THE STRUCTURE OF THE ATOM

1. THE STRUCTURE OF THE ATOM ATOMIC MODELS

2. 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.

3. 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

4. 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.

5. 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

6. 2 OTHER FUNDAMENTAL IDEAS WERE INVESTIGATED... THE LAW OF DEFINITE PROPORTION

7. 2 OTHER FUNDAMENTAL IDEAS WERE INVESTIGATED... THE LAW OF MULTIPLE PROPORTIONS

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

9. 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 IDEAS ARE NOW CALLED THE ATOMIC THEORY OF MATTER AND CAN BE SUMMARIZED AS:

10. ELEMENT 1 ELEMENT 2 ELEMENT 3 ELEMENT 4

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

13. 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.

14. 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

15. 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

16. 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

17. 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

18. J. J. THOMPSON I PLAY WITH ELECTRONS

19. 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.

21. 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 OFIDENTICAL NEGATIVELY CHARGED PARTICLES • WHICH WERE LATER CALLED ELECTRONS

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

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

24. HIS OIL DROP EXPERIMENT SHOWED THAT EVEN THOUGH THE ATOMS OF DIFFERENT ELEMENTS ARE VERY DIFFERENT, THEIR ELECTRONS AREIDENTICAL. • 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

25. 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.

26. OTHER SIGNIFICANT DISCOVERIES… • IT WAS SOON DISCOVERED THAT THE POSITIVE PARTICLES MASS IS ABOUT2000 TIMESTHAT OF THE ELECTRON

27. 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 THEELECTRON, PROTON, AND NEUTRON.

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

29. 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.

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

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

32. 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.

36. 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

37. BASED ON THE RESULTS, RUTHERFORD SUGGESTED A NEW THEORY OF THE ATOM. • HE PROPOSED THAT THE ATOM ISMOSTLY 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

38. A LITTLE REVIEW! • 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

39. THE ELECTRONS SURROUND THE NUCLEUS AND OCCUPY MOST OF THE ATOM’S VOLUME • HOW, THEN, ARE ATOMS OF HYDROGEN DIFFERENT FROM THOSE OF OXYGEN? • A SUBATOMIC PARTICLES FROM ONE TYPE OF ATOM LOOKS LIKE PARTICLES FROM OTHER ATOMS proton from H electron for H proton from C electron for C proton from O electron for O

40. IT’S THE NUMBERS OF PROTONS IN AN ATOM THAT MAKES ATOMS DIFFERENT. • AN OXYGEN ATOM HAS 8 PROTONS IN ITS NUCLEUS • A HYDROGEN ATOM HAS 1 PROTON IN ITS NUCLEUS • THEATOMIC NUMBEROF AN ELEMENT IS THE NUMBER OF PROTONS IN THE NUCLEUS OF AN ATOM OF THAT ELEMENT • SINCE OXYGEN’S NUCLEUS HAS 8 PROTONS ITS ATOMIC # IS 8 • THE PROTONS IDENTIFY THE TYPEOF ATOM, THE ONLY ATOM WITH 8 PROTONS IS OXYGEN

42. NAME SYMBOL ATOMIC # PROTONS NEUTRONS MASS # ELECTRONS

43. FOR EACH ELEMENT LISTED, THE NUMBER OF PROTONS EQUALS THE NUMBER OF ELECTRONS • REMEMBER ATOMS ARE ELECTRICALLY NEUTRAL • IN AN ATOM, THE NUMBER OF ELECTRONS MUST EQUAL THE NUMBER OF PROTONS • A HYDROGEN ATOM HAS 1 ELECTRN • AN OXYGEN ATOM HAS 8 ELECTRNS • THE MASS OF AN ATOM IS CONCEN. IN ITS NUCLEUS AND DEPENDS ON THE NUMER OF PROTONS AND NEUTRONS

44. THE TOTAL NUMBER OF PROTONS AND NEUTRONS IN AN ATOM IS CALLED THE MASS NUMBER • A CARBON ATOM, WHICH HAS 6 PROTONS AND 6 NEUTRONS, HAS A MASS NUMBER OF 12 • IF YOU KNOW THE ATOMIC NUMBER AND MASS NUMBER OF AN ATOM OF ANY ELEMENT, YOU CAN DETERMINE THEATOM’S COMPOSITION • THE COMPOSITION OF ANY ATOM CAN BE REPRESENTED IN SHORTHAND NOTATION:

45. CHEMICAL COMPOSITION SHORTHAND # OF PROTONS + # OF NEUTRONS MASS NUMBER Cl 35 ATOMIC NUMBER 17 NUMBER OF PROTONS

46. ISOTOPES • EVERY CHLORINE ATOM HAS 17 PROTONS, WITHOUT EXCEPTION, • HOWEVER, NOT EVERY CHLORINE ATOM HAS 18 NEUTRONS. • ATOMS WITH THE SAME NUMBER OF PROTONS BUT CONTAIN DIFFERENT NUMBERS OF NEUTRONS ARE CALLED ISOTOPES. • BECAUSE ISOTOPES OF AN ELEMENT HAVE DIFFERENT NUMBERS OF NEUTRONS THEY HAVEDIFFERENT MASS NUMBERS.

47. ISOTOPES ARE CHEMICALLY ALIKE BECAUSE THEY HAVE IDENTICAL NUMBERS OF PROTONS AND ELECTRONS • IT’S THE ELECTRONS AND PROTONS THAT ARE RESPONSIBLE FOR CHEMICAL BEHAVIOR BERYLLIUM ISOTOPES proton neutron electron

49. ATOMIC MASS Cl Cl 35 37 17 17 20 18 NEUTRONS NEUTRONS ATOMIC NUMBER EXAMPLE OF AN ISOTOPE

50. IONS • AN ELEMENT’S ATOMS ARE NOT ALWAYS NEUTRAL IN CHARGE. • WHEN AN ATOM LOSES OR GAINS ONE OR MORE OF ITS ELECTRONS IT BECOMES ION. • AN ION THAT HAS MORE ELECTRONS THAN PROTONS HAS ANEGATIVEELECTRICAL CHARGE • AN ION THAT HAS FEWER ELECTRONS THAN PROTONS HAS A POSITIVE ELECTRICAL CHARGE NOTE: IT’S THE PROTONS THAT DEFINE THE TYPE OF ATOM IT IS, BUT THE ELECTRONS DEFINE THE ATOM’S CHARGE.