1

Topic 3 – National Chemistry Summary Notes

Bonding, Structure and Properties of Substances

Covalent Bonds

Most atoms do not exist as single atoms. They are mainly found combined with

other atoms in compounds.

They are held together in these compounds by chemical bonds. Atoms prefer to

be bonded to each other because this gives them a stable electron

arrangement.

A stable electron arrangement is achieved when the outer energy level (electron

level or electron shell) is full. The noble gases all have full outer energy levels

and are very stable. Remember you were told in the last topic that chemical

reactions are all to do with outer electrons? If the outer energy level is full

then the atom is un-reactive.

Most atoms need 8 electrons in the outer energy level to be full (the exceptions

are Hydrogen and Helium who only need 2). This is sometimes described as a

stable octet.

You can use target diagrams, like the one shown below for sodium, to show the

arrangement of electrons in an atom:

The electron arrangement for sodium is : 2,8,1

You will need to be able to draw similar diagrams for the first 20 elements.

LI 1

2

In this first part of the topic we are going to look at how two non-metal atoms

form bonds called covalent bonds. When two or more non-metal atoms are

joined by covalent bonds we call them molecules.To help us we are going to use

simple diagrams showing just the outer electrons of atoms rather than target

diagrams.

When non-metal atoms (like the example for hydrogen shown below) form

covalent bonds they do this by overlapping their outer energy levels in order to

share one pair of electrons.

This gives each atom a full outer energy level (electron level).

Note: In some textbooks energy levels are also called electron shells.

Covalent Bonds in Elements

There are a group of 7 elements in the Periodic Table that exist as two atoms

joined together by covalent bonds in a Diatomic Molecule. They are called The

Diatomic Elements and are shown below:

Iodine - In

Oxygen - October

Chlorine - Children

Bromine - Buy

Nitrogen - Nasty

Hydrogen - Halloween

Fluorine - Food

LI 2 E

A diatomic molecule is a

molecule containing two

non-metal atoms joined

together by covalent bonds.

A diatomic element is a

when a diatomic molecule is

formed from two identical

atoms

3

When you look at the outer energy levels of each element you can see how many

electrons their atoms would have to share in order to fill up their outer energy

level.

Hydrogen has room for one electron to fill up its’ outer energy

level, so it will share one electron pair.

Chlorine, Bromine, Iodine and Fluorine all have 7 electrons in the

outer energy level and therefore have room for one more, so they will also share

one electron pair.

Every time a pair of electrons is shared a covalent bond is made. The bonds

between these atoms are described as single bonds because only one pair of

electrons are shared:

The single covalent bond can also be represented using element symbols and

lines:

Cl-Cl, H-H, Br-Br, I-I, F-F one dash represents a single bond.

4

Double and Triple Covalent Bonds

Oxygen atoms have room to share two electron pairs. This means that a double

covalent bond is formed between two oxygen atoms. You can draw an overlapping

orbital diagram to show this sharing:

O=O

Nitrogen atoms have 5 outer energy level electrons so they need to share 3

pairs of electrons in order to fill the level. This requires a triple bond:

How would you show two nitrogen atoms joining up to form a diatomic molecule

using symbols and dashes?

Answer:

5

Covalent Bonds in Compounds

Note: You should remember from earlier work that a compound is a substance

made up of two or more different elements joined together.

As a general rule if a compound is made up of non-metal elements only then it

will have covalent bonding. Examples:

Methane- CH4

or

Ammonia – NH3

or

Water – H2O

or

Note: Covalent bonds are very strong. A covalent bond is when the two

positive nuclei involved in the bond are held tightly together by their

common attraction for the shared pair of electrons.

LI 3

E

6

Shapes of Covalent Molecules

Molecules are three dimensional structures with specific shapes. We need to

know the shapes of some of the simple two element molecules:

Molecule Arrangement of Atoms Shape

Hydrogen oxide

(water)

Bent (and flat or planar)

Hydrogen fluoride

Linear

Nitrogen hydride

(ammonia)

Pyramidal

Carbon hydride

(methane)

Tetrahedral

The arrangement of atoms in methane and ammonia are trying to represent the

real shape using a perspective diagram.

LI 4 E

2

7

Discrete Molecular Covalent versus Giant Network Structures

Most covalent substances exist as molecules with a definite number of atoms

present eg methane CH4. These are called discrete covalent molecular

structures. Some covalent substances, however, take the form of huge

repeating structures, called giant covalent networks.

Example 1 : Diamond – a covalent network element

Diamond contains only carbon atoms joined together by covalent bonds.

The carbon atoms are held very tightly giving Diamond great hardness and a

very high melting point.

Example 2 : Quartz – a covalent network compound

Quartz contains atoms of silicon and oxygen in the ratio of one silicon to two

oxygen. Its’ formula is SiO2 and it’s proper name is silicon dioxide.

The arrangement of atoms in quartz makes the structure very rigid and gives it

a high melting and boiling point. To melt a covalent network you have to break

that network of covalent bonds which are very strong!!!

LI 5 E

8

Ionic Bonds

Ionic bonds are a special bond that forms between a metal atom and a non-metal

atom.

