3. ATOMS 1.ATOMS ARE SMALLEST ELEMENT OF MATTER. 2. ATOMS ARE MADE UP OF THREE SUBATOMIC PARTICLES : ELECTRONS, PROTONS AND NEUTRONS. 3. PROTON AND NEUTRON ARE PRESENT IN A SMALL NUCLEUS AT THE CENTER OF ATOM. 4.ELECTRON ARE PRESENT OUTSIDE THE NUCLEUS. 5. ATOMS HAVE FOUR SHELL K, L, M, N.

4. DISCOVERY OF ELECTRON THE EXISTENCE OF ELECTRON IN AN ATOM WAS SHOWN BY J.J THOMSAN IN 1897.THOMSON PASSED ELECTRICITY AT HIGH VOLTAGE THROUGH A GAS AT VERY LOW PRESSURE TAKEN IN A DISCHARGE TUBE.STREAMS OF MINUTE PARTICLES WERE GIVEN OUT BY CATHODE (NEGATIVE ELECTRODE).THESE STREAMS OF PARTICAL ARE CALLED CATHODE RAYS (BECAUSE THEY COME OUT OF CATHODE). THE MASS AND CHARGE OF THE CATHODE RAY ELECTRON

5. PARTICLES DOES NOT DEPEND ON THE NATURE OF GAS TAKEN IN THE DISCHARGE TUBE. CATHODE RAYS CONSISTS OF SMALL, NEGATIVELY CHARGED PARTICLES CALLED ELECTRONS. SINCE ALL THE GASES FROM CATHODE RAYS, IT WAS CONCLUDE THAT ALL THE ATOMS CONTAIN NEGATIVELY CHARGED PARTICLES CALLED ELECTRONS. THOMSON EXPLAINED THE FORMATION OF CATHODE RAYS AS FOLLOWS. THE GAS TAKEN IN THE DISCHARGE TUBE CONSISTS OF ATOM, AND ALL THE ATOMS CONTAIN ELECTRONS. WHEN HIGH ELECTRICAL VOLTAGE IS APPLIED, THE ELECTRIAL ENERGY PUSHES OUT SOME OF THE ELECTRON FROM THE ATOMS OF THE GAS. THESE FAST MOVING ELECTRON FROM CATHODE RAYS. THUS, THE FORMATION OF CATHODE RAYS SHOWN THAT ONE OF THE SUBATOMIC PARTICLE PRESENT IN ALL THE ATOMS IS THE

6. NEGATIVELY CHARGED ‘ELECTRON’. WE CAN NOW DEFINE AN ELECTRON AS FOLLOWS. BASICALLY THE ELECTRON IS A NEGATIVELY CHARGED OPARTICLE FOUND IN THE ATOMS OF ALL THE ELEMENTS. THE ELECTRON ARE LOCATED OUTSIDE THE NUCLEUS IN AN ATOM. ONLY HYDROGEN ATOM CONTAINS ONE ELECTRON, ALL OTHER ATOMS CONTAIN MORE THAN ONE ELECTRON. AN ELECTRON IS USUALLY REPRESENTED BY THE SYMBOL E - (E FOR ELECTRON AND MINUS SIGN FOR NEGATIVE CHARGE).

7. DISCOVERY OF PROTON PROTON SOMEWHERE BETWEEN THOMSON AND CHADWICK, PHYSICISTS REALIZED THAT THERE ARE POSITIVELY CHARGED CONSTITUENTS OF THE NUCLEUS, WHICH WE CALL 'PROTONS'. THE WAY THIS HAPPENED WAS A GRADUAL PROCESS, AND THAT IS WHY IT IS HARD TO SAY EXACTLY WHO DISCOVERED THE PROTON, ALTHOUGH IF YOU HAD PUT A NAME AGAINST IT, IT WOULD BE RUTHFORD, SORT OF. AFTER THE DISCOVERY OF THE ELECTRON, IT WAS REALIZED THAT THERE MUST

8. BE POSITIVE CHARGE CENTERS WITHIN THE ATOM TO BALANCE THE NEGATIVE ELECTRONS AND CREATE ELECTRICALLY NEUTRAL ATOMS. RUTHERFORD'S DISCOVERY OF THE NUCLEUS DEMONSTRATED THAT THESE POSITIVE CHARGES WERE CONCENTRATED IN A VERY SMALL FRACTION OF THE ATOMS' VOLUME. IN 1919 RUTHERFORD DISCOVERED THAT HE COULD CHANGE ONE ELEMENT INTO ANOTHER BY STRIKING IT WITH ENERGETIC ALPHA PARTICLES (WHICH WE NOW KNOW ARE JUST HELIUM NUCLEI). IN THE EARLY 1920'S RUTHERFORD AND OTHER PHYSICISTS MADE A NUMBER EXPERIMENTS, TRANSMUTING ONE ATOM INTO ANOTHER. IN EVERY CASE, HYDROGEN NUCLEI WERE EMITTED IN THE PROCESS. IT WAS APPARENT THAT THE HYDROGEN NUCLEUS PLAYED A FUNDAMENTAL ROLE IN ATOMIC STRUCTURE, AND BY COMPARING NUCLEAR MASSES TO CHARGES, IT WAS REALIZED THAT THE POSITIVE CHARGE OF ANY NUCLEUS COULD BE ACCOUNTED FOR BY AN INTEGER NUMBER OF HYDROGEN NUCLEI. BY THE LATE 1920'S PHYSICISTS WERE REGULARLY REFERRING TO HYDROGEN NUCLEI AS 'PROTONS'.

