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Chemistry Matters –Chapter 7

Covalent Bonding and Metallic Bonding

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Why do Covalent Bonds form?

When non-metals react with one another, their atoms share valence electrons.

After bonding, each atom attains the electronic configuration of a noble gas.

The bond formed between atoms that share electrons is called a covalent bond.

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What are Molecules?

When atoms combine by sharing electrons, molecules are formed.

A molecule is a group of two or more non-metal atoms held together by

covalent bonds.

Covalent bonds, like ionic bonds, are STRONG bonds.

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Molecules of Elements

Many non-metallic elements exist as molecules made up of two or more identical atoms which bond by sharing electrons in order to become stable.

Examples:Hydrogen (H2), chlorine (Cl2), oxygen (O2), nitrogen (N2), fluorine (F2), bromine (Br2).

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Example 1:Hydrogen (H2)

Two hydrogen atoms share a pair of electrons so that each atom has

two electrons in its valence shell. As a

result, both atoms have a stable duplet configuration.

A hydrogen atom hasone valence electron.

H (1) H (1) H – H or H2

Single covalent bond consisting of 2 shared electrons.

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HydrogenThe sharing of two electrons forms a single covalent bond.

A single covalent bond or a single bond is usually represented by a single line ‘–’ in the structural formula.

A structural formula shows how the atoms are arranged and the bond between them.

A molecular formula only shows the number of each kind of atom in a molecule.

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ChlorineA chlorine atom has seven valence electrons.

To attain an octet structure, two chlorine atoms combine to share a pair of electrons (two electrons).

Example 2:

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Chlorine

Like hydrogen, chlorine has a single covalent bond.

Q. How many electrons are shared in a single covalent bond? Two

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OxygenAn oxygen atom has six valence electrons.

Each oxygen atom shares two of its electrons with another oxygen atom.

It needs two more electrons to form a stable octet structure.

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Oxygen4 electrons are shared between two oxygen atoms.

Such a bond is called a double covalent bond.

A double bond is represented by ‘==’ in a structural formula.

O = O

Q. How many electrons are shared in a double covalent bond?

Four electrons (two pairs of electrons)

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Let’s Practice!

Q. What can you deduce about the bonding in this particle?

• It is a molecule in which the atoms are bonded by a single covalent bond.

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Let’s Practice!

Q. The valency of an atom is the number

of electrons from each atom that is shared.

What is the valency of each atom here?

One.

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Let’s Practice!

Examples of this kind of molecule – Cl2, F2, Br2, I2.

Q. How many electrons are shared the a single

covalent bond?Two.

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Let’s Practice!

(Group VII elements)

Q. Give the names and molecular formulae of

molecules of the type shown here.

Fluorine (F2), chlorine (Cl2), bromine (Br2), iodine (I2).

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Molecules of Compounds

When 2 or more different types of atoms form covalent bonds, a molecular compound or covalent compound is formed.

Water (H2O), methane (CH4),, ammonia (NH3) and carbon dioxide (CO2)are examples of covalent compounds.

(Molecules are always covalent, not ionic!)

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Example 1: Water

Water is formed by the reaction of hydrogen with oxygen such that all three atoms attain noble gas configurations.

Each water molecule contains two single covalent bonds.

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The hydrogen atoms are now electronically like helium and the oxygen atom like neon.

Two hydrogen – oxygen (H – O) single covalent bonds are formed.The water molecule is V-shaped, with the H-O-H bond angle of 105.

Water molecule (H2O)

+

105.

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Example 2: MethaneMethane (CH4) contains carbon and hydrogen.

Methane has four single covalent bonds.

In a methane molecule, the carbon atom has an octet configuration while each hydrogen atom has a duplet configuration.

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Methane, CH4

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Example 3: Ammonia, NH3

Q. How many electrons does each hydrogen atom need to become stable? 1

Q. How many electrons does each nitrogen atom need to become stable? 3

Ammonia molecule, NH3 (with 3 single covalent N-H bonds)

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Example 4: Carbon dioxide (CO2)

Each carbon atom shares two electrons each with two oxygen atoms.

Each oxygen atom shares two electrons.

6p

6p

8p 8p

8p 8p

Two double covalent (C=O) bonds

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A carbon dioxide molecule (CO2) contains two double covalent bonds. Each double bond has 4 shared electrons or 2 pairs of shared electrons.

Different ways of representing the carbon dioxide molecule.

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Structures of Covalent Substances

There are 2 forms in which

covalent substances exist:

(1) Simple (small) molecules

(2) Giant structures

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Simple (small) Covalent Molecules

Also known as simple molecular structures.

Most covalent substances exist in this form e.g.Hydrogen (H2), Oxygen (O2), nitrogen (N2), chlorine (N2), Iodine (I2), Methane (CH4), water (H2O), carbon dioxide (CO2), Hydrogen chloride (HCl), ammonia (NH3).

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Methane (CH4)In a molecule of methane, CH4, the four C–H covalent bonds are strong.

However, weak van der Waals’ forces between methane molecules hold them together loosely. Therefore, methane exists as a gas at room temperature and pressure.

Ex 1: Simple Covalent Molecules

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Simple Covalent Molecules have low melting & boiling points

Little energy is needed to overcome the weak forces between molecules.

(In melting or boiling which are PHYSICAL CHANGES, no chemical bonds are broken; e.g. in methane C-H bonds are NOT broken).

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Within each iodine molecule, the iodine atoms are held together by strong covalent bonds.

