5. bonding and intermolecular...

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Lewis Structures

Valence Electrons

The Octet Rule

3D Diagrams & VSEPR

© Boardworks Ltd 20086 of 8

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.

http://upload.wikimedia.org/wikipedia/commons/7/71/Atomo_di_alluminio.svg

http://upload.wikimedia.org/wikipedia/commons/7/71/Atomo_di_alluminio.svg


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)


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


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


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


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.


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.


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.


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…


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


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



1. Carbon

2. Nitrogen

3. Phosphorus

4. Boron

5. Oxygen

6. Hydrogen

7. Chlorine

8. Argon

9. Iodine

10. Helium


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

http://www-tech.mit.edu/cgi-bin/imagemap/Projects/Chemicool/pertable.map

http://www-tech.mit.edu/cgi-bin/imagemap/Projects/Chemicool/pertable.map


Lewis Structure Rules for BONDING


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


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


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.



• Some electrons are already paired up within a

single atom and are not shared.

• These are called non-bonding / unshared

pairs.



Some elements

are able to share

more than one

pair of electrons

to form double

and triple bonds


Valence

Shell

Electron

Pair

Repulsion

Theory

Planar triangular

Tetrahedral

Trigonal pyramidal

Bent

http://people.bath.ac.uk/ch3tam/inorganic/vsepr/movies/NH3.mov


http://people.bath.ac.uk/ch3tam/inorganic/vsepr/movies/SnCl2.mov



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.


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


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


• Each shape has a name

Names of Shapes:• tetrahedral

• trigonal pyramidal

• Bent

• Linear

• trigonal planar

VSEPR Overview


Triangular PlanarTetrahedral

Trigonal pyramidalLinear

Bent or V

Models



http://people.bath.ac.uk/ch3tam/inorganic/vsepr/movies/CH4.mov

http://people.bath.ac.uk/ch3tam/inorganic/vsepr/movies/CH4.mov

http://people.bath.ac.uk/ch3tam/inorganic/vsepr/movies/BF3.mov

http://people.bath.ac.uk/ch3tam/inorganic/vsepr/movies/BF3.mov




C

H

H

H

H methane, CH4

Bonds are all evenly spaced electrons

109.5°

C

H

H

H

H

Tetrahedral


NH H

HC

H

H

H

H

Less repulsion between the bonding pairs of electrons.. ammonia

NH3

....

..

Trigonal Pyramidal


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)


C

H

H

H

H NH H

H

OH H.. ..

..


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


Linear

• The shape is linear with the X–A–X bond

angle being 180°


Trigonal Planar

• The shape is linear with the X–A–X bond

angle being 180°


Lewis StructureVSEPR


Practice Questions


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

5. bonding and intermolecular...

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