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PIONEERS OF THE ATOM DEMOCRITUS LAVOISIER DALTON THOMPSON MILLIKEN RUTHERFORD BOHR THE EARLY ATOM It’s all Greek to me …… DEMOCRITUS IS THE MAN ! DEMOCRITUS WAS A GREEK PHILOSOPHER THAT BEGAN WONDERING WHAT WAS THE UNIVERSE MADE OF. HE PROPOSED THAT ALL THE MATTER IN THE UNIVERSE IS COMPOSED OF ‘TINY PARTICLES’. THESE PARTICLES ARE ‘INDIVISIBLE’. HE CALLED THESE PARTICLES ‘ATOMOS’ (MEANING ‘INDIVISIBLE’). BUT HE HAD AN OPPONENT…… ARISTOTLE = SKEPTIC Democritus is an ignoramus! ARISTOTLE WAS MUCH MORE POPULAR AND HE OPPOSED THE IDEA OF ‘ATOMS’ ARISTOTLE = SKEPTIC Democritus is an ignoramus! ARISTOTLE BELIEVED THAT MATTER WAS ‘CONTINUOUS’. ARISTOTLE’S FOUR BASIC ELEMENTS EARTH, AIR, FIRE and WATER BECAUSE OF ARISTOTLE’S GREAT INFLUENCE…… DEMOCRITUS’ ATOMS ...’ATOMS’ DIED FOR ABOUT 1,400 YEARS UNTIL... IN THE 1700’S… 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 PHYSICAL AND CHEMICAL PROPERTIES THAN THOSE OF THE ELEMENTS THAT FORM THEM. 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 MASS BALANCES. ANTOINE LAVOISIER Matter can be neither created nor destroyed! HE BEGAN TO GATHER EVIDENCE ABOUT HOW MATTER BEHAVED BEFORE AND AFTER A REACTION. 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’. JOHN DALTON INVESTIGATED TWO OTHER FUNDAMENTAL IDEAS The Law of Definite Proportion The Law of Multiple Proportions LAW OF DEFINITE PROPORTIONS LAW OF MULTIPLE PROPORTIONS A CHEMICAL COMPOUND IS ALWAYS COMPOSED OF THE SAME ELEMENTS IN THE SAME PROPORTION BY MASS. CARBON MONOXIDE + C = O CO IF TWO DIFFERENT COMPOUNDS ARE COMPOSED OF THE SAME TWO ELEMENTS, THEN THE RATIO OF OF THE ELEMENTS IS ALWAYS A RATIO OF SMALL WHOLE NUMBERS. CARBON MONOXIDE CARBON DIOXIDE + C O + + C = O CO = O CO2 IN 1808, JOHN DALTON PROPOSED AN EXPLANATION FOR THE LAW OF THE CONSERVATION OF MASS AND ‘DEFINITE PROPORTIONS. HE ALSO FORMULATED THE LAW OF ‘MULTIPLE PROPORTIONS’. HE REASONED THAT ELEMENTS WERE COMPOSED OF ‘ATOMS’ AND THAT ONLY WHOLE NUMBERS OF ATOMS CAN COMBINE TO FORM COMPOUNDS HIS IDEAS ARE NOW CALLED THE ‘ATOMIC THEORY OF MATTER’ AND CAN BE SUMMARIZED AS: 1.ALL MATTER IS COMPOSED OF EXTREMELY SMALL PARTICLES CALLED ATOMS 2.ATOMS OF A GIVEN ELEMENT ARE IDENTICAL IN SIZE, MASS, AND OTHER PROPERTIES. ATOMS OF DIFFERENT ELEMENTS DIFFER IN SIZE, MASS, & OTHER PROPERTIES ELEMENT 2 ELEMENT 3 ELEMENT 4 3.ATOMS CANNOT BE SUBDIVIDED, CREATED, OR DESTROYED. 4.ATOMS OF DIFFERENT ELEMENTS COMBINE IN SIMPLE WHOLE NUMBER RATIOS TO FORM CHEMICAL COMPOUNDS. 5.IN CHEMICAL REACTIONS, ATOMS ARE COMBINED, SEPARATED, OR REARRANGED. + + DALTON AND HIS CONTEMPORARIES THE EARLIEST THEORISTS THOUGHT THE ATOM WAS HARD AND ROUND, MUCH LIKE TINY MARBLES OR BALL BEARINGS. 