3.0 INTRODUCTION
The advances in modern electronics engineering have been
made possible by the achievements in the study of the structure of materials.
This has, for instance, made it possible to come up with high quality
conductors and insulators for very high voltage transmission. The development
of semi-conductor technology has brought rapid changes in the way electricity
is used. This chapter is devoted to the study of the elementary structure of
materials and their electric properties. It serves as a basis for the study of
the sources of electricity and the behaviour of electric circuits.
3.1 ATOMIC THEORY
Matter is anything that has weight and it occupies space. It
exists in three forms:
(a)
Solid.
(b)
Liquid.
(c)
Gas.
Matter can be in its pure form or it can be in a compound
form. In its pure form, it is called the element. Elements can be either metals
or non-metals. Metallic elements include copper, iron, aluminium, etc.
Non-metallic elements include hydrogen, oxygen, carbon, etc. There are 92
naturally occurring elements.
Substances made up of more than one element are called
compounds. For instance, water is 1 compound of hydrogen and oxygen; common
salt is a compound of sodium (a metal) and chloride (a gas).
The smallest particle of an element that can take part in a
chemical reaction is called an atom.
When two or more atoms join together, they form a molecule.
A molecule is the smallest particle of a substance that can exist on its own.
Atoms of a molecule can be from the element like oxygen or
from two or more different elements like oxygen and hydrogen. The former is called simple molecule whereas
the latter complex molecule.
An atom is made of three types of particles:
(a)
The protons.
(b)
The neutrons.
(c)
The electrons.
The protons and neutrons form the nucleus of the Atom.
·
Protons carry a positive electric charge
·
Neutrons carry no charge.
·
Electrons carry negative charge.
Charges of the same type repel one another whereas those of
the unlike type attract.
Law of charges:
Unlike charges attract and like charges repel.
Thus, electrons try to keep away from each other while they
are attracted to the protons in the nucleus of an atom.
Mass of a Proton is equivalent to that of a neutron whereas
the proton is 1800 the mass of an electron.
This means that the mass of an atom is concentrated at its centre, that
is, nucleus.
The number of protons in the nucleus of an atom determines
what the element is and its atomic number.
Atomic number = no. of protons
As noted it easier to remove an electron from an atom than a
proton or neutron due to their atomic locations.
Generally an atom has the same number of electrons rotation
around its nucleus as the protons in its nucleus. This makes the atom
electrically neutral.
If an electron is lost, the atom becomes net positively
charged because the positively charged protons outnumbers the electrons. Similarly, a net negatively charged atom is
obtained when it gains an electron or more.
An electrically charged atom is called an ion. When it is net positively
charged it called an anion and if it is net negatively charged it is called a
cation.
 |
| Basic atomic model |
Atomic Structure
The path
traced by an electron around the nucleus is called an orbit. The orbits
that are of the same distances from the nucleus forms a shell. The number
of electrons occupying each shell is limited by the formula:
n = no. of
electrons
N = Shell
no.
This can be
represented as follows:
First (innermost or ground) shell
— Two
Second shell — Eight
Second shell — Eight
Third shell — Eighteen
Fourth shell — Thirty-two
The
innermost shells are filled first before the outer ones can be occupied.
The number
of the electrons in the outermost shells of the atoms determines the electrical
behaviour of the elements. As seen from
the formula this means in most cases the outermost shell is rarely completely
filled.
The hydrogen
atom is the smallest and lightest of all the atoms. It has one proton, one
electron, and no neutron. Because it has one proton it is said to have atomic
number of 1.
 |
| Hydrogen Atom |
The basic method of drawing atomic model
given atomic number and number of electrons involves:
Breaking the electronic number using the
formula
This is done until all the shells sum to the
total number of electrons forming electronic configuration.
Consider the construction of electronic
configuration (E.C.) of various common electrical elements below.
