THE STRUCTURE OF THE ATOM

1. THE STRUCTURE OF THE ATOM ATOMIC MODELS

2. PIONEERS OF THE ATOM • DEMOCRITUS • LAVOISIER • DALTON • THOMPSON • MILLIKEN • RUTHERFORD

3. THE EARLY ATOM IT’S ALL GREEK TO ME... • DEMOCRITUS IS THE MAN!!! • YOU KNOW IT!

4. DEMOCRITUS • WAS A GREEK PHILOSOPHER THAT BEGAN WONDERING WHAT WAS THE UNIVERSE MADE UP OF. • HE PROPOSED THAT ALL THE MATTER IN THE UNIVERSE IS COMPOSED OF TINY PARTICLES • THESE PARTICLES ARE INDIVISABLE • HE CALLED THESE PARTICLES ATOMOS (MEANS INDIVISABLE) • BUT HE HAD AN OPPONENT…

5. ARISTOTLE = SCEPTIC DEMOCRITUS IS AN IGNORAMUS! ARISTOTLE WAS MUCH MORE POPULAR AND HE OPPOSED THE IDEA OF ATOMS!

6. ARISTOTLE = SCEPTIC DEMOCRITUS IS AN IGNORAMUS! ARISTOTLE BELIEVED THAT MATTER WAS CONTINUOUS.

7. DEMOCRITUS’ ATOMS

8. SO...CHEMISTRY DIED FOR ABOUT 1400 YEARS UNTIL...

9. SO...CHEMISTRY DIED FOR ABOUT 1400 YEARS IN THE 1700’S

10. FOUNDATIONS OF ATOMIC THEORY • THE PREDOMINATE BELIEF BY THE MID 1700’S WAS THE MODERN DEFINITION OF AN ELEMENT AS A SUBSTANCE THAT CANNOT BE BROKEN DOWN BY ORDINARY CHEMICAL MEANS • IT WAS ALSO BELIEVED THAT ELEMENTS COMBINE TO FORM COMPOUNDS THAT HAVE DIFFERENT PHYS AND CHEM PROPERTIES THAN THOSE OF THE ELEMENTS THAT FORM THEM.

11. FOUNDATIONS OF ATOMIC THEORY • HOWEVER, THERE WAS CONTROVERSY AS TO WHETHER ELEMENTS ALWAYS COMBINE IN THE SAME RATIO WHEN FORMING A PARTICULAR COMPOUND. • IN THE 1790’S, THE STUDY OF MATTER WAS REVOLUTIONIZED BY A NEW EMPHASIS ON THE QUANTITATIVE ANALYSIS • SCIENTISTS’ INVESTIGATIONS WERE MADE MORE ACCURATE BY NEW IMPROVED BALANCES

12. ANTOINNE LAVOISIER TOOK ADVANTAGE… MATTER CAN BE NEITHER CREATED NOR DESTROYED. HE BEGAN TO GATHER EVIDENCE ABOUT HOW MATTER BEHAVED BEFORE AND AFTER A REACTION.

13. ANTOINNE LAVOISIER TOOK ADVANTAGE… MATTER CAN BE NEITHER CREATED NOR DESTROYED. HE DISCOVERED THAT THE TOTAL MASS OF THE INGREDIENTS OF A CHEMICAL REACTION WAS THE SAME AS THE TOTAL MASS OF THE RESULTS

14. 2 OTHER FUNDAMENTAL IDEAS WERE INVESTIGATED... THE LAW OF DEFINITE PROPORTION

15. 2 OTHER FUNDAMENTAL IDEAS WERE INVESTIGATED... THE LAW OF MULTIPLE PROPORTIONS

16. 2 OTHER FUNDAMENTAL IDEAS WERE INVESTIGATED... I’M BETTER LOOKING THAN PROUST!

17. + = CARBON MONOXIDE C O CO + = CARBON MONOXIDE C O CO + + = CARBON DIOXIDE O C O CO2

18. DALTON’S THOUGHTS • IN 1808, JOHN DALTON PROPOSED AN EXPLANATION FOR THE LAW OF CONS OF MASS, THE LAW OF DEFINITE PROPORTIONS, & FORMULATED THE LAW OF MULTIPLE PROPORTIONS. • HE REASONED THAT ELEMENTS WERE COMPOSED OF ATOMS & THAT ONLY WHOLE #’S OF ATOMS CAN COMBINE TO FORM COMPNDS • HIS THEORY CAN BE SUMMARIZED IN 5 STATEMENTS.

19. ELEMENT 1 ELEMENT 2 ELEMENT 3 ELEMENT 4

20. + +

21. DALTON AND HIS CONTEMPORARIES THE EARLIEST THEORISTS THOUGHT THE ATOM WAS HARD AND ROUND, MUCH LIKE TINY MARBLES OR BALL BEARINGS.