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 SUBATOMIC PARTICLES THAT DETERMINES THE IDENTITY 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 PRESSURE. CARRIED OUT IN TUBES CALLED CATHODE-RAY TUBES J.J. THOMPSON Around 1900 THOMPSON NOTICED THAT WHEN CURRENT WAS PASSED THROUGH A CATHODE RAY TUBE, THE SURFACE OF THE TUBE DIRECTLY OPPOSITE THE CATHODE GLOWED. THOMPSON THEORIZED THAT THE GLOW WAS CAUSED BY A STREAM OF ‘PARTICLES’. HE CALLED THIS STREAM A ‘CATHODE RAY’. 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 THOMPSON’S INVESTIGATIONS GAVE EVIDENCE THAT THE CATHODE RAY COULD BE DEFLECTED BY APPLYING A POSITIVELY CHARGED ELECTRIC FIELD. THIS HELPED THOMPSON DISCOVER THAT THE CATHODE RAY WAS ‘NEGATIVELY CHARGED’. + 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. HE PROPOSED THAT THE ATOM LOOKS SOMETHING LIKE A CHOCOLATE CHIP COOKIE. THE CHIPS WOULD BE HIS ‘ELECTRONS’. THE COOKIE PART IS POSITIVE MATTER, TO CANCEL OUT THE NEGATIVE ELECTRONS. THOMPSON WAS ‘ENGLISH’……… HE CALLED HIS MODEL THE ‘PLUM PUDDING ATOM’ IN 1909, ROBERT MILLIKAN, PERFORMED AN 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……HYDROGEN. 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 THE TYPE OF ELECTRON. HE ALSO CALCULATED THAT THE ELECTRON’S MASS IS 9.109x10-31kg THOMPSON’S & MILLIKAN’S IDEAS ATOMS ARE IN FACT DIVISIBLE. ELECTRONS ARE PRESENT IN ATOMS OF ALL ELEMENTS. ONE OF THE ATOM’S FUNDAMENTAL PARTICLES IS NEGATIVE 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. BUT, NO ONE KNEW WHAT THE INSIDE OF AN ‘ATOM’ WAS REALLY LIKE UNTIL…… ERNEST RUTHERFORD What’s inside an atom? IN 1911, RUTHERFORD PERFORMED ONE OF THE MOST BRILLIANT EXPERIMENTS EVER. IN THE EXPERIMENT, RUTHERFORD SHOT A NARROW BEAM OF ALPHA PARTICLES AT A VERY THIN SHEET OF GOLD FOIL. IF ATOMS WERE LIKE ‘PLUM PUDDING’, THE ALPHA PARTICLES SHOULD HAVE PASSED EASILY THROUGH THE GOLD, WITH ONLY A SLIGHT DEFLECTION. BUT, THAT IS NOT WHAT HAPPENED…… WHAT HAPPENED? THE MAJORITY OF THE PARTICLES PASSED ‘STRAIGHT THROUGH’ THE GOLD ATOMS. MORE SURPRISINGLY, A SMALL NUMBER OF THE PARTICLES ‘BOUNCED OFF’ THE GOLD FOIL AT VERY LARGE ANGLES. AND SOME EVEN BOUNCED BACK DIRECTLY AT THE SOURCE! BASED ON THE RESULTS, RUTHERFORD SUGGESTED A NEW THEORY OF THE ATOM. HE PROPOSED THAT THE ATOM IS MOSTLY EMPTY SPACE. HE CONCLUDED THAT ALL THE POSITIVE CHARGE AND ALMOST ALL THE MASS IS CONCENTRATED IN A SMALL CORE CALLED THE ‘NUCLEUS’. IN 1932, THE ENGLISH PHYSICIST JAMES CHADWICK CONFIRMED THE EXISTANCE OF YET ANOTHER SUBATOMIC PARTICLE THE NEUTRON. A PARTICLE WITH NO CHARGE. IT HAS A MASS NEARLY EQUAL TO THAT OF THE PROTON. THEREFORE, THE SUBATOMIC PARTICLES ARE THE ELECTRON, PROTON, AND NEUTRON. electron e -1 0 9.11x10-28 proton + p +1 1 1.67x10-24 neutron n0 0 1 1.67x10-24 A LITTLE REVIEW THERE ARE THREE FUNDAMENTAL PARTICLES THAT MAKE UP THE ATOM. THEY ARE ELECTRONS, PROTONS, AND NEUTRONS. THE PROTONS AND THE NEUTRONS MAKE UP THE CENTER OF THE ATOM, WHICH IS CALLED THE NUCLEUS. THE PROTONS & NEUTRONS MAKE UP THE MASS OF THE ATOM. 