Element
|
Atomic no.
|
Neutrons
|
Electrons
|
E.C.
|
Carbon
|
6
|
6
|
6
|
{2,4}
|
Oxygen
|
8
|
8
|
8
|
{2,6}
|
Neon
|
10
|
10
|
10
|
{2,8}
|
Aluminium
|
13
|
14
|
13
|
{2,8,3}
|
Using the above we thus draw an atomic model,
say Neon as:
 |
| Neon Atomic Model |
The electrons found in the outermost cell of
electrically neutral atom are called valency electrons. These electrons determine the Group of
element as displayed in Periodic Table.
Periodic table
The number of electrons an element can gain
or lose to stabilize its outermost shell is its valence. This can be either negative if gain of
electrons are anticipated or positive for the vice versa.
Group
|
Element
|
Symbol
|
Atomic No.
|
Outermost Shell Electron
|
Metals
|
Silver
|
Ag
|
47
|
1
|
Copper
|
Cu
|
29
|
1
|
|
Gold
|
Au
|
79
|
1
|
|
Aluminium
|
Al
|
13
|
3
|
|
Iron
|
Fe
|
26
|
2
|
|
Semi- conductors
|
Carbon
|
C
|
6
|
4
|
Silicon
|
Si
|
14
|
4
|
|
Germanium
|
Ge
|
32
|
4
|
|
Active gases
|
Hydrogen
|
H
|
1
|
1
|
Oxygen
|
O
|
8
|
6
|
|
Insert gases
|
Helium
|
He
|
2
|
2
|
Neon
|
Ne
|
10
|
8
|
Common elements used in electrical engineering
3.3 ELECTRICAL PROPERTIES OF MATERIALS
From an
electrical point of view, most materials can be classified into three groups;
namely:
(a)
Conductors.
(b)
Semi-conductors.
(c)
Insulators.
3.3.1 Conductors
According to
atomic theory, an electron can escape from the parent atom and move about
between the atoms. This is called a "free electron”. Once an electron escapes from an atom, the
atom becomes net positively charged. It may then attract a free electron to
become a neutral atom again.
Metals are
characterized by many free electrons. Materials having many free electrons
easily allow an electric current to flow through them and are called
conductors. Electrons are the electric charge carriers in solid conductors.
Metals are normally good conductors.
Some of the
conductors, in order of their conductivity, are silver, copper, gold, aluminium, tungsten and brass. Silver is the
best conductor of all metals, but is only rarely used because it is very
expensive. It is used in precision instruments and in special switches as a
coating but not in wires. The most commonly used conductors are copper and
aluminium because of their abundance.
Copper is
used in flexible cables such as wiring whereas aluminium is used mostly in
overhead transmission cables because it is lighter than copper. It is also
cheaper.
When an
electric field is applied it sets up an electric force.
Without an
electric field applied to a conductor, the electrons move in all directions, so
it cannot be said that there is an electric current. However, if a potential is
applied across the ends of a conductor, the free electrons will tend to move in
the same direction. This is what happens when a conductor is connected between
the terminals of an electric cell.
The
electromotive force (e.m.f.) of the cell is the field. The electrons are
repelled from the negative terminal and attracted towards the positive terminal
of the cell. Some electrons will leave the piece of metal and pass into the
positive terminal. Those from the negative terminal will pass into the
conductor. In this process, an electric charge will be transported or conducted
through the conductor.
Other than
metals, other conductors of electricity are electrolytes and some gases.
3.3.2 Semi-conductors
The atoms of
some elements, such as carbon, silicon and germanium, do not have free
electrons at low temperature. However, at room temperature, a few electrons
acquire enough energy to escape from bonds and become free. As the temperature
increases, more electrons becomes free; consequently rendering the substance a
good conductor. Materials which are poor conductors at low temperatures and
good conductors at higher temperatures are called semi-conductors. Electrons
that are set free by the temperature rise are termed thermally generated
electrons. As an increase in temperature
provides more charge carriers (electrons), the resistivity of semi-conductors
decreases with increasing temperature.
Substances whose resistivity decrease with increase in temperature are
said to negative temperature co-efficient.
3.3.3 Insulators
An insulator
is a substance that does not conduct electricity (except at very high
temperature). Many non-metallic compounds are insulators. The outstanding
characteristic is that they contain no free electrons. In an insulator, the
electrons are bound to their atom by very strong forces.
