1. We have a quiz over Covalent Bonding (you may want to use this time to study!)
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 NEW room code: KILBANESTP
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2. Chemical Bonding and VSEPR
Today’s topic:
(Also our LAST topic of the semester!!)
Chemical Bonding and VSEPR 2

3. The Shapes of Molecules
The shape of a molecule has an important bearing on its reactivity and behavior.
The shape of a molecule depends a number of factors. These include:
Atoms forming the bonds
Bond distance
Bond angles 3

4. Valence Shell Electron Pair Repulsion
Valence Shell Electron Pair Repulsion (VSEPR) theory can be used to predict the geometric shapes of molecules.
VSEPR revolves around the principle that electrons repel each other.
One can predict the shape of a molecule by finding a pattern where electron pairs are as far from each other as possible. 4

5. Bonding Electrons and Lone Pairs
In a molecule some of the valence electrons are shared between atoms to form covalent bonds. These are called bonding electrons.
Other valence electrons may not be shared with other atoms. These are called non-bonding electrons or they are often referred to as lone pairs. 5

6. VSEPR
In all covalent molecules electrons will tend to stay as far away from each other as possible
The shape of a molecule depends on:
the number of regions of electron density (BONDS) it has on its central atom,
whether these are bonding or non-bonding electrons (lone pairs). 6

7. Lewis Dot Structures
Lewis Dot structures are used to represent the valence electrons of atoms in covalent molecules
Dots are used to represent only the valence electrons.
Dots are written between symbols to represent bonding electrons 7

8. Lewis Dot Stucture for SO3
The diagram below shows the dot structure for sulfur trioxide. The bonding electrons are in shown in red and lone pairs are shown in blue. 8

9. Violations of the Octet Rule
Violations of the octet rule usually occur with B and elements of higher periods. Some common examples include: Be, B, P, S, and Xe.
SF4
BF3
Be: 4
B: 6
P: 8 OR 10
S: 8, 10, OR 12
Xe: 8, 10, OR 12

10. VSEPR Predicting Shapes

11. VSEPR: Predicting the shape
Once the dot structure has been established, the shape of the molecule will follow one of basic shapes depending on:
How many bonds the central atom forms
The number of regions of electron density that are occupied by bonding electrons vs. lone pairs 11

12. VSEPR: Predicting the shape
As #2 said, the bond angles relate to the amount of bonding electrons vs. lone pairs.
2 Lone Pairs (LP/LP) = largest repulsion
Lone pair to bonded pair (LP/BP) = medium repulsion
2 bonded pairs (BP/BP) = smallest or least repulsion 12

13. Basic Molecular shapes
The most common shapes of molecules are shown at the right 13

14. Linear Molecules
Linear molecules have only two regions of electron density, or bonding sites.
Examples: CO2 , BeH 14

15. Trigonal Planar
Trigonal planar molecules have three regions of electron density, or bonding sites.
All are occupied by other atoms
Examples: SO3, BH3 15

16. Bent: a type of trigonal planar
Angular or bent molecules have at least 3 regions of electron density, but only two are occupied (at least one is a lone pair!)
Examples: SO2 , H2O 16

17. Tetrahedral
Tetrahedral molecules have four regions of electron density.
All are occupied by other atoms
Examples: CH4, NH3 17