Between the iodine molecules, there are only weak van der Waals’ forces holding the molecules together.

Ex 2: Simple Covalent Molecules

Iodine (I2)

Weak force

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Q. Why does iodine sublime easily when heated gently?

Weak force between molecules

Sublimation is a physical change and only bonds between iodine molecule are broken. The intermolecular (van der Waals’ forces are weak, so little energy is needed to overcome them.

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Physical Properties of Simple Molecular (Covalent) Compounds

1. Volatile – low melting and boiling points

2. Soluble in organic solvents; most are insoluble in water.

3. Cannot conduct electricity in any state.(Some exceptions are: carbon (graphite), ammonia, ethanol, sugar, chlorine, hydrogen chloride, sulphur dioxide.)

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This is because of the weak van der Waal’s forces between the molecules, so little energy is needed to overcome them during changes of state.

Q. Why are many covalent substances liquids or gases at room temperature?

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Q. Why do covalent substances not conduct electricity even when molten or in aqueous solution?

(Exceptions: solid Carbon, in the form of graphite, conducts electricity. Hydrogen chloride, sulfur dioxide and ammonia react with water to form solutions that conduct electricity. )

Covalent substances contain neutral molecules. There are no mobile, charged ions or electrons to conduct electricity.

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Giant Molecular (Covalent) Structures

Diamond Graphite

Examples:

Silicon(IV) oxide (silicon dioxide)

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Giant Covalent (Molecular) Structures

Ex 1: Diamond

It is a form (allotrope) of carbon.

Each carbon atom is bonded to 4 other carbon atoms by strong covalent bonds.

Tetrahedral structure

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How to draw the diamond structure.

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1. Diamond is a very hard substance. It is not easily scratched or worn out.

Physical properties of diamond

2. It has high melting point (3500°C) and high boiling point (4800 °C).

4. It is insoluble in water.

3. Diamond does not conduct electricity.

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Q. Why is Diamond hard?

A crystal of diamond contains millions of carbon atoms joined by strong covalent bonds.

A large amount of energy is required to break these strong covalent bonds. This explains why diamond is so hard.

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Q. Why is the melting point of diamond high?

A large amount of energy is required to break the millions of strong covalent bonds so that the atoms are separated.

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Q. Why diamond does not conduct electricity?

All the valence electrons of the carbon atoms are used for bonding.

There are therefore no free electrons that are available move through the structure to conduct electricity.

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Uses of diamond

Synthetic diamonds produced under high pressures and temperatures are used at the tips of drills and other cutting tools.

They are used for drilling, grinding and polishing very hard surfaces.

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GraphiteGraphite is another form (allotrope) of carbon.

It is made of layers of carbon atoms.

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Graphite - StructureWithin each layer, each carbon atom forms strong covalent bonds (C – C) with three other carbon atoms.

The atoms form rings of six carbon atoms that are joined together to form two-dimensional flat layers.

Each layer is a giant molecule.

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Graphite - Structure

Strong covalent bond (C-C) between carbon atoms

The layers of carbon atoms are held together by weak van der Waals’ forces.

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Why does graphite have high melting and boiling points?

In graphite, the (C-C) bonds within each layer are strong and difficult to break.

Hence, graphite has high melting and boiling points.

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Why is graphite soft and slippery?

The forces of attraction between the layers of carbon are very weak.

The layers can slide over each other. This explains why graphite is soft and slippery.

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In graphite, each carbon atom has one outer (valence) shell electron that is not used to form covalent bonds, and can move through the layer to conduct electricity!

Why is graphite able to conduct electricity?

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Why is graphite able to conduct electricity?

These electrons are delocalised (free) , that is, they can move along the layers from one carbon atom to the next when graphite is connected to a battery.

Hence, graphite is a good conductor of electricity.

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Uses of graphite

1. As a dry lubricant.

3. To make inert electrodes for electrolysis.

2. As brushes for electric motors (to reduce friction).

4. Baked with clay to make pencil lead.

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Sand is actually silicon (IV) oxide.

It is also commonly known as silica.

The formula of silicon (IV) oxide is SiO2. It

is also known as silicon dioxide.

Silicon (IV) oxide

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Each silicon atom is

bonded to four oxygen

atoms in a tetrahedral

structure and

each oxygen atom is

bonded to two silicon

atoms.

Silicon (IV) oxide - structure

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Why does Silicon (IV) oxide have a high melting point?

This is because the silicon and

oxygen atoms are all held together

by strong covalent bonds in a giant

covalent (molecular) structure.

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Writing the formulae of covalent compounds

Ex 1:What is the chemical formula of carbon dioxide?

The word ‘dioxide’ means ‘two oxygen atoms’.

We assume the first element named (carbon) contains only one atom unless otherwise stated.

So the formula is CO2.

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Ex 2: What is the chemical formula of dinitrogen monoxide?

So the formula is N2O.

Dinitrogen means 2 nitrogen atoms.

Monoxide means 1 oxygen atom.

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Ex 3: What is the chemical formula of dinitrogen tetroxide?

So the formula is N2O4.

Dinitrogen means 2 nitrogen atoms.

Tetroxide means 4 oxygen atoms.

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Formulae of covalent substances are not always so easily derived.

Chemical formulae of some common covalent substances which cannot be derived from their names include:

ammonia (NH3), hydrogen peroxide (H2O2), methane (CH4), ozone (O3),water (H2O).