Atomic Theory Definition

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 Definition

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