The bonds form when a metal atom transfers electrons(s) to a non-metal atom.

This means the atoms are able to achieve the same stable, full outer energy

level arrangement as the nearest noble gas.

Ions

Atoms which have lost or gained electrons become charged particles called

ions.

How do you know what kind of ion an atom will become?

Atoms in groups 1,2 and 3 all LOSE electrons to become positively

charged.

Atoms in groups 5,6 and 7 all GAIN electrons to become negatively

charged.

Atoms in group 4 do not usually form ions!!

In General

Metals lose electrons forming positive ions.

Non-metals gain electrons forming negative ions.

Activity!! Complete the following:

Group

Number

1 2 3 4 5 6 7 0

Charge

on ion

+

example

Na+

LI 6

9

How Does An Ionic Bond Form?

Your teacher will show you how to use target diagrams to illustrate how metal

atoms react with non-metal atoms to form ionic bonds.

Atoms are electrically neutral (no charge) but electrons are negatively charged.

When metal atoms lose electrons becoming positively charged and non-metal

atoms gain electrons to become negatively charged these oppositely charged

ions attract each other forming a strong bond.

This is often described as an electrostatic force of attraction between

positive and negative ions.

LI 7

E

The sodium atom

gives away its’ outer

electron to the

chlorine atom.

The sodium atom is

now a positive ion

with a full outer

energy level.

The chlorine atom is

now a negative ion

with a full outer

energy level

10

Ionic Lattice Structures

A tiny amount of an ionic solid compound contains millions of ions. These millions

of ions are held together by ionic bonds, that is :

“The electrostatic force of attraction between positive and negative ions”

These forces are so great that the ions group together with positive ions

surrounding negative ions and negative ions surrounding positive ions.

This produces a regular, geometric structure called an Ionic Lattice.

The lattice structure of a sodium chloride crystal.

In an ionic lattice the ions are held tight and cannot move.

LI 8 E

Ionic compounds form lattice structures

of oppositely charged particles.

11

Melting Points of Substances

The bonding present in a substance has a big effect on a number of different

properties. Melting points (and boiling points) are an important property. How

easy it is to melt or boil a substance determines what the physical state will be

at room temperature. ( room temperature is aprox 25oC )

Physical States of Ionic Compounds

Have a look at page 6 of the Data Booklet. Find the melting points and boiling

points of the following substances and then decide what state they will be at

room temperature:

As you can see all these ionic compounds are solids at room temperature. In fact

all ionic compounds are solids at room temperature.

The high melting and boiling points are due to the strong ionic bonds

which need to be broken. A lot of heat energy has to be put in to allow

the ions to break free from each other and move around. Therefore all

ionic substances are solids at room temperature.

Name of Compound Melting Point oC Boiling Point oC Physical State at

Room Temperature

Barium chloride

Calcium oxide

Magnesium chloride

Potassium iodide

LI 9

E

12

Physical States of Covalent Compounds

Most covalent compounds have low melting points and can be found as gases,

liquids and soft solids at room temperature.

Name of Compound Melting Point oC Boiling Point oC Physical State at

Room Temperature

Water (hydrogen

oxide)

Methane (carbon

hydride)

Carbon dioxide

Silicon dioxide **

** Discrete Covalent Molecular compounds all have low melting and

boiling points but The Giant Covalent Network compounds like

silicon dioxide have very high melting points because very strong

covalent bonds have to be broken in order to melt the compound.

To melt a Discrete Molecular Covalent compound there are only

weak forces of attraction between molecules that are easy to

break.

Comparing Electrical Conductivity of Substances

A substance can conduct electricity if it can allow some kind of charged

particles to flow through it.

An electric current (electricity) is a flow of charged particles.

From practical work in science you may remember that all metals conduct

electricity but what about non-metal elements, ionic compounds and covalent

compounds?

Using a simple electrical circuit with a light bulb to indicate whether the

substance conducts electricity you will test a range of solids first and fill in the

following table:

E

LI 10

13

Electrical Conductivity of Solid Substances:

Name of

Substance

Metallic

Bonding

(please tick)

Covalent

Bonding

(please tick)

Ionic

Bonding

(please tick)

Does it Conduct

Yes/No

Sulphur

Copper

Iron

Tin

Carbon

(graphite)

Wax (contains

carbon and

hydrogen)

Sodium

Chloride

Lead bromide

Sugar (carbon,

hydrogen and

oxygen)

The solid to be tested has

to be attached in series in

the circuit using wires and

crocodile clips.

14

Summary of Conductivity of Solids

1. All the metals do conduct electricity.

2. The only non-metal that can conduct is carbon in the form of graphite.

3. Ionic compound do not conduct as solids.

4. Covalent compounds do not conduct as solids.

Electrical Conductivity of Liquid Substances

A range of liquids were tested. These included solutions of both ionic and

covalent substances, some pure liquids and a teacher demonstration of two

“melts”. “Melts” simply means solids that have to be heated first to melt them

before they were tested using a simple circuit. They can also be described as

“molten”.

