THE HISTORY OF AN ATOMTHE HISTORY OF AN ATOM

What is an Atom?What is an Atom?Atom is a portion of an element that takes Atom is a portion of an element that takes

part in chemical reactions.part in chemical reactions.

HISTORY OF AN ATOMHISTORY OF AN ATOM

According to John Dalton, the matter is According to John Dalton, the matter is made of very tiny particles called atoms. made of very tiny particles called atoms. And atoms cannot be broken down further.And atoms cannot be broken down further.

That is atoms were not composed of That is atoms were not composed of simpler constituents. simpler constituents.

They were imagined to be like marblesThey were imagined to be like marbles

HISTORY CONT….HISTORY CONT….

20 Year period from 1895-1915 changed many 20 Year period from 1895-1915 changed many basic concepts related to atoms- basic concepts related to atoms-

from indivisible to divisible atomfrom indivisible to divisible atom

EXPERIMENTEXPERIMENT PASSING ELECTRICITY THROUGH GAS PASSING ELECTRICITY THROUGH GAS

Electricity, passing through a tube containing a gas at low Electricity, passing through a tube containing a gas at low pressure, resulted in the production of new kind of rays. pressure, resulted in the production of new kind of rays.

CATHODE RAYSCATHODE RAYSThese rays, like light, These rays, like light,

traveled in a straight line, traveled in a straight line, cast shadows of obstacles, cast shadows of obstacles,

but unlike light could be bent by electric but unlike light could be bent by electric and magnetic fields. and magnetic fields.

It was thus clear that these rays consisted It was thus clear that these rays consisted of charged particles. of charged particles.

Where did these particles come from?Where did these particles come from?

To start with we only had gas atoms. Do To start with we only had gas atoms. Do these atoms contain charged particles? In these atoms contain charged particles? In the absence of any clue, scientists called the absence of any clue, scientists called them them cathode rayscathode rays because they because they seemed to come out of cathode (the seemed to come out of cathode (the negative electrode).negative electrode).

FROM CATHODE RAYS TO X-RAYSFROM CATHODE RAYS TO X-RAYS

W.H. Roentgen (1845-1923) was W.H. Roentgen (1845-1923) was also studying the passage of also studying the passage of electricity through gases. He electricity through gases. He found in 1896 that when these found in 1896 that when these cathode rays struck the glass cathode rays struck the glass walls of the tube a new kind of walls of the tube a new kind of rays were emitted. Since rays were emitted. Since Roentgen did not know what Roentgen did not know what these rays were, he called them these rays were, he called them called called x-raysx-rays..

X-RAYSX-RAYS

X-Rays X-Rays

traveled in straight line.traveled in straight line. Were not bent by electric or Were not bent by electric or

magnetic field.magnetic field. They could pass through opaque They could pass through opaque

materials like black paper and materials like black paper and expose a photographic plate expose a photographic plate wrapped in it.wrapped in it.

CATHODE RAYS & X-RAYSCATHODE RAYS & X-RAYS Cathode rays were later renamed as electrons. Thus , Cathode rays were later renamed as electrons. Thus ,

they are beam of electrons. they are beam of electrons.

X-Rays are electromagnetic waves with very short X-Rays are electromagnetic waves with very short wavelengths. They are emitted when fast moving wavelengths. They are emitted when fast moving electrons are suddenly stopped by some obstruction.electrons are suddenly stopped by some obstruction.

HENRI BECQUEREL’S HENRI BECQUEREL’S EXPERIMENTEXPERIMENT

Henri Becquerel, a French Henri Becquerel, a French Scientist, tried to find out Scientist, tried to find out whether phosphorescent whether phosphorescent materials emitted any radiation.materials emitted any radiation.

He conducted an experiment He conducted an experiment in a dark room with a in a dark room with a phosphorescent material- phosphorescent material- Uranium salt a and a Uranium salt a and a photographic plate. When he photographic plate. When he developed the plates he found developed the plates he found the clear image of the salt the clear image of the salt placed next to it.placed next to it.

Becquerel’s ConclusionBecquerel’s Conclusion He immediately He immediately

concluded that the salts concluded that the salts gave out penetrating gave out penetrating radiation spontaneously. radiation spontaneously. This radiation was This radiation was capable of penetrating capable of penetrating through thick layers of through thick layers of paper.paper.

From Where are these rays emitted?From Where are these rays emitted?Obviously these came from atom.Obviously these came from atom.

J.J. THOMSON’S ‘PLUM PUDDING J.J. THOMSON’S ‘PLUM PUDDING MODEL’ OF AN ATOMMODEL’ OF AN ATOM

J.J. Thomson, through his J.J. Thomson, through his work on cathode rays work on cathode rays discovered thatdiscovered that

atoms were in fact divisible,atoms were in fact divisible, being partially composed of being partially composed of

very light negatively-charged very light negatively-charged particles (which proved particles (which proved identical to each other in identical to each other in properties, no matter the properties, no matter the element they originated element they originated from), which were later from), which were later named electrons. named electrons.

An atom – a sphere of positive charge containing electrons

Ernest Rutherford showed that radiation from uranium Ernest Rutherford showed that radiation from uranium salt contained at least two components which he called salt contained at least two components which he called alpha (alpha (αα) and beta () and beta (ββ) radiation. ) radiation.

Using strong magnetic fields, scientists were able to Using strong magnetic fields, scientists were able to show that show that αα–rays were positively charged and –rays were positively charged and ββ – rays – rays were negatively charged.were negatively charged.

ALPHA AND BETA PARTICLESALPHA AND BETA PARTICLES

RUTHERFORD’S EXPERIMENTRUTHERFORD’S EXPERIMENT

CENTRAL NUCLEUSCENTRAL NUCLEUS The earlier “plum pudding” The earlier “plum pudding”

theory of atomic structure was theory of atomic structure was shattered and a new theory of shattered and a new theory of central nucleus was put central nucleus was put forward. forward.

The atom’s mass is not The atom’s mass is not uniformly distributed. Rutherford uniformly distributed. Rutherford hypothesized that most of the hypothesized that most of the mass and all of its positive mass and all of its positive charge are centered in a very charge are centered in a very small region called nucleus. small region called nucleus.

Ultimately what we reached at…Ultimately what we reached at…The The

atomic atomic structurestructure