Some excellent
electrical insulating materials are mica, porcelain, glass, rubber, dry paper,
Bakelite, PVC.
It is
important to observe that not all insulators are alike in their insulating
qualities. The best of them contain no free electrons, while less perfect ones
contain a limited number of free electrons. Insulating properties also depend
on the temperature of the material.
![]()
 |
| Porcelain |
 |
Porcelain insulation on power
transmission
|
Other insulating materials
include wood, paper, cotton, mineral insulating material and air; but
each material is suitable only for a specific use. Insulating materials are
also used as dielectric materials in the construction of capacitors. A capacitor is a device used to store an
electric charge.
3.3.4 Tabulated Classification and common usage of materials
GROUP
|
NAME
|
RESISTIVITY
|
COMMON USE
|
Conductors
|
Silver
|
1.63 x 10-8
|
Contacts
|
Copper
|
1.72 x 10-8
|
Cables
|
|
Gold
|
2.20 x 10-8
|
Contacts and integrated
circuits
|
|
Aluminium
|
2.67 x lO-8
|
Power line cables
|
|
Brass
|
8 x 10-8
|
Terminals
|
|
Semi-conductors
|
Carbon
|
4 x 10-5
|
Machine brushes
|
Silicon
|
1 x 10-2
|
Most semi-conductor
devices
|
|
Germanium
|
46 x 10-2
|
Transistors diodes
|
|
Gallium
|
1.7 X 10-1
|
Light emitting diodes
|
|
Insulators
|
Porcelain
|
1 X 109
|
Line support insulators
|
PVC
|
1 x 107
|
Cable insulators
|
|
Rubber
|
1 x 106
|
Cable insulators
|
|
Mica
|
5 x 1014
|
Heating element insulator
|
|
Polyester
|
1 x 1011
|
Capacitor dielectrics
|
|
Glass
|
1 x 107
|
Power line insulators
|
3.4
BASIC ATOMIC THEORY KEY POINTS
1.
Matter is
anything that occupies space and has mass.
2.
The smallest
particle of matter that can take part in a chemical reaction is called an atom.
3.
An atom
consists of protons, neutrons and electrons.
4.
The protons
and neutrons form the nucleus of the atom.
5.
Protons and
neutrons are collectively known as nucleons.
6.
The atomic
number of an element indicates the number of protons in the nucleus of its atom.
7.
The number
of electrons in the outermost orbit is the valence electrons.
8.
Electrons
carry a negative charge; while protons carry a positive charge. Charged atoms
or molecule are called ions.
9.
Most metals
contain free electrons which, at room temperatures, roam the spaces between the
atoms.
10.
In most
substances, atoms team up to form molecules.
11.
Electrons
moving in a given direction give rise to an electric current. An electric
circuit is a closed path through which an electric current may flow.
12.
Electrically,
materials can be grouped as:
a.
conductors,
b.
semi-conductors,
and
c.
insulators.
13.
Conductors
are those materials which have free electrons. The most common conductors are
made of copper and aluminium.
14.
Semi-conductors
have few free electrons at low temperatures, but have many free electrons at
higher temperatures.
15.
The most
commonly used semi-conductors are silicon and germanium. They are used for
making diodes, transistors and integrated circuits.
16.
Insulators
have no free electrons and do not conduct an electric current.
3.5
BASIC ATOMIC THEORY REVIEW EXERCISES
1. What is an
atom?
2. State the
characteristics of an atom.
3.
How do atoms of different
elements differ?
4.
List ten elements.
5.
Define the following: Molecule,
element and compound.
6.
Deduce electronic configuration
and draw the atomic structure of a neutral atom whose atomic number is 14 with
14 neutrons.
7.
What is the principal
characteristic of good conductors of electricity?
8.
List four good conductors,
starting with the best.
9.
Explain the meaning of the term
"poor conductor" and give five examples of such materials.
10.
How do insulators differ from the conductors
of electricity?
11.
What are the characteristics of a
pure semi-conductor?
12.
Name the two most common
semi-conductor materials and state
their main applications in industry.
