Basic Atomic Theory

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:

Where

n = no. of electrons

N = Shell no.

This can be represented as follows:

First (innermost or ground) shell — Two
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.

Using electronic configuration, atomic model is drawn.

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.

Sheet mica is used for the heating elements of toasters, because it can withstand high temperatures without allowing any excess current leakage.

PVC Colour Coded Flexible Cable

Porcelain, is one of the best insulators available today and is used in high voltage transmission lines.Porcelain, like mica, has high insulating properties at high temperatures. It is mechanically stronger and can be moulded into large insulators. 

Porcelain

It is used for insulating the line support of electric transmission lines. PVC and rubber are excellent insulators where cable flexibility is required. They are mostly used in low and medium voltage rating cables. They have the disadvantage that their resistances decrease at temperatures above 80° C. They are pro­duced in many colours, making the colour coding of cables possible.  Cable colour coding is the process whereby given coloured insulations are used to signify cable application is specific areas.

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.