Substance Ionic Bonding

(please tick)

Covalent

Bonding

(please tick)

Does it

Conduct

Yes/No sugar solution

(carbohydrate)

sodium chloride

solution

copper chloride

solution

hexane

(carbon hydride)

nickel sulphate

solution

molten wax

molten lead

bromide

ethanol

LI11

15

Summary of Conductivity of Liquids

1. Ionic substances DO conduct when in solution.

2. Ionic substances DO conduct when molten.

3. Covalent solutions, pure liquids and melts do NOT conduct.

Problems With Testing Conductivity of a Melt!!

Testing conductivity of a melt can often give false positive results. After

watching the teacher demonstration, draw a labelled diagram of the apparatus

used and clearly indicate and explain the possible source of error.

Explanation:

Circuit to Test Conductivity of a Melt:

16

Explanation of Conductivity of Substances

Why do covalent substances not conduct in any state or in solution?

Covalent substances do not contain any charged particles. They are made up of

neutral atoms joined together. Electricity cannot flow unless there are charged

particles able to flow (move).

Why do ionic substances only conduct in solution or melt?

Ionic substances do contain charged particles but they are not able to move in a

solid, therefore ionic solids cannot conduct.

Ionic solutions do conduct because dissolving the substance has broken down

the ionic lattice. The ions are now free to move.

Ionic melts conduct because the addition of heat energy has allowed the ions to

vibrate more, overcoming the electrostatic forces holding them together and

shake free of each other. The ions are free to move.

Why do metals conduct in any state?

We will be looking at metals in more detail later in the course but they have a

special structure where their outer energy level electrons are loosely shared

between atoms in a metallic lattice. These electrons are easy to move and since

they are charged particles their movement allows electricity to flow through

easily. Some text books describe these electrons as a “sea of delocalised

electrons”.

Metallic lattice

LI12

E

17

Summary of Physical State and Conductivity of Structures

Type of Structure Physical

State at

Room

Temperature

Conductivity

Liquid Solid

Ionic

High melting

point solids

Discrete Covalent

Molecular

Low melting

point, gas,

liquids and soft

solids

Giant Covalent

Network

Very high

melting point

solids

Metallic

Wide range of

melting points

but all solids

except mercury

18

Topic 3 – Pupil Self-evaluation Note: if the success criteria is in italics then this is extension level work.

Learning Intention

Success Criteria

“ I can:” 1 I am going to find out how

atoms can join together in a

covalent bond.

draw target diagrams to show electron arrangements of atoms.

draw outer electron diagrams to show overlapping energy levels.

state that a covalent bond is a shared pair of electrons.

state that atoms share electrons to get a full, stable, outer energy

level.

state that non-metal atoms form covalent bonds.

state that when two or more non-metal atoms are joined together by

covalent bonds then a molecule if formed.

2 I am going to find out about

the diatomic molecules.

name the 7 diatomic elements describe what “diatomic element” and “diatomic molecule” means. draw diagrams to represent single, double and triple covalent bonds.

3 I am going to find out about

how compounds are held

together by covalent bonds.

draw simple diagrams of two element compounds.

state that covalent bonds are very strong.

describe what holds a covalent bond together. 4 I am going to find out about

the shapes of simple

covalent molecules.

describe the shapes of water, methane, ammonia and hydrogen fluoride molecules.

draw perspective diagrams of these molecules. 5 I am going to find out about

different covalent

structures.

describe what a Giant Network Covalent and a Discrete Molecular Covalent structure is like.

give an example of a compound that exists as Giant Network Covalent state that melting point and boiling point are related to structure.

6 I am going to find out how state that ionic bonds form between a non-metal atom and a metal

E

19

atoms can change in order

to join together in ionic

bonds.

atom.

state that atoms become ions by losing or gaining electrons.

state what charge an ion will have from groups 1 to 0.

7 I am going to find out how

an ionic bond forms.

draw diagrams to explain how an ionic bond forms.

explain that an electrostatic force of attraction holds the ionic bond together.

8 I am going to be able to

describe an ionic lattice.

explain that an ionic solid has a regular, geometric structure called an ionic lattice and describe it.

9 I am going to be able to use

the data booklet to find out

the melting and boiling

points of substances and

relate this to physical state

and type of bonding.

use the data booklet to find the melting and boiling points of selected

substances.

state that all ionic substances are solids at room temperature.

explain why ionic substances are all solids.

10 I am going to find out how

electrical conductivity of a

solid substance relates to

its’ bonding.

describe how I could test a solid substance for electrical conductivity.

state the conductivity of metal solids.

state the conductivity of covalent or ionic solids.

11 I am going to find out how

electrical conductivity of a

liquid substance is related

to the bonding.

state the conductivity of ionic substances when in solution.

state the conductivity of covalent substances as solution or as liquid.

state that ionic melts conduct but covalent melts do not conduct.

explain a problem that can occur when testing melts for conductivity.

12 I am going to be able to

describe why a substance

conducts.

explain that for an ionic substance to conduct then the ions need to be free to move.

describe conductivity in a metal as a flow of electrons.