22. STRUCTURE OF THE ATOM • ALTHOUGH DALTON THOUGHT ATOMS WERE INDIVISIBLE, INVESTIGATORS IN THE LATE 1800’S PROVED OTHERWISE • IT SOON BECAME CLEAR THAT ATOMS ARE ACTUALLY COMPOSED OF SEVERAL BASIC TYPES OF SMALLER PARTICLES • AND IT’S THE NUMBERS AND ARRANGEMENTS OF THESE SUB-ATOMIC PARTICLES THAT DETERMINES THE IDENTITY OF THE ATOM.

23. STRUCTURE OF THE ATOM • THE FIRST DISCOVERY OF A SUBATOMIC PARTICLE RESULTED FROM THE INVESTIGATIONS INTO THE RELATIONSHIP BETWEEN ELECTRICITY AND MATTER. • IN THE LATE 1800’S, MANY EXPERIMENTS WERE PERFORMED IN WHICH ELECTRIC CURRENT WAS PASSED THROUGH VARIOUS GASES AT LOW PRESS • CARRIED OUT IN TUBES CALLED CATHODE-RAY TUBES

24. STRUCTURE OF THE ATOM • INVESTIGATORS NOTICED THAT WHEN CURRENT WAS PASSED THROUGH A CATHODE RAY TUBE, THE SURFACE OF THE TUBE DIRECTLY OPPOSITE THE CATHODE GLOWED. CATHODE RAY TUBE

25. CATHODE RAY STRUCTURE OF THE ATOM • THEY HYPOTHESIZED THAT THE GLOW WAS CAUSED BY A STREAM OF PARTICLES • THEY CALLED THE STREAM A CATHODE RAY CATHODE RAY TUBE

26. CATHODE RAY STRUCTURE OF THE ATOM • THE CATHODE RAY TRAVELED FROM THE CATHODE TO THE ANODE WHEN CURRENT WAS PASSED THROUGH THE TUBE. • THE CATHODE RAY TUBE OPENED THE DOOR FOR J.J. THOMPSON CATHODE ANODE CATHODE RAY TUBE

27. J. J. THOMPSON I PLAY WITH ELECTRONS

28. J. J. THOMPSON • THOMPSON’S INVESTIGATIONS SUPPLIED EVIDENCE THAT THE CATHODE RAY COULD BE DEFLECTED BY APPLYING A POSITIVELY CHARGED ELECTRIC FIELD • THIS HELPED THOMPSON DISC- OVER THAT THE CATHODE RAY WAS NEGATIVELY CHARGED.

30. THOMPSON WAS ABLE TO MEASURE THE RATIO OF THE CHARGE OF CATHODE RAY PARTICLES TO THEIR MASS • HE FOUND THAT THE RATIO WAS THE SAME REGARDLESS OF THE METAL USED AS THE CATHODE • THOMPSON CONCLUDED THAT ALL CATHODE RAYS ARE COMPOSED OF IDENTICAL NEGATIVELY CHARGED PARTICLES • WHICH WERE LATER CALLED ELECTRONS

31. THOMPSON’S ATOMIC MODEL • HE PROPOSED THAT THE ATOM LOOKS SOMETHING LIKE A CHOCOLATE CHIP COOKIE THE CHIPS WOULD BE HIS ELECTRONS THE COOKIE PART IS POS. MATTER TO CANCEL OUT THE NEG. ELECTRONS

32. THOMPSON’S EXPERIMENTS REVEALED THAT THE ELECTRON HAS A VERY LARGE CHARGE IN RELATION TO ITS MASS • IN 1909, ROBERT MILLIKAN, PERFORMED AN INGENIOUS EXPERIMENT TO CALCULATE THE MASS OF AN ELECTRON • HE DISCOVERED THAT THE MASS OF THE ELECTRON IS ABOUT 1/2000TH THE MASS OF THE SIMPLEST ATOM (HYDRO.)

33. HELLO, MILLIKAN DAT’S RIGHT! I SAVED THE DAY!

34. HIS OIL DROP EXPERIMENT SHOWED THAT EVEN THOUGH THE ATOMS OF DIFFERENT ELEMENTS ARE VERY DIFFERENT, THEIR ELECTRONS ARE IDENTICAL. • AN ELECTRON IS AN ELECTRON • THE DIFFERENCE BETWEEN TWO DIFFERENT ATOMS IS THEIR NUMBER OF ELECTRONS NOT THEIR TYPE OF ELECTRON. • HE ALSO CALCULATED THAT THE ELECTRON’S MASS IS 9.109x10-31kg

35. SO FAR WE’VE LEARNED… • THOMPSON’S & MILLIKAN’S IDEAS: • ATOMS ARE IN FACT DIVISABLE • ELECTRONS ARE PRESENT IN ATOMS OF ALL ELEM. • ONE OF THE ATOM’S FUNDMNTL PARTICLES IS NEG. CHARGED • ATOMS ARE ELECTRICALLY NEUTRAL, SO THERE MUST BE A (+) CHARGE TO BALANCE OUT THE (–) • BECAUSE ELECTRONS ARE BASICALLY MASSLESS THERE MUST BE SOMETHING ELSE THAT ACCOUNTS FOR THE ATOMS MASS.