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 IT’S THE NUMBERS OF PROTONS IN AN ATOM THAT MAKES ATOMS DIFFERENT. AN OXYGEN ATOM HAS 8 PROTONS IN IT’S NUCLEUS. A HYDROGEN ATOM HAS 1 PROTON IN IT’S NUCLEUS. THE ‘ATOMIC NUMBER OF’ AN ELEMENT IS THE NUMBER OF PROTONS IN THE NUCLEUS OF AN ATOM OF THAT ELEMENT. SINCE OXYGEN’S NUCLEUS HAS 8 PROTONS IT’S ATOMIC NUMBER IS 8. THE PROTONS IDENTIFY THE TYPE OF ATOM, THE ONLY ATOM WITH 8 PROTONS IS OXYGEN. ATOMS OF THE FIRST TEN ELEMENTS Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon H He Li Be B C N O F Ne 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 0 2 4 5 6 6 7 8 10 10 1 4 7 9 11 12 14 16 19 20 1 2 3 4 5 6 7 8 9 10 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 ELECTRON. AN OXYGEN ATOM HAS 8 ELECTRONS. THE MASS OF AN ATOM IS CONCENTRATED IN IT’S NUCLEUS AND DEPENDS ON THE NUMBER OF PROTONS AND NEUTRONS. 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 THE ATOM’S COMPOSITION. THE COMPOSITION OF ANY ATOM CAN BE REPRESENTED IN SHORTHAND NOTATION: CHEMICAL COMPOSITION SHORTHAND # OF PROTONS + # OF NEUTRONS Cl MASS NUMBER 35 ATOMIC NUMBER 17 NUMBER OF PROTONS 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 HAVE DIFFERENT MASS NUMBERS. 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 SODIUM ISOTOPES EXAMPLE OF AN ISOTOPE ATOMIC MASS 35 17 Cl 18 NEUTRONS 37 17 Cl 20 NEUTRONS ATOMIC NUMBER 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 A NEGATIVE ELECTRICAL 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. ‘LOST’ AN ELECTRON = + CHARGE ‘GAINED’ AN ELECTRON = - CHARGE SOME ATOMS GAIN ELECTRONS ATOM’S IONIC CHARGE = NUMBER OF PROTONS – NUMBER OF ELECTRONS - - - - - - - O - - - - - - O-2 - ATOMS, IONS, AND ISOTOPES ATOMS NEUTRAL AND ARE DEFINED BY THE # OF PROTONS IN THEIR NUCLEUS 3 p+ = Li ATOM, ETC. IONS HAVE AN ELECTRICAL CHARGE DETERMINED BY # PROTONS - # ELECTRONS N-2 = 7 p+ - 9 e- ; ETC. ISOTOPES TWO ATOMS WITH THE SAME # OF PROTONS, BUT DIFFERENT #’S OF NEUTRONS OR MASSES CALCIUM-40 & CALCIUM-44 NIELS BOHR INVESTGATED THE ‘ARRANGEMENT’ OF ELECTRONS How are electrons arranged inside an atom? BOHR FOUND THAT ELECTRONS TRAVEL IN DISCRETE ‘ORBITS’ ABOUT THE NUCLEUS. THE CHEMICAL PROPERTIES OF AN ATOM ARE DETERMINED BY THE NUMBER OF ELECTRONS IN AN ATOM’S OUTER ORBIT. ELECTRON SHELLS AND ATOMIC STRUCTURE THE FIRST SHELL WILL HOLD UP TO 2 ELECTRONS. THE SECOND AND THIRD SHELLS WILL HOLD UP TO 8 ELECTRONS. SOMETIMES SHELLS ARE MADE OF SUB-SHELLS. ATOMS WITH ELECTRONS OUTSIDE OF FILLED SHELLS OR WITH MISSING ELECTRONS ARE VERY CHEMICALLY REACTIVE. ATOMS WITH ‘FILLED’ SHELLS ARE STABLE, OR NON-REACTIVE. THESE ARE CALLED ‘INERT GASES’