9. DISCOVERY OF NEUTRONS AFTER THE DISCOVERY OF PROTON AND ELECTRONS, IT WA NOTICED THAT ALL THE MASS OF AN ATOMCANNOT BE ACCOUNTED FOR ON THE BASIS OF ONLY PROTONS AND ELECRTONS PRESENT IN IT. FOR EXAMPLE, A CARBON ATOM CONTAINS 6 P[ROTONS AND 6 ELECTRONS. NOW, THE MASS OF ELECTRONS IS SO SMALL IT CAN BE IGNORED. SO, THE ATOMIC MASS OF CARBON SHOULD BE ONLY 6 U, WHICH IS THE MASS OF 6 PROTONS. THIS, HOWEVER IS WRONG BECAUSE THE ACTUAL ATOMIC MASS OF CARBON IS 12 U. THEN, HOW DO WE EXPLAIN THIS EXTRA MASS OF 6 UNITS ?

10. THIS PROBLEM WAS SOLVED BY THE DISCOVERY OF ANOTHER SUBATOMIC PARTICLE BY JAMES CHADWICK IN 1932. THIS PARTICLE IS CALLED NEUTRON. THE NEUTRON IS NATURAL PARTICLE FOUND IN THE NUCLUES OF ANY ATOM. ATOMS OF ALL THE ELEMENT CONTAIN NEUTRON EXCEPT ORDINARY HYDROGEN ATOM WHICH DOES NOT CONTAIN ANY NEUTRON. THUS, THE SUBATOMIC PARTICLES NOT PRESENT IN A HYDROGEN ATOM IS NEUTRON. A HYDROGEN ATOM CONTAINS ONLY ONE PROTON AND ONE ELECTRON. A NEUTRON IS REPRESENTED BY THE SYMBOL N.

11. THOMSON’S MODAL OF THE ATOM WHEN J. J THOMSAN PROPESED HIS MODEL OF THE ATOM IN 1903, THEN ONLY ELECTRON AND PROTON WERE KNOWN TO BE PREENT IN THE ATOM. ACCORDING TO THOMSON’S MODEL OF THE ATOM : 1.ANY ATOM COSISTS OF A SPHERE (OR BALL) OF POSITIVE CHARGE WITH NEGATIVELY CHARGE ELECTRON EMBEDDED IN IT. 2.THE POSITIVE AND NEGATIVE CHARGE IN ANY ATOM ARE EQUAL IN MAGNITUDE, DUE TO WHICH ANY ATOM IS ELECTRICALLY NATURAL. IT HAS NO OVERALL POSITIVE OR NEGATIVE CHARGE.

12. THE COLOURED AREA IN SPHERE (OR BALL) CONTAINS ALL THE POSITIVE CHARGE IN THE ATOM. THE NEGATIVELY CHARGED ELECTRON ARE SPREAD THROUGHOUT THE POSITIVE CHARGED. THE TOTAL NEGATIVE CHARGED OF ELECTRONS IS EQUAL TO THE TOTAL POSITIVE CHARGED OF THE SPHERE. THESE EQUAL AND OPPOSITE CHARGES BALACE EACH OTHER DUE TO WHICH ANY ATOM BECOME ELECTRICALLY NEUTRAL ON THE HOLE. THOMSON’S MODAL OF THE ATOM IS SIMILAR TO THAT OF A CHRISMAS PDDING. THE ELECTRONS EMBEDDED IN A SPHERE OF POSITIVE CHARGE ARE LIKE THE CURRENTS (DRY FRUITS) IN A SPHERICAL CHRISMAS PUDDING. WE CAN ALSO COMPARE THOMSON’S MODAL OF ATOM TO A WATERMALEN. THE RED, EDIBLE PART OF WATERMALEN REPRESENT THE SPHERE OF POSITIVE CHARGE WHEREAS THE BLACK SEEDS EMBEDDED IN WATERMALEN ARE LIKE THE ELECTRONS. ALTHROUGH THOMSON’S MODAL OF ATOM EXPLAIN THE ELECTRICALLY NEUTRAL NATURE OF ATOM BUT IT COULD NOT EXPLAIN THE RESULT OF VARIOUS EXPARIMENT CARRIED OUT BY OTHER SCINTISTS SUCH AS RATHERFORD’S EXPARIMENT.