36. OTHER SIGNIFICANT DISCOVERIES… • IN 1886, WHEN E. GOLDSTEIN OBSERVED A CATHODE-RAY TUBE AND FOUND RAYS TRAVELLING IN THE OPPOSITE DIRECTION OF THAT OF THE CATHODE RAYS • HE CALLED THE RAYS CANAL RAYS AND CONCLUDED THAT THEY WERE COMPOSED OF POS. PARTICLES • IT WAS SOON DISCOVERED THAT THE POSITIVE PARTICLES MASS IS ABOUT 2000 TIMES THAT OF THE ELECTRON

37. OTHER SIGNIFICANT DISCOVERIES… • IN 1932, THE ENGLISH PHYSICIST JAMES CHADWICK CONFIRMED THE EXISTANCE OF YET ANOTHER SUBATOMIC PARTICLE. • THE NEUTRON, WHICH IS A PARTICLE WITH NO CHARGE • HAS A MASS NEARLY EQUAL TO THAT OF THE PROTON • THEREFORE THE SUBATOMIC PARTICLES ARE THE ELECTRON, PROTON, AND NEUTRON.

39. Properties of Subatomic Particles RELATIVE RELATIVE ACTUAL SYMBOL PARTICLE CHARGE MASS (g) MASS

40. THE ATOMIC NUCLEUS • WHEN SUBATOMIC PARTICLES WERE DISCOVERED, SCIENTISTS WONDERED HOW THESE PARTICLES WERE PUT TOGETHER IN AN ATOM. • THIS WAS A DIFFICULT QUESTION TO ANSWER, GIVEN HOW TINY ATOMS ARE. • MOST SCIENTISTS THOUGHT IT LIKELY THAT THE ELECTRONS WERE EVENLY DISTRIBUTED THROUGHOUT AN ATOM FILLED UNIFORMLY WITH POSITIVELY CHARGED MATERIAL.

41. ERNEST RUTHERFORD HOW IS THE ATOM BUILT? IN 1911, RUTHERFORD PERFORMED ONE OF THE MOST BRILLIANT INVESTIGATIONS EVER CONCIEVED.

42. RUTHERFORD’S FAMOUS EXPERIMENT • RUTHERFORD SET OUT TO TEST THE THOMPSON MODEL OF THE ATOM • THE TEST USED RELATIVELY MASSIVE RADIOACTIVE ALPHA PARTICLES • ALPHA PARTICLES () ARE HELIUM ATOMS THAT HAVE LOST THEIR 2 ELECTRONS AND HAVE A DOUBLE POSITIVE CHARGE BECAUSE OF THE 2 REMAINING PROTONS

43. IN THE EXPERIMENT, RUTHERFORD DIRECTED A NARROW BEAM OF ALPHA PARTICLES AT A VERY THIN SHEET OF GOLD FOIL. • ACCORDING TO THE PREVAILING THEORY, THE ALPHA PARTICLES SHOULD HAVE PASSED EASILY THROUGH THE GOLD, WITH ONLY A SLIGHT DEFLECTION DUE TO THE POSITIVE CHARGE THOUGHT TO BE SPREAD OUT IN THE GOLD ATOMS.

47. WHAT HAPPENED? • THE MAJORITY OF THE  PARTICLES PASSED STRAIGHT THROUGH THE GOLD ATOMS, WITHOUT ANY DEFLECTION • EVEN MORE SURPRISINGLY, A SMALL FRACTION OF THE  PARTICLES BOUNCED OFF THE GOLD FOIL ATOMS AT VERY LARGE ANGLES • SOME EVEN BOUNCED BACK AT THE SOURCE

48. BASED ON THE RESULTS, RUTHERFORD SUGGESTED A NEW THEORY OF THE ATOM. • HE PROPOSED THAT THE ATOM IS MOSTLY EMPTY SPACE • –THIS EXPLAINED THE LACK OF DEFLECTION OF SOME OF THE PARTICLES • HE CONCLUDED THAT ALL THE POSITIVE CHARGE AND ALMOST ALL THE MASS IS CONCENTRATED IN A SMALL CORE • –HE CALLED THIS REGION THE NUCLEUS

50. WHAT DO WE KNOW? • THERE ARE THREE FUNDAMENTAL PARTICLES THAT TOGETHER MAKE UP THE ATOM. • COMPOSED OF ELECTRONS, PROTONS, AND NEUTRONS • THE PROTONS AND THE NEUTRONS MAKE UP THE CENTER OF THE ATOM • CENTER OF THE ATOM IS CALLED THE NUCLEUS • THE PROTONS & NEUTRONS OCC-UPY THE MASS OF THE ATOM