5. Bonding and Intermolecular...
Transcript of 5. Bonding and Intermolecular...
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Lewis Structures
Valence Electrons
The Octet Rule
3D Diagrams & VSEPR
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Electron Shell (Bohr) Diagrams
• A shell diagram
shows all electrons,
but we are most
interested in the
electrons in the
outermost energy
level called the
valence level or
valence shell.
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Bohr Models: How to
• Used to represent a model of an atom.
• To draw a Bohr model follow these steps:
(We will use Helium as an example)
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Making a Bohr Model Using Helium
1. Look to the periodic table and determine
how many protons, neutrons and
electrons are in 1 atom of helium.
P=____ N=_____ E=_____
2. Draw a circle and label the # of P and N in
the inside of the circle
P= 2
N= 2
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Making a Bohr Model Using Helium
3. Draw your 1st electron shell.
4. Draw up to 2 electrons in the 1st shell.
5. If you need to add more electrons, you need to
add more electron shells! Remember…2, 8, 8!!!
P= 2
N= 2
P= 2
N= 2
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Valence Electrons are:
The electrons in the outermost shell
Responsible for atomic bonding
Equal to the last digit of the group number
How many valence electrons in this atom? What
group would it be in?
Valence electrons REVISITED
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Lewis Dot Diagrams for the Elements
A Lewis dot structure for an atom consists of the
symbol for the element and one dot for each
valence electron.
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How to Draw a Lewis Structure
1) Find your element on the periodic table.
2) Determine the number of valence
electrons by looking at the group
(column)
3) This is how many electrons you will draw.
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Lewis Structures
1) Write the element
symbol.
2) Carbon is in the 14th
group, so it has 4
valence electrons.
3) Starting at the right,
draw 4 electrons, or
dots, counter-
clockwise around the
element symbol.
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Lewis Structures
What would the Lewis Dot Structure for
Phosphorus look like?
Maximum 2 dots per side
(4 sides = 8 dots)
It does not matter which
side you start on.
You have to fill in each
side with one dot before
you double up…
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Represent the number of valence electrons as dots
Valence number is the same as the Periodic Table Group Number
H
Li Be B C N O F Ne
He
Na; Is2, 2s2, 2p6, 3s1 = [Ne] 3s1
Lewis Structure = Na
For example,
Groups 1 2 3 4 5 6 7 8
n = 1
n = 2
Lewis Symbols
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Electron Dot Diagrams
Draw electron dot diagrams of the following elements:
1. Carbon 6. Hydrogen
2. Nitrogen 7. Chlorine
3. Phosphorus 8. Argon
4. Boron 9. Iodine
5. Oxygen 10. Helium
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Electron Dot Diagrams
1. Carbon
2. Nitrogen
3. Phosphorus
4. Boron
5. Oxygen
6. Hydrogen
7. Chlorine
8. Argon
9. Iodine
10. Helium
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Elements want to achieve the
stable electron configuration
of the nearest noble gas
Octet Rule:
Atoms tend to gain, lose or share electrons until they are
surrounded by 8 electrons
Stable “Octets”
Ne
n = 2
n = 3
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Lewis Structure Rules for BONDING
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Lewis Structures of Simple Ionic Compounds
• Determine the charge expected for each ion
• Add or removed specific numbers of electrons
• Arrange non-metal atoms symmetrically around the metal ion
Example of Ionic Bonding
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Ionic Bonding refers to electrostatic forces between ions, usually a
metal cation and a non-metal anion
Covalent Bonding results from the sharing of two electrons between
two atoms (usually non-metals) resulting in molecules
Two Types of Bonding
H H H H
Cl Cl ClCl
N N NN
+
number of electrons around each atom = He
+
number of electrons around each atom = Ar
+
number of electrons around each atom = NeTriple bond
Each covalent bond contains 2 electrons
Octet Rule Applies
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Covalent Bonding in Carbon
H
CH H
H
methane
Carbon has 4 valence electrons
C
H
H
H
H
H C
Ne
Neon
Stable Octet required
Example of Covalent Bonding
Lewis structures show
how Valence Electrons
are distributed in a
molecule.
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Electron Dot Diagrams
• Some electrons are already paired up within a
single atom and are not shared.
• These are called non-bonding / unshared
pairs.
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Electron Dot Diagrams
Some elements
are able to share
more than one
pair of electrons
to form double
and triple bonds
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Valence
Shell
Electron
Pair
Repulsion
Theory
Planar triangular
Tetrahedral
Trigonal pyramidal
Bent
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Converting Lewis Structures into 3D
The process of inferring a three-dimensional
shape from a Lewis structure is based on a
very simple premise:
Valence electrons represent regions of negative
charge that repel each other.
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Converting Lewis Structures into 3D
• Any group of valence electrons associated with a central
atom will tend to orient themselves in three-dimensional
space around that atom so as to minimize the repulsion
between them.
• Examples : lone pair, bonding pair, or multiple pairs
involved in a double or triple bond.
While remaining attached to the
central atom, these groups of
electrons will position
themselves as far away from
each other as possible.
This is the fundamental principle
behind the valence shell
electron pair repulsion (VSEPR)
theory
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VSEPR Theory
• Based on Electron Dot (Lewis structures)
• Theory predicts shapes of compounds• abbreviated VSEPR
• VSEPR (pronounced “vesper”) stands for Valence Shell Electron Pair Repulsion
• VSEPR predicts shapes based on electron pairs repelling (in bonds or by themselves)
• Electrons around central nucleus repel each other. So, structures have atoms maximally spread out
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• Each shape has a name
Names of Shapes:• tetrahedral
• trigonal pyramidal
• Bent
• Linear
• trigonal planar
VSEPR Overview
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Triangular PlanarTetrahedral
Trigonal pyramidalLinear
Bent or V
Models
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C
H
H
H
H methane, CH4
Bonds are all evenly spaced electrons
109.5°
C
H
H
H
H
Tetrahedral
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NH H
HC
H
H
H
H
Less repulsion between the bonding pairs of electrons.. ammonia
NH3
....
..
Trigonal Pyramidal
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C
H
H
H
H NH H
H
OH H.. ..
..
109.5° (109.5°) 109.5° (107°) 109.5° (104.5°)
water, H2O
Bent (or angular)
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C
H
H
H
H NH H
H
OH H.. ..
..
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H
CH H
H
NH H
H
OH H
Bent or V
2 unshared pairs of e’s at top of O
repel bonds and force them to
bend
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Linear
• The shape is linear with the X–A–X bond
angle being 180°
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Trigonal Planar
• The shape is linear with the X–A–X bond
angle being 180°
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Lewis StructureVSEPR
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Practice Questions
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Groups of
charge
Lone electron
pairs
Electronic
geometry
Molecular
shape
2 0 Linear Linear
3 0 Trigonal planar Trigonal planar
3 1 Trigonal planar Bent
4 0 Tetrahedral Tetrahedral
4 1 TetrahedralTrigonal
pyramid
4 2 Tetrahedral Bent