13. RATHERFORD’S MODAL OF AN ATOM BY 1911 THE COMPONENTS OF THE ATOM HAD BEEN DISCOVERED. THE ATOM CONSISTED OF SUBATOMIC PARTICLES CALLED PROTONS AND ELECTRONS. HOWEVER, IT WAS NOT CLEAR HOW THESE PROTONS AND ELECTRONS WERE ARRANGED WITHIN THE ATOM. J.J. THOMSON SUGGESTED THE"PLUM PUDDING" MODEL. IN THIS MODEL THE ELECTRONS AND PROTONS ARE UNIFORMLY MIXED THROUGHOUT THE ATOM:

14. RUTHERFORD TESTED THOMSON'S HYPOTHESIS BY DEVISING HIS "GOLD FOIL" EXPERIMENT. RUTHERFORD REASONED THAT IF THOMSON'S MODEL WAS CORRECT THEN THE MASS OF THE ATOM WAS SPREAD OUT THROUGHOUT THE ATOM. THEN, IF HE SHOT HIGH VELOCITY ALPHA PARTICLES (HELIUM NUCLEI) AT AN ATOM THEN THERE WOULD BE VERY LITTLE TO DEFLECT THE ALPHA PARTICLES. HE DECIDED TO TEST THIS WITH A THIN FILM OF GOLD ATOMS. AS EXPECTED, MOST ALPHA PARTICLES WENT RIGHT THROUGH THE GOLD FOIL BUT TO HIS AMAZEMENT A FEW ALPHA PARTICLES REBOUNDED ALMOST DIRECTLY BACKWARDS. THESE DEFLECTIONS WERE NOT CONSISTENT WITH THOMSON'S MODEL. RUTHERFORD WAS FORCED TO DISCARD THE PLUM PUDDING MODEL AND REASONED THAT THE ONLY WAY THE ALPHA PARTICLES COULD BE DEFLECTED BACKWARDS WAS IF MOST OF THE MASS IN AN ATOM WAS CONCENTRATED IN A NUCLEUS. HE THUS DEVELOPED THE PLANETARY MODEL OF THE ATOM WHICH PUT ALL THE PROTONS IN THE NUCLEUS AND THE ELECTRONS ORBITED AROUND THE NUCLEUS LIKE PLANETS AROUND THE SUN.

15. BOHR’S MODAL OF AN ATOM IN 1913 NIELS BOHR CAME TO WORK IN THE LABORATORY OF ERNEST RUTHERFORD. RUTHERFORD, WHO HAD A FEW YEARS EARLIER, DISCOVERED THE PLANETARY MODEL OF THE ATOM ASKED BOHR TO WORK ON IT BECAUSE THERE WERE SOME PROBLEMS WITH THE MODEL: ACCORDING TO THE PHYSICS OF THE TIME, RUTHERFORD'S PLANETARY ATOM SHOULD HAVE AN EXTREMELY SHORT LIFETIME.

16. BOHR THOUGHT ABOUT THE PROBLEM AND KNEW OF THE EMISSION SPECTRUM OF HYDROGEN. HE QUICKLY REALIZED THAT THE TWO PROBLEMS WERE CONNECTED AND AFTER SOME THOUGHT CAME UP WITH THE BOHR MODEL OF THE ATOM. BOHR'S MODEL OF THE ATOM REVOLUTIONIZED ATOMIC PHYSICS. THE BOHR MODEL CONSISTS OF FOUR PRINCIPLES:1)ELECTRONS ASSUME ONLY CERTAIN ORBITS AROUND THE NUCLEUS. THESE ORBITS ARE STABLE AND CALLED "STATIONARY" ORBITS.2)EACH ORBIT HAS AN ENERGY ASSOCIATED WITH IT. FOR EXAMPLE THE ORBIT CLOSEST TO THE NUCLEUS HAS AN ENERGY E1, THE NEXT CLOSEST E2 AND SO ON.3)LIGHT IS EMITTED WHEN AN ELECTRON JUMPS FROM A HIGHER ORBIT TO A LOWER ORBIT AND ABSORBED WHEN IT JUMPS FROM A LOWER TO HIGHER ORBIT.4)THE ENERGY AND FREQUENCY OF LIGHT EMITTED OR ABSORBED IS GIVEN BY THE DIFFERENCE BETWEEN THE TWO ORBIT ENERGIES. WITH THESE CONDITIONS BOHR WAS ABLE TO EXPLAIN THE STABILITY OF ATOMS AS WELL AS THE EMISSION SPECTRUM OF HYDROGEN. ACCORDING TO BOHR'S MODEL ONLY CERTAIN ORBITS WERE ALLOWED WHICH MEANS ONLY CERTAIN ENERGIES ARE POSSIBLE. THESE ENERGIES NATURALLY LEAD TO THE EXPLANATION OF THE HYDROGEN ATOM SPECTRUM: