetrical distribution of the bonds leads to cancellation of the charges.

17. Polar and Non-polar Molecules

19. 1. Linear with More than 2 Atoms
Sometimes molecules made of more than 2 atoms are shaped linearly
Example: CO2
The oxygens pull the electrons in equal, but opposite, directions
The molecule is sometimes nonpolar and sometimes polar

20. 2. Trigonal Planar This shape resembles a triangle
Example: CH2O and BF3
Boron is an exception to the octet rule
The bond angle is 120°
The molecule can be polar or nonpolar depending on the nature of the atoms bonded to the central atom

21. 2. Bent There are two shapes that are called bent
The first one has one lone pair on the central atom and two bonded atoms (trigonal planar family)
Example: NOCl (nitrosyl chloride)
Bond angle: 117°
The molecule is polar

22. 3. Tetrahedral
This shape usually occurs when 4 atoms are bonded to the central atom
Examples: CF4, CH4 (methane)
If the 4 bonded atoms are identical, and the central atom obeys the octet rule, the molecule is nonpolar
Bond angle is 109.5°

23. 3. Trigonal Pyramidal
There is one lone pair on the central atom and three bonded atoms
Examples: NH3, PCl3
Bond angle is 107°
The molecule is polar

24. 3. Bent
The second “bent” molecular geometry has two lone pairs on the central atom
Example: H2O
The bond angle is 105° (some books report 104.5°)
The molecule is polar