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Sunday, 21 July 2019

Atomic Structure of Elements

Classification of Materials:


Any electrical equipment has to use materials which serve the purpose under arduous conditions without loosing the reliability which is an essential condition. If therefore becomes necessary to have a thorough knowledge of materials and its characteristic quantities with respect to its role. The parameters of the material are temperature, pressure, frequency, reaction to atmosphere, ageing, stress, fatigue etc. The variable parameters are established from experimental results and only those which are of importance are reflected in the respective topics.

It is possible that a material which are used for electrical engineering application are also used for mechanical, civil etc., and when it falls under electrical engineering materials, its properties and behaviour to its role in electrical engineering forms the prime importance. For example, copper is used both for electrical and also for refrigeration and air conditioning. Electrical properties of copper are of interest in electrical engineering materials whereas in refrigeration and air conditioning the point of interest is different.

At the time of designing of electrical machines and equipment a designer has to take all precautions to select the right type of material for each component of his product. To fulfill this requirement, he should have a thorough knowledge and understanding of the properties and behaviors of electrical engineering materials. Materials used for electrical applications may have limitations and it would be dangerous if its usability is not studied thoroughly for the conditions of requirements. For example, the maximum working temperature of material is fixed by experimental results and if in its role in electrical equipment exceeds this limit; it would fail sustaining heavy damage to the equipment itself.

The factors such as cost, availability, feasibility for manufacture without involving complicated process and its peripheral conditions should be kept in mind before selecting any material to serve a purpose or role in the equipment.



Atomic Structure of Element

To understand the behavior of a material as a conductor, insulator or semi-conductor, it is necessary to study their atomic structure. Every material is thought to be made of smallest and tiny indivisible particles which are not further divisible and is called an Atom. Every element has number of atoms in it. An atom of any element consists of:

(a) electrons which are negatively charged, light in weight and movable.
(b) protons which are immovable
(c) neutrons have no charge but heavy and immovable.

Each electron has a negative electrical charge of 1.602 x 10-19 Coulombs and protons within the nucleus have a positive charge of the same magnitude. Since opposite charges attract, a force of attraction exists between the oppositely charged electrons and nucleus. The force of attraction is balanced by the centrifugal force due to the motion of the electrons around the nucleus. Compared to the mass of nucleus, electrons are very tiny particles having negligible mass and therefore can be considered as having no mass. The nucleus of an atom is largely a cluster of protons and neutrons. A neutron has no charge at all. For a given atom, the number of protons in the nucleus is equal to the number of orbiting electrons.

In an atom, the number of negatively charged electrons is always equal to the positively charged protons contained in the nucleus. These electrons fall within orbits that are clearly defined for all types of material. Due to some external force if a negatively charged electron move out of the orbit, another adjacent electron takes its place. The electrons in any orbit are normally balanced as shown in Figure.

Sample Atomic Structure

For a more complex atom, the number of electrons is needed to balance the electrical charge and so the orbit increases to accommodate the electrons. For example, in a Hydrogen atom only one electron exists and the electron occupy in a single orbit. Similarly in the helium atom the two electrons share the same orbit. But in the Lithium atom there are three electrons and therefore the number of only two electrons can occupy in the first orbit. The pattern of orbits are set even in the more complex atoms. The first, the innermost orbit can have maximum of two electrons and the last or the outermost orbit has a maximum of eight electrons. Orbits between the innermost and outermost may contain various numbers of electrons but the outermost never exceed eight electrons. The properties are determined by the number of electrons in the outermost orbit and are classified as conductors, insulators, and semi-conductors depending on the number of electrons in the outermost orbit.

Electronic Configuration :

The positively charged proton and the negatively charged electron are equal in magnitude and is equal to 1.6 x 10-19 Coulomb. The mass of the proton or neutron is 1.672 x 10-27 kg whereas the mass of an electron is 9.107 x 10-31 kg. Therefore, a proton is 1836 times heavier than an electron. The orbit in which the electrons revolve is called shell. Different shells have different planes. The number of electrons in each shell can accommodate 2n2 where n is the number of shell counting from the innermost shell. A sample shell configuration is shown in Fig.

Taking an example, potassium has 19 protons in the nucleus and 19 electrons in the orbit. Its atomic number is therefore 19. Applying the formula 2n2, the number of electrons in the orbit is given by

(a) innermost orbit (K shell) 2n2 = 2 X 12 = 2 electrons
(b) second orbit (L shell) 2n2 = 2 X 22 = 8 electrons

The balance is nine. Third orbit (M shell) can occupy 2 x 32 = 18 electrons but there are only nine electrons. For a stable condition even number of electrons should be present in the shell. Therefore, 8 electrons occupy M shell and the remaining one in the N shell.

Take the case of Germanium where the atomic number is 32. It contains 32 electrons in the orbit. Apply the same formula, the number of electrons in each shell is as follows :

K shell = 2n2 = 2 x 12 = 2 electrons
L shell = 2n2 = 2 x 22 = 8 electrons
M shell = 2n2 = 2 x 32 = 18 electrons

and the balance is four electrons which is even in number occupy in the N shell.

Atomic Structure of Elements:

Atomic
Element
Proton
Electrons per level
K
L
M
N
O
P
Q
1
Hydrogen (H)
1
1






2
Helium (He)
2
2






3
Lithium (Li)
3
2
1





5
Boron (B)
5
2
3





6
Carbon (C)
6
2
4





7
Nitrogen (N)
7
2
5





8
Oxygen (O)
8
2
6





10
Neon (Ne)
10
2
8





11
Sodium (Na)
11
2
8
1




12
Magnesium (Mg)
12
2
8
2




13
Aluminium (Al)
13
2
8
3




14
Silicon (Si)
14
2
8
4




15
Phosphorous (P)
15
2
8
5




16
Sulphur (S)
16
2
8
6




17
Chlorine (Cl)
17
2
8
7




18
Argon (A)
18
2
8
8




19
Potassium (K)
19
2
8
8
1



24
Chromium (Cr)
24
2
8
12
2



26
Iron (Fe)
26
2
8
14
2



27
Cobalt (Co)
27
2
8
14
3



28
Nickel (Ni)
28
2
8
16
2



29
Copper (Cu)
29
2
8
18
1



30
Zinc (Zn)
30
2
8
18
2



32
Germanium (Ge)
32
2
8
18
4



33
Arseni (As)
33
2
8
18
5



34
Selenium (Se)
34
2
8
18
6



36
Krypton (Kr)
36
2
8
18
8



47
Silver (Ag)
47
2
8
18
18
1


48
Cadmium (Cd)
48
2
8
18
18
2


50
Tin (Sn)
50
2
8
18
18
4


51
Antimony (Sb)
51
2
8
18
18
5


74
Tungsten (W)
74
2
8
18
32
12
2

78
Platinum (Pt)
78
2
8
18
32
16
2

79
Gold (Au)
79
2
8
18
32
18
1

80
Mercury (Hg)
80
2
8
18
32
18
2

82
Lead (Pb)
82
2
8
18
32
18
4




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