Chapter # 03 [Atom; The fundamental particle]

 

UNIT# 03

ATOM; THE FUNDAMENTAL PARTICLE

Structure of an atom was more complicated to understand for the scientists for many years. From 1808, many scientists explained the structure of the atom but finally, it was accepted in 1913.

Scientists played a functional role in the description of atom. As it includes further particles termed as Electron, Proton and neutron. They are named so, because of the charges they contain.

Q#	Questions	Year
Q# 01	State Dalton’s atomic theory.	[2016][2011][2005]
Q# 02	State the postulates of Modern Atomic theory.
Q# 03	Explain Rutherford’s atomic model.	[2011][2009]
Q# 04	Explain the Discharge tube experiment.
Q# 05	How the discovery of proton was made possible?
Q# 06	What is mean by Neutron? Give its properties.
Q# 07	Give properties of Cathode rays.	[2016][2013][2008] [2006]
Q# 08	Write note on Isotopes.	[2018][2016][2015] [2012][2009] [2005]
Q# 09	Define Isotope.	[2008][2006]
Q# 10	Define radioactivity.	[2017][2007][2006]
Q# 11	Give properties of alpha, beta and gamma rays.	[2012][2010][2007]
Q# 12	Draw a labeled diagram to show the separation of alpha, beta and Gamma rays.	[2007]

 Q. State Dalton’s atomic theory.

DALTON’S THEORY [1808]

IMPORTANT POSTULATES

Following are the main postulates of Dalton’s atomic theory:

1.      All elements are made up of small indivisible particles and indestructible particles called atoms.

2.      All atoms in a single element always identical in all respect having same size, mass and chemical properties.

3.      When atoms of more than one element combine in a simple whole ratio, compounds are formed.

4.      Atoms are neither created nor destroyed during chemical reaction.

 DEFECTS OF DALTON’S THEORY [1808]

·        According to Dalton’s atomic theory, atom is indivisible, indestructible particles but according to modern atomic theory atom can be sub divided into electron, proton and neutron

·        According to Dalton’s atomic theory, different atoms of same element are identical in all respects (having same size, mass and chemical properties) but contrary isotopes of elements differ in mass.

 

Q. State Modern atomic theory.

MODERN ATOMIC THEORY

IMPORTANT POSTULATES

Modern atomic theory failed the Dalton’s atomic theory by providing the following facts:

1.      Atom is the complex organization; even composed of smaller particles called electron, proton and neutron.

2.      Atom cannot have the same size, mass and chemical properties; this point was failed by the discovery of isotopes.

3.      Compounds are formed when atoms of more than one element combined in a simple whole number ration.

4.      Atom can neither created nor destroyed during any chemical reaction.

 

Q. Describe the construction and working of Discharge tube.

DISCHARGE TUBE EXPERIMENT [DISCOVERY OF ELECTRON]

INTRODUCTION

The fundamental negative charged sub particle was discovered by J.J. Thomson in 1897.

 CONSTRUCTION

The discharge tube consists of a glass tube, fitted with two metal electrodes connected to a high voltage source and a vacuum pump.

 WORKING

When the tube is evacuated and a current of high potential is passed between the electrodes streaks of bluish light extending from negative electrodes (cathode) towards positive electrodes (anode). The rays appear to travel in a straight line. These rays were deflected towards the positive plate in an electric and magnetic field. The high voltage is slowly increased till a charge was observed.

CONCLUSION

The name electron was given to these rays because of containing negative charge. 

 

Q. Give the properties of Cathode rays.

PROPERTIES OF CATHODE RAYS

By performing the discharge tube experiment, following properties of the electron came to see.

1.      They are negatively charged particles.

2.      These rays can cause a light paddle wheel to rotate.

3.      They produce florescence.

4.      Charge / mass ratio is same as electron.

5.      They possess kinetic energy.

6.      These rays deflected towards positive plate, when passed through electric and magnetic field.

 

Q. Describe the experiment for the discovery of Proton.

DISCOVERY OF PROTON [GOLDSTEIN EXPERIMENT]

Discovery of proton was made possible by the similar instrument used in the discovery of electron i.e. Discharge tube, Goldstein confirmed the identity of proton but the properties of proton was studied by J.J Thomson.

 EXPERIMENT

Discharge tube was filled with hydrogen gas while cathode was perforated. Light was observed when high voltage is applied at low pressure.

CONCLUSION

It was concluded that light was observed due to the ionization of gas.

 

Q. Give the properties of positive rays.

PROPERTIES OF POSITIVE RAYS

1.      These rays travel in straight line from anode to cathode.

2.      They have positive charge

3.      Charge / mass ratio is smaller the electrons.

Q. Give the properties of proton.

PROPERTIES OF PROTON

1.      These are positive charged particles

2.      The mass of proton is 1836 times heavier than electron [9.109 X 10 ^-31 Kg].

3.      The charge on proton is 1.602 X 10^-19 coulomb

 

Q. Describe the discovery of neutron.

DISCOVERY OF NEUTRON

In 1932, James Chadwick another researcher discovered the third sub-atomic particle i.e. neutron through artificial radioactivity. It was found that it has no charge but mass is same as mass of proton.

 

Q. Give the properties of positive rays.

PROPERTIES OF NEUTRON

1.      It is neutral particle.

2.      Its mass is 1.76 X 10^-27 Kg.

3.      It has no charge.

 

Q. Compare electron, proton and neutron.

Comparison between electron, proton and neutron
Property	Electron	Proton	Neutron
Discovery	J.J. Thomson	Rutherford	Chadwick
Charge	Negative	Positive	Neutral
Mass	9.1 X 10-31Kg	1.6727 X 10-27 Kg	1.6750 X 10-27 Kg
Symbol	e	p	n
Location	Orbit	Nucleus	Nucleus

 

Q. Define radioactivity. Explain the experiment to show the discovery of radioactive rays.

RADIOACTIVITY

The phenomenon in which invisible radiations excrete from the radioactive elements is called radioactivity and the substance is called radioactive element.

FOR EXAMPLE

·        Uranium

·        Polonium

·        Thorium

 

TYPES OF RADIOACTIVE RAYS

EXPERIMENT

Rutherford took lead box and placed radioactive substance in it. Photographic plate was placed in front of the box. Radioactive substance resolves into three different radiations on the basis of their movement.

Q. Give the properties of alpha, beta and gamma rays.

TYPE I [ALPHA RAYS]

1.      These rays deflected by electric and magnetic field.

2.      They contain double positive charge

3.      They cannot pass from matter

 

TYPE II [BETA RAYS]

1.      These rays deflected towards the negative plate, thus named beta rays

2.      They contains negative charge

3.      They can pass up to 1-2 m from matter

 

TYPE III [GAMMA RAYS]

1.      These rays deflected by electric and magnetic field.

2.      They do not contain any charge

3.      They have high penetration ability

 

Q. Give the properties of alpha, beta and gamma rays.

Comparison between alpha, beta and gamma rays
Property	Alpha rays	Beta rays	Gamma rays
Ionizing power	Highest Ionization Power	Weaker Ionizing Power Than Alpha	Weakest Ionization Power
Penetration	Lowest Penetration	A Few Cm Penetration Ability	Highest Penetration
Speed	0.1 C	0.9 C	C
Motion in electric and magnetic fields slight deflection	Slight Deflection	Large Deflection	No Deflection

 

Q. Explain Rutherford’s atomic model.

RUTHERFORD ATOMIC MODEL

EXPERIMENT

In 1911, Rutherford passed the beam of the alpha particles through a very thin gold metal foil. He founded that most of the alpha particles passed through it without any deflection. However, some of them deflected at large angles and very few of them bounced back.

CONCLUSION

1.      Most of the portion of atom is empty called extra nuclear part.

2.      Positive charge in an atom is present in the center, known as Nucleus.

 

PREDICTION

He predicted about the structure of atom that atom consists of two major parts

1.      Nucleus  which consists of proton and neutron

2.      Extra nuclear part. This includes the revolving electron around the nucleus.

 

Q. Give the weakness or defects of Rutherford’s atomic model.

DEFECTS OR WEAKNESS IN RUTHERFORD ATOMIC MODEL

On the atomic model of Rutherford, few scientists claimed that:

1.      If electron is revolving around the nucleus constantly so it should falls in the nucleus.

2.      If electron emits energy than it should form continuous spectrum but in actual line spectrum is produced.

 

Q. Explain Neil Bohr’s atomic model.

NEIL BOHR’S ATOMIC MODEL

In 1913, Neil Bohr proposed that the theoretical explanation of line spectra. His assumptions are mentioned below:

1.      Electron in an atom move only in certain energy levels so an electron in an allowed energy state will not radiate energy continuously and therefore will not fall in the nucleus.

2.      Electron only release and gain energy when it jumps from higher to lower energy level or vice versa.

3.      When electron release (dissipate) energy, it jumps back to lower energy level because of such movement, color is observed on screen called line spectrum

4.      Quantum energy is directly proportional to the frequency of radiation i.e. ∆E = E₂ – E₁

 

Q. What is Isotopes? Write note on Isotopes of Hydrogen.

ISOTOPES

Those elements which have same charge number but different mass number are called Isotopes.

OR

Atoms of an element having same number of protons but different number of neutrons are called isotopes.

 

ISOTPOES OF HYDROGEN

There are three isotopes of hydrogen.

Protium

2.     Deuterium

3.     Tritium

 PROTIUM

It has one proton and no neutron in the nucleus.

It is used in ordinary water.

 

DEUTRIUM

It has one proton and one neutron in the nucleus.

It is used in heavy water.

 

TRITIUM

It has one proton and two neutrons in the nucleus.

It is used in crackers. It is also known as radioactive isotope of Hydrogen.

 

Q. Give the isotopes of Oxygen.

ISOTOPES OF OXYGEN

ISOTOPE	ELECTRON	PROTON	NEUTRON
8O16	8	8	8
8O17	8	8	9
8O18	8	8	10

 

Q. Give the isotopes of Uranium.

ISOTOPES OF URANIUM

ISOTOPE	ELECTRON	PROTON	NEUTRON
92U234	92	92	142
92U235	92	92	143
92U236	92	92	144

 

Q. Give the isotopes of Carbon.

ISOTOPES OF CARBON

ISOTOPE	ELECTRON	PROTON	NEUTRON
6C12	6	6	6
6C13	6	6	7
6C14	6	6	8

 

Q. What do you know about electronic configuration?

ELECTRONIC CONFIGURATION

Electron revolves around the nucleus in various circular orbits called shell or energy level or orbit.

The distribution of electron in different energy level is called electronic configuration.

Shells of an atom are:

K= 2

L =8

M= 18

N= 32

O=50

P= 72

S,p,d and f are sub shell which contain number of electrons as follows:

s=2,

p=6,

d=10,

f=14