1. THE HISTORY OF AN ATOM

2. What is an Atom? Atom is a portion of an element that takes part in chemical reactions. Atom is a portion of an element that takes part in chemical reactions.

3. HISTORY OF AN ATOM According to John Dalton, the matter is made of very tiny particles called atoms. And atoms cannot be broken down further. According to John Dalton, the matter is made of very tiny particles called atoms. And atoms cannot be broken down further. That is atoms were not composed of simpler constituents. That is atoms were not composed of simpler constituents. They were imagined to be like marbles They were imagined to be like marbles

4. HISTORY CONT…. 20 Year period from 1895-1915 changed many basic concepts related to atoms- from indivisible to divisible atom from indivisible to divisible atom

5. EXPERIMENT PASSING ELECTRICITY THROUGH GAS Electricity, passing through a tube containing a gas at low pressure, resulted in the production of new kind of rays.

6. CATHODE RAYS These 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 and magnetic fields. but unlike light could be bent by electric and magnetic fields. It was thus clear that these rays consisted of charged particles. It was thus clear that these rays consisted of charged particles.

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

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

9. X-RAYS X-Rays X-Rays traveled in straight line. traveled in straight line. Were not bent by electric or magnetic field. Were not bent by electric or magnetic field. They could pass through opaque materials like black paper and expose a photographic plate wrapped in it. They could pass through opaque materials like black paper and expose a photographic plate wrapped in it.

10. CATHODE RAYS & X-RAYS Cathode rays were later renamed as electrons. Thus, they are beam of electrons. Cathode rays were later renamed as electrons. Thus, they are beam of electrons. X-Rays are electromagnetic waves with very short wavelengths. They are emitted when fast moving electrons are suddenly stopped by some obstruction. X-Rays are electromagnetic waves with very short wavelengths. They are emitted when fast moving electrons are suddenly stopped by some obstruction.

11. HENRI BECQUEREL’S EXPERIMENT Henri Becquerel, a French Scientist, tried to find out whether phosphorescent materials emitted any radiation. Henri Becquerel, a French Scientist, tried to find out whether phosphorescent materials emitted any radiation. He conducted an experiment in a dark room with a phosphorescent material- Uranium salt a and a photographic plate. When he developed the plates he found the clear image of the salt placed next to it. He conducted an experiment in a dark room with a phosphorescent material- Uranium salt a and a photographic plate. When he developed the plates he found the clear image of the salt placed next to it.

12. Becquerel’s Conclusion He immediately concluded that the salts gave out penetrating radiation spontaneously. This radiation was capable of penetrating through thick layers of paper. He immediately concluded that the salts gave out penetrating radiation spontaneously. This radiation was capable of penetrating through thick layers of paper. From Where are these rays emitted? Obviously these came from atom.

13. J.J. THOMSON’S ‘PLUM PUDDING MODEL’ OF AN ATOM J.J. Thomson, through his work on cathode rays discovered that J.J. Thomson, through his work on cathode rays discovered that atoms were in fact divisible, atoms were in fact divisible, being partially composed of very light negatively-charged particles (which proved identical to each other in properties, no matter the element they originated from), which were later named electrons. being partially composed of very light negatively-charged particles (which proved identical to each other in properties, no matter the element they originated from), which were later named electrons. An atom – a sphere of positive charge containing electrons

14. Ernest Rutherford showed that radiation from uranium salt contained at least two components which he called alpha ( α ) and beta ( β ) radiation. Ernest Rutherford showed that radiation from uranium salt contained at least two components which he called alpha ( α ) and beta ( β ) radiation. Using strong magnetic fields, scientists were able to show that α –rays were positively charged and β – rays were negatively charged. Using strong magnetic fields, scientists were able to show that α –rays were positively charged and β – rays were negatively charged. ALPHA AND BETA PARTICLES

15. RUTHERFORD’S EXPERIMENT

16. CENTRAL NUCLEUS The earlier “plum pudding” theory of atomic structure was shattered and a new theory of central nucleus was put forward. The earlier “plum pudding” theory of atomic structure was shattered and a new theory of central nucleus was put forward. The atom’s mass is not uniformly distributed. Rutherford hypothesized that most of the mass and all of its positive charge are centered in a very small region called nucleus. The atom’s mass is not uniformly distributed. Rutherford hypothesized that most of the mass and all of its positive charge are centered in a very small region called nucleus.

17. Ultimately what we reached at… The atomic structure The atomic structure