Chemistry 11

Unit 9 Chemical Bonding #4

Name:____________________________ Block:_________

Types of intermolecular force

The molecules in simple covalent substances are not entirely isolated from one another. There are forces of attraction between them. These are called , “inter” means “between”, or Van der Waals forces.

There are three main types of intermolecular forces:

· – temporary dipolar attractions between neighbouring atoms, for example, between I2 molecules in iodine crystals.

· – for example, found between HCl molecules in hydrogen chloride.

· – for example, found between H2O molecules in water.

Van der Waals forces

How do Van der Waals forces hold Molecules together?

Electrons are constantly moving…

At any one time the electrons may not be evenly distributed

The temporary dipole in one molecule can…

The dipole induction occurs between all neighboring molecules.

Strength of van der Waals forces

The strength of van der Waals forces as molecular size increases.

This is illustrated by the boiling points of group 7 elements.

increases down the group, so the become further from the nucleus.

They are attracted by the nucleus and so are easier to induce.

London Forces are the weakest type of bonding

The an atom or molecule has altogether, the existing between it and a neighbouring atom or molecule.

ASSIGNMENT #4A: HEBDEN PG 181 #74-77

Permanent dipole–dipole forces

Within any substance containing molecules, each molecule has a pand a n pole — a .

Because of these partial charges, the molecules in the liquid and solid phases will naturally orient themselves so that the positive pole of one molecule will be next to and .

If molecules contain bonds with a , the molecules may align so there is between the opposite charges on neighbouring molecules.

Permanent dipole–dipole forces (dotted lines) occur in hydrogen chloride (HCl) gas (shown below)

This network of dipole-dipole forces will result in because will be required to overcome the attractions between the molecules.

The permanent dipole–dipole forces are approximately the strength of a covalent bond.

What is hydrogen bonding?

When hydrogen bonds to nitrogen, oxygen or fluorine, a than in other polar bonds.

This is because these atoms are due to their and small size.

When these atoms bond to hydrogen, electrons are withdrawn from the H atom, making it .

The H atom is very small so the positive charge is more concentrated, making it easier to link with other molecules.

Hydrogen bonds are therefore particularly .

In molecules with OH or NH groups, a on nitrogen or oxygen is attracted to the slight positive charge on the hydrogen on a neighbouring molecule.

Hydrogen bonding makes the and points of water than might be expected.

It also means that alcohols (OH groups) have much than alkanes of a similar size.

Boiling points of the hydrogen halides

The boiling point of hydrogen fluoride is much higher than that of other hydrogen halides, due to fluorine’s .

The means that between molecules of hydrogen fluoride is much than the permanent dipole–dipole forces between molecules of other hydrogen halides. is therefore required to separate the molecules of hydrogen fluoride.

ASSIGNMENT #4B: HEBDEN Pg182-183 #79-84

Lewis Dot Diagrams for the Elements

A Lewis dot structure for an atom consists of the for the element and .

How to Draw a Lewis Structure

1) Find your element on the periodic table.

2) Determine the number of by looking at the group (column)

This is how many electrons you will draw. 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…

To write Lewis structures for the atoms, follow these steps:

1. Write the element symbol.

2. For hydrogen place a single dot next to the symbol. For helium place a pair of dots.

3. For 2nd period elements and beyond: for each valence electron present, place one dot around that symbol at one of the four positions of the compass: west, east, north, or south.

4. 4. If more than four valence electrons are present, begin pairing the dots only after the four compass positions are filled.

Stable “Octets”

Octet Rule:Atoms tend to gain, lose or share electrons until they are surrounded by 8 electrons

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: MgCl2 (draw diagram)

Electron Dot Diagrams

· Some electrons are already paired up within a single atom and are .

· These are called / unshared pairs.

· Some elements are able to share more than one pair of electrons to form bonds

Converting Lewis Structures into 3D

The process of inferring a three-dimensional shape from a Lewis structure is based on a very simple premise

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 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 from each other as possible.

This is the fundamental principle behind the (VSEPR) theory

VSEPR OVERVIEW

Name of Shape

Chemical Formula

Bond Angle

Example Molecule

Tetrahedral

Trigonal Pyramidal

Name of Shape

Chemical Formula

Bond Angle

Example Molecule

Bent

Linear

Trigonal Planar

1

10

050100150

-50-100-150-200

boiling  point  (°C)

Br2F2 Cl2 I2element

0

50

100

150

-50

-100

-150

-200

b

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p

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(

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Br

2

F

2

Cl

2

I

2

element

hydrogen  bond lone  pair

hydrogen

bond

lone pair

02040

-20-40-60-80-100

boiling  point  (°C)

HF HCl HBr HI

0

20

40

-20

-40

-60

-80

-100

b

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HF HCl HBr HI