07/12/2016
Review:
When drawing Lewis Structures we represent
electrons as either bonding pairs or lone pairs.
Chemistry 11
Unit 2: From Structures to Properties
7 December 2016
Co-ordinate Covalent Bonds
Resonance Structures
Resonance structures are chemical structures that
When an atom is able to donate both electrons for a
bond we say that the bond is co-ordinated, or a
co-ordinate bond has formed
give the same order of the elements in a chemical
structure but show differences in their bonding
electrons. For example, Let’s draw SO2:
For example, if ammonia (NH3) gains a hydrogen ion
(H+1) then it forms NH4+1, ammonium
We have 6 + 6 + 6 valence electrons to give 18 eWe want 8 + 8 +8 = 24 electrons to satisfy octets
H
24 18
3
2
H
+
+
H
H
H
+
N
H
Resonance Structures
So which structure is it?
The answer is both
We represent both (or all resonance structures) by
writing them out and placing two headed arrows in
between
We call these resonance hybrids
We can also represent these resonance hybrids with a
dotted line:
H
..
O
..
..
S
..
O
..
..
O
..
..
S
..
O
..
..
H
..
N
18 e- 6 e- bonding
- 12 e- lone pairs
Valence-Shell Electron Pair
Repulsion Theory
When trying to devise what a molecule’s actual shape
was, it made sense to look at the force of repulsion that
exists between one electron and another
Electrons that affect shape in a molecule exist as either
bonding pairs (BPs) or lone pairs (LPs)
Electrons will arrange themselves in a configuration
that will maximize the distance between electron
groups. These electron groups are called electron
domains. This is the key idea behind VSEPR theory.
We use VSEPR around the central atom only!
1
07/12/2016
VSEPR Theory
VSEPR Theory
In a molecule electron domains are simply zones or
It is important to note that electron domains exist as
areas we will find electrons. This means that single
bonds, double bonds, and triple bonds all count as a
single electron domain each
Take:
areas around the molecule we would likely find
electrons.
In the below diagram water we can see a mixture of
BPs and LPs
We have 2 bonding domains
We have 2 lone pair domains
In total this molecule has 4 electron
domains (2 bonding, 2 LPs)
These zones equally repel each other
Electron Domain Geometries
Based on the number of electron domains we can
It only has 2 electron domains (bonding) on the
central atom
How to Determine the VSEPR
shape:
1.
determine the electron domain geometry (shape)
This is a general shape that reflects the location of all of
the electrons relative to each other.
# Electron
Domains
Electron Domain Geometry
2
Linear
3
Trigonal Planar
4
Tetrahedral
5
Trigonal bipyramidal*
6
Octahedral*
Example: VSEPR – Electron Domains
What is the electron domain geometry predicted by
VSEPR for CO2?
Solution:
1. Draw Lewis Structure
2.
3.
4.
5.
Draw the Lewis structure making sure to include all
bonding and lone pairs.
Find the central atom
Around the central atom, count the number of
bonding pairs (BPs) and Lone Pairs (LPs).
For Electron Domain Geometry add the number
of BPs to the number of LPs and find the total
number of electron domains.
Use the table of Electron domains and find which
shape matches the number of electron domains you
have.
Try This:
What is the electron domain geometry of a molecule
of CH3Cl (chloromethane)?
2. Count electron Domains
There are 2 electron domains, both are Bonding . Remember even
though they are double bonds each double bond counts only for 1!
3. Use the table to find the proper geometry
2 Electron Domains matches linear geometry
• There are 4 electron domains
• 4 electron domains matches tetrahedral
electron domain geometry
2
07/12/2016
Molecular Domain Geometries
Although electron domain geometries give you a great
idea of the orientation of the electrons, it doesn’t give
you a picture of what the molecule actually looks like.
Molecular Domain Geometry is the shape of the
molecule only considering the atoms orientation and
not where the lone pairs are located.
Molecular Domains are found by differentiating
electron domains into either Bonding Domains or
Nonbonding Domains
Example – Molecular Domain
Geometry
What is the molecular domain geometry of CCl2O?
Draw the structure!
How to find the Molecular Domain
Geometry?
Draw the complete Lewis Structure making sure to
include Lone pairs
2. Count the number of bonding domains and
nonbonding domains (lone pairs).
3. Using your VSEPR chart, find the molecular
domain that matches the number of bonding
domains and nonbonding domains
4. Write the Molecular geometry!
It is possible the Molecular Domain Geometry
(MDG) and Electron Domain Geometry (EDG) are
the same name!
1.
Try This- MDG
What is the electron domain geometry and
molecular domain geometry of ammonia, NH3 ?
Draw the Structure!
Count the bonding Domains
3 bonding domains
Count the nonbonding Domains
0 nonbonding domains
Find the match for this shape
EDG is Trigonal Planar, MDG is also trigonal planar
Count the number of Bonding Domains
There are 3 Bonding Domains
Count the number of nonbonding domains
There are 1 non bonding domains
Find the EDG and MDG!
The EDG is tetrahedral and the MDG is trigonal pyramidal
3