complete chapter#03 (atomic structure)

7/31/2019 Complete Chapter#03 (Atomic Structure)

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3.1. DEFINITIONS

ATOMIC NUMBER(Z)DEFINITION

“The number of protons present in the nucleus of an atom is referred as Atomic number”.

NOTATION

Atomic no. is indicated by symbol “Z”.

EXAMPLE

Atomic number of hydrogen = Z = 1.

Atomic number of sodium = Z = 11.

ATOMIC MASS (A)

Atomic mass of atom is infecting the mass of its nucleus.

DEFINITION

“The sum of the number of protons and neutrons present in the nucleus of an atom is called Atomicmass or Mass number of an element”.

NOTATION

Atomic mass is indicated by “A”.

EXAMPLE

Atomic mass of hydrogen = A = 1.

Atomic mass of carbon = A = 12.

3.2. DALTON’S ATOMIC THEORYINTRODUCTION

In 1808, John Dalton an English school teacher offered a scientific theor y about atom now known as

Dalton’s atomic theory.

FUNDAMENTAL POSTULATESThe fundamental postulates of Dalton’s atomic theory are as:

Matter is compressed of tiny indivisible particle called atom.

Atom can neither be created nor destroyed.

Atoms of a particular element are identical in size, shape, mass and all other properties and differ

from atoms of other elements in these properties.

Chemical combination takes place between small whole numbers of atoms.

3.3. MODERN ATOMIC THEORY (DEFECTS OF DALTON’S ATOMIC

THEORY)As a result of latest research on the structure of atom, following defects are observed:

Atom consists of charge particles called protons, electrons and neutral particles called neutrons.

The atom of same element may differ in atomic masses and such atoms of element are called

isotopes.

The ratio between atoms of various elements in many molecules of organic compounds is not simple.

Matter can be converted into energy by EINSTIEN’S famous equation (E=mc2).

3.4. DISCOVERY OF ELECTRONINTRODUCTION

In 1897, J. J Thomson discovered a negative charged particle known as electron.

CONSTRUCTION

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ATOMIC STRUCTURE



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Discharge tube is a simple glass tube filled with a gas.

It is about 30cm in length and 4cm in diameter.

It is closed at both ends.

It is fitted with two metallic electrodes.

The electrode connected to negative terminal of battery is called Cathode.

The electrode connected to positive terminal of battery is called Anode.

These two electrodes are

connected with a high

voltage source.

A vacuum pump is also

connected to reduce the

pressure inside the tube.

DIGRAM OF

DISCHARGE TUBE

EXPERIMENT A high voltage current was passed through electrodes at a very low pressure 0.1 mm of Hg in a

discharge tube.

When high voltage (10,000 volts) is applied across the two metallic electrodes sealed in a discharge

tube.

The rays bring to originates from cathode which are called cathode rays.

PROPERTIES OF CATHODE RAYS

On the basis of experiments J.J Thomson observe the following properties.

Cathode rays travel in straight lines away from cathode.

Cathode rays are negatively charge as they bent towards the positive pole.

The rays upon striking glass or certain other materials to glow.

The charge to mass ratio (e/m) of these particles was found equal to that of electron (e/m=1.76x108)c/g.

The part of each particle (negatively) was found equal to 1/1837 of the lightest hydrogen atom.

They can produce mechanical pressure indicating they possess kinetic energy.

RESULT

On the basis of above properties it was concluded that cathode rays are negatively charged particles are

called electrons.

3.5. DISCOVERY OF PROTONS (CANAL RAYS)INTRODUCTION

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In 1886 Goldstein discovered positive rays in a gas discharge tube experiment.

CONSTRUCTION

It is

a simple glass

tube filled with

a gas.

It isabout 30cm in

length and 4cm

in diameter.

It is

fitted with two

metallic

electrodes.



These two electrodes are connected with a high voltage battery source.



A vacuum pump is also connected to reduce the pressure inside the tube.

DIAGRAM OF DISCHSRGE TUBE

EXPERIMENT

A high voltage electric current was passed through gasses at a very low pressure in a discharge tube.

During the passage of electricity it was observed Goldstein that some radiations behind the perforated cathode.

These rays were found to be positively charged and hence called positive or canal rays.

PROPERTIES OF POSITIVE RAYS

On the basis of experiments observation Goldstein found that the positive rays possess the following properties.

These rays travel in positive direction than that of cathode rays. The gas ionized due to the bombardment of electrons coming from cathode.

Canal rays are deflected in electric and magnetic field in opposite direction than that of electron.

The deflections shows that these rays are positively charge particles.

These rays are fundamental particles of an atom.

Charge and mass ratio (e/m) depends upon the nature of the gas present in the discharge tube.

These rays are 1836 times heavier than cathode rays.

RESULT

On the basis of these properties it was concluded that canal rays are positively charged particles called

protons.

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3.6. DISCOVERY OF NEUTRONSBASIS FOR THE DISCOVERY OF NEUTRON

The sum of the masses of protons and electron in a atom is less then the total mass of an atom. This

shows that there must be another heavy particle in the atom.

INTRODUCTION

In 1932, James Chadwick discovered experimentally a new particle in the nucleus of atom and named itneutron.

PROPERTIES OF NEUTRON

According to Chadwick following are the properties of neutron.

The neutron does not have any electrical charge.

It mass is 1.675x10-24gram.

Like proton and electron, it is a fundamental particle of atoms.

The number of protons and neutrons are considered as the total mass of an atom.

The number of neutrons is either equal to or greater than the number of protons in the atoms of all

other elements.

It does not show any deflection in electric or magnetic field.

3.6. RUTHERFORD ATOMIC STRUCTUREINTRODUCTION

In 1911, Lord Rutherford carried out an experiment in order to determine the structure of an atom.

DIGRAMATICALLY

EXPERIMENT

Rutherford took a thin sheet of gold foil and bombarded it with alpha particles obtained from radioactive

source.

A photographic plat (Zinc) was placed behind the foil to observe the extent of scattering of (alpha)

particles.

OBSERVATION

He observed that:

Most of alpha particles passed undefeated through the sheet and produced laminations on screen.

Very few alpha particles underwent small deflection after passing through gold sheet.

A few alpha particles bounced back from the gold foil to their original track.

CONCLUSION

Since majority of the alpha particles passed undefeated, shows that major portion of atom is empty.

The total mass of the atom is present in the nucleus.

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The size of nucleus is very small as compared to the size of atom.

Positive charge is present in the nucleus.

The electrons revolve around the nucleus.

WEAKNESS (DEFECTS)

According to classical electromagnetic theory, the

revolving electrons will loss energy continuously.

Therefore the orbit of the electrons will become smaller and smaller until the electrons fall into the nucleus. Hence

atom should be unstable.

If the electrons emit energy continuously they

should form a continuous spectrum but actually line

spectrum is formed.

When electrons absorb energy it moves to higher energy level and when it loses energy it return to lower

energy level and radiations is emitted.

When electron jumps from higher energy level (E 2) to lower energy lever (E 1) emission if energyoccur in a fixed amount called quanta.

STRUCTURE OF SODIUM ATOM

3.7. APPLICATION OF BOHAR’S ATOMIC MODEL

INTRODUCTION Neil Bohar, a Danish Physicist in 1913 offered a theoretical explanation known as Bohar’s atomic theory

The significance or applications of Bohr’s atomic theory are as follows.

Bohr’s theory classified and explained many defects of Rutherford’s atomic model.

According to this theory number of electrons in an orbit can be determined by formula “2n2” where“n” is the number of shells.

EXAMPLE

In K shell (1st orbit) = 2x12 = 2.

In L shell (2nd orbit)= 2x22) =8.

In M shell (3rd orbit)= 2x32)=18.

The theory clearly shows how electrons are arranged in fixed orbit with particular number in each orbit.

Bohr’s theory also helps to discuss the periodicity of chemical properties of elements.

It explains the emission of line spectrum.

3.8. FUNDAMENTAL PARTICLES OF ATOMAtom is composed of numerous sub atomic particles but only fundamental particles take part in chemical

reaction.These fundamental particles are as under:

Electron.

Proton.

Neutron.

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PROPERTIES OF FUNDAMENTAL PARTICLES

The characteristics of these particles are described below.

ELECTRON (e- )

It is a negative charge particle.

It has a mass equal to 9.11x10-31 kg.

It has charge equal to 1.6022x10-19 columb. It is present around the nucleus.

PROTON (P+ )

It is a positively charge particle.


It has charge equal to 1.6022x10-19 coulomb.

It us 1836 times heavier than electron.


It is present in the nucleus.

3.9. NEUTRON (N0) It has no charge.

The actual mass of neutron is 1.76x10-27 kg.

It is 1842 times heavier than an electron.

ISOTOPES

“The atoms of same atomic elements having same atomic number but different atomic masses are called

Isotopes”.

EXAMPLES

ISOTOPES OF HYDROGEN

There are three isotopes of hydrogen named protium, deuterium and tritium.

1H1 (P1) 1H

2(1D2) 1H

3 (1T3)

ISOTOPES OF OXYGEN

Oxygen atoms has three isotopes having mass number 16.17 and 18 respectively

8O16

8O17 8O

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ISOTOPES OF URANIUM

There are three common is common isotopes of Uranium with mass number 234.235 and 236

respectively.

92U234

92U235

92U236

APPLICATION OF ISOTOPES

Isotopes are used as tracers in physical, chemical and biological researches as well as in treatment and

diagnoses of disease like cancer.

3.10. ELECTRONIC CONFIGURATION“A particular distribution of electrons among the different shells of an atom is called Electronic

configuration”.

NOTE

The maximum number of electrons in the given shell is governed by the formula 2n2. Where “n” is the

number of shells thus first orbit can have a maximum of “2” electrons and second have “8” electrons.

3.11. RADIOACTIVITY

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INTRODUCTION

In 1896, Henry Becquerel working with uranium mineral called as pitch blend, found that invisible

radiations coming out of this mineral.

NATURE OF RADIOACTIVITY (TYPES OF RAYS)

In 1902, British physicist Earnest Rutherford determined the nature of radioactive rays by the following

experiment and showed that , it is composed of three types of rays.

DEFINITION

“The spontaneous disintegration of a nucleus of an atom from which invisible radiation are emitted is

called Radioactivity”.

DIGRAMATICALLY

EXPERIMENT

A sample of radioactive substance was placed in a lead block, between the two oppositely charged

plates. The radiations were resolved into three components.

One component was deflected towards the negative plate, prove that it carried a positive charge and

were named Alpha ( ) rays.

One component was deflected towards the positive plate, prove that it carried a negative charge and

were named Beta ( ) rays.

The third type carried no charge no mass and were not deflected in the electric field. These were,

named as Gamma ( ) rays.

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complete chapter#03 (atomic structure)

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