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A Dr. Reich Chemistry History Lesson September 2007 The History of the Atom.

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1 A Dr. Reich Chemistry History Lesson September 2007 The History of the Atom

2 Sheep to Shawl Democritus ugh… a while ago John Dalton J.J. Thomson Wilhelm C. Röentgen Henri Becquerel 1852–1908 Marie Curie 1867–1934 Ernest Rutherford Robert Millikan Niels Bohr Erwin Schr Ö dinger Werner Heisenberg James Chadwick 1891–1974

3 Democritus The material cause of all things that exist is the coming together of atoms and void. Atoms are too small to be perceived by the senses. They are eternal and have many different shapes, and they can cluster together to create things that are perceivable. Differences in shape, arrangement, and position of atoms produce different things. By aggregation they provide bulky objects that we can perceive with our sight and other senses. We see changes in things because of the rearrangement of atoms, but atoms themselves are eternal. Words such as ‘nothing’, ‘the void’, and ‘the infinite’ describe space. Individual atoms are describable as ‘not nothing’, ‘being’, and ‘the compact’. There is no void in atoms, so they cannot be divided. I hold the same view as Leucippus regarding atoms and space: atoms are always in motion in space. ( ~460, Greece-371 BCE

4 His Buddy Aristotle Aristotle emphasized that nature consisted of four elements: air, earth, fire, and water. Aristotle

5 The Greek Gods Empedocles associates the Elements with four Gods: Hera (Earth), Persephone (Water), Zeus (Air) and Hades (Fire),

6 Fast Forward to Dalton Sept. 6, 1766, England-July 27, 1844 Dalton's Atomic Theory 1) All matter is made of atoms. Atoms are indivisible and indestructible. 2) All atoms of a given element are identical in mass and properties 3) Compounds are formed by a combination of two or more different kinds of atoms. 4) A chemical reaction is a rearrangement of atoms.

7 Dalton’s Creek, err, element set

8 Dalton’s Laws “I see no sufficient reason why we may not conclude that all elastic fluids under the same pressure expand equally by heat and that for any given expansion of mercury, the corresponding expansion of air is proportionally something less, the higher the temperature” 1801 “I am nearly persuaded that the circumstance depends on the weight and number of the ultimate particles of the several gases." 1805

9 So what changed?

10 Crooks Tube Current moved from the cathode to the anode through a vacuum It moved in lines similar to light But magnets could deflect the current The new path could be used to calculate a mass to charge ratio but couldn’t calculate mass or charge

11 J. J. Thompson Dec. 18, 1856, England - Aug. 30, 1940

12 Wilhelm Conrad Röntgen March 27, 1845, Germany - Feb. 10, 1923 “On the evening of November 8, 1895, he found that, if the discharge tube is enclosed in a sealed, thick black carton to exclude all light, and if he worked in a dark room, a paper plate covered on one side with barium platinocyanide placed in the path of the rays became fluorescent even when it was as far as two metres from the discharge tube. During subsequent experiments he found that objects of different thicknesses interposed in the path of the rays showed variable transparency to them when recorded on a photographic plate. When he immobilised for some moments the hand of his wife in the path of the rays over a photographic plate, he observed after development of the plate an image of his wife's hand which showed the shadows thrown by the bones of her hand and that of a ring she was wearing, surrounded by the penumbra of the flesh, which was more permeable to the rays and therefore threw a fainter shadow. This was the first "röntgenogram" ever taken. In further experiments, Röntgen showed that the new rays are produced by the impact of cathode rays on a material object. Because their nature was then unknown, he gave them the name X-rays. Later, Max von Laue and his pupils showed that they are of the same electromagnetic nature as light, but differ from it only in the higher frequency of their vibration.” (


14 Henri Becquerel Dec. 15, 1952, France – Aug. 25, 1908 Following a discussion with Henri Poincaré on the radiation which had recently been discovered by Röntgen (X-rays) and which was accompanied by a type of phosphorescence in the vacuum tube, Becquerel decided to investigate whether there was any connection between X-rays and naturally occurring phosphorescence. He had inherited from his father a supply of uranium salts, which phosphoresce on exposure to light. When the salts were placed near to a photographic plate covered with opaque paper, the plate was discovered to be fogged. The phenomenon was found to be common to all the uranium salts studied and was concluded to be a property of the uranium atom. Later, Becquerel showed that the rays emitted by uranium, which for a long time were named after their discoverer, caused gases to ionize and that they differed from X-rays in that they could be deflected by electric or magnetic fields. For his discovery of spontaneous radioactivity Becquerel was awarded half of the Nobel Prize for Physics in 1903, the other half being given to Pierre and Marie Curie for their study of the Becquerel radiation. (

15 Check Your Sock Drawer Further investigation, on the 26th and 27th of February, was delayed because the skies over Paris were overcast and the uranium-covered plates Becquerel intended to expose to the sun were returned to a drawer. On the first of March, he developed the photographic plates expecting only faint images to appear. To his surprise, the images were clear and strong. This meant that the uranium emitted radiation without an external source of energy such as the sun. Becquerel had discovered radioactivity, the spontaneous emission of radiation by a material. He himself stated to the French Academy of Sciences "There is an emission of rays without apparent cause. The sun has been excluded" (

16 A Nobel Next to Socks

17 Pause for Dramatic Effect What We Know Up to Now! There are different elements Elements are made up of atoms Atoms have positive and negative components Some elements are radioactive

18 Anyone for a Curie November 7, 1867, France – July 4, 1934 She helped to discover Polonium and Radium. To do this she and her husband took Pitchblend, which contained the radioactive element Uranium by the ton and carefully isolated the radioactive components over several years. What they found was that there were more than one radioactive element in it because they found an element that was far more radioactive than the element uranium. She got a Nobel in Physics for radioactivity. She got a Nobel in chemistry for discovering two new elements.

19 A Wonderful Place to Work With her Husband

20 Raw Materials PitchblendeRadium There is 1 gram of radium in 7 tons of pitchblende “It therefore appeared probable that if pitchblende, chalcolite, and autunite possess so great a degree of activity, these substances contain a small quantity of a strongly radioactive body, differing from uranium and thorium and the simple bodies actually known. I thought that if this were indeed the case, I might hope to extract this substance from the ore by the ordinary methods of chemical analysis.” [Curie 1961 (1903), p. 16].

21 How Did She Do It?

22 In her own words If we assume that radium contains a supply of energy which it gives out little by little, we are led to believe that this body does not remain unchanged, as it appears to, but that it undergoes an extremely slow change. Several reasons speak in favor of this view. First, the emission of heat, which makes it seem probable that a chemical reaction is taking place in the radium. But this is no ordinary chemical reaction, affecting the combination of atoms in the molecule. No chemical reaction can explain the emission of heat due to radium. Furthermore, radioactivity is a property of the atom of radium; if, then, it is due to a transformation, this transformation must take place in the atom itself. Consequently, from this point of view, the atom of radium would be in a process of evolution, and we should be forced to abandon the theory of the invariability of atoms, which is the foundation of modern chemistry [M. Curie 1904].

23 Ernest Rutherford Aug. 30, 1871 New Zealand – Oct. 19, 1937

24 In Simpler terms…

25 So what just happened? It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper, and it came back to hit you "On consideration, I realized that this scattering backwards must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center carrying a charge."

26 WTH?

27 So what’s an atom look like? The Planetary Model

28 So is there anything cool I can do with this? "We must conclude that the nitrogen atom is disintegrated under the intense forces developed in a close collision with a swift alpha particle, and that the hydrogen atom which is liberated formed a constituent part of the nitrogen nucleus." -Ernest Rutherford

29 So how’d he do it? "In the last year of war, in April Rutherford sent off a paper that, had he done nothing else, would earn him a place in history... As was usual with Rutherford's experiments, the apparatus was simple to the point of being crude: a small glass tube inside a sealed brass box fitted at one end with a zinc-sulphide scintillation screen. The brass box was filled with nitrogen and then through the glass tube was passed a source of alpha particles- helium nuclei- given off by radon, the radioactive gas of radium. The excitement came when Rutherford inspected the activity on the zinc-sulphide screen: the scintillations were indistinguishable from those obtained from hydrogen. How could that be, since there was no hydrogen in the system? This led to the famously downbeat sentence in the fourth part of Rutherford's paper: 'From the results so far obtained it is difficult to avoid the conclusion that the long-range atoms arising from collision of alpha particles with nitrogen are not nitrogen atoms but probably atoms of hydrogen... If this be the case, we must conclude that the nitrogen atom is disintegrated.' The newspapers were not so cautious. Sir Ernest Rutherford, they shouted, had split the atom. He himself realized the importance of his work. His experiments had drawn him away, temporarily, from antisubmarine research. He defended himself to the overseers' committee: 'If, as I have reason to believe, I have disintegrated the nucleus of the atom, this is of greater significance than the war.' In a sense, Rutherford had finally achieved what the old alchemists had been aiming for, transmuting one element into another." (Watson, The Modern Mind, ). Rutherford's experiment convinced him that the nitrogen nucleus was composed of hydrogen nuclei; the hydrogen nucleus, therefore, must be an elementary particle. Rutherford named it the ' proton,' from the Greek 'protos,' meaning 'first.' Printing and the Mind of Man, 411. Particle Physics... Chronological Bibliography: "Discovery of the Proton

30 Robert A. Millikan March 22, 1868, U.S.A. – Dec Established the charge of an electron and Determined the atomic structure of electricity

31 So What’d He Do?

32 Well, what’s that thingy?

33 So, what’d it look like inside?

34 So, what changed?

35 A Nagging Concern Rutherford’s Model Positive Protons and Oppositely Charged Electrons What should happen? Collapse!

36 Niels Bohr Oct. 7, 1885, Denmark – Nov. 18, 1962

37 Spectrum of Hydrogen

38 A colorful spectrum of hydrogen

39 Bohr’s Idea What if the path of the electrons was fixed…

40 What energies would exist? What would that mean we would see?

41 A story of what would happen

42 A colorful spectrum of hydrogen

43 This is what he thought it would look like!

44 This model describes small atoms But……… When they looked at bigger atoms then hydrogen the model didn’t predict the light patterns accurately. Something was wrong with the model.

45 Erwin Schrodinger and Werner Heisenberg

46 Let’s talk about light, Baby!

47 What if it’s a particle?

48 1 Photon leads to 1 electron

49 But light moves in waves

50 KinderUni2005/waves.gif

51 Here’s the experiment


53 Heisenberg Uncertainty This uncertainty leads to some strange effects. For example, in a Quantum Mechanical world, I cannot predict where a particle will be with 100 % certainty. I can only speak in terms of probabilities. For example, I can only say that an atom will be at some location with a 99 % probability, and that there will be a 1 % probability it will be somewhere else

54 Probability Density An abstract function, called wave function or probability amplitude (formula sign Y), describes the states of a particle or a physical system. Y is dependent on position and time. The wave function itself does not have any explicit meaning, but its square (more precisely the square of its absolute value) describes the probability of measuring the various possible positions of a particle. In addition, it provides information about the probability distribution of all other physical quantities (for example impulse and energy). Y satisfies the Schrödinger Equation, whose solution describes the behavior over time of a physical system. Schrödinger’s original formulation for particles with a given total energy E is as follows:

55 A compilation of individual electrons In 1927 Heisenberg formulated an idea, which agreed with tests, that no experiment can measure the position and momentum of a quantum particle simultaneously. Scientists call this the "Heisenberg uncertainty principle." This implies that as one measures the certainty of the position of a particle, the uncertainty in the momentum gets correspondingly larger. Or, with an accurate momentum measurement, the knowledge about the particle's position gets correspondingly less. The visual concept of the atom now appeared as an electron "cloud" which surrounds a nucleus. The cloud consists of a probability distribution map which determines the most probable location of an electron. For example, if one could take a snap-shot of the location of the electron at different times and then superimpose all of the shots into one photo, then it might look something like the view at the top.

56 This is what the probability density looks like for the first orbits described

57 James Chadwick Oct. 20, 1891, England – July 24, 1974

58 Chadwick’s Setup In 1930, the German physicists Walther Bothe and Herbert Becker noticed something odd. When they shot alpha rays at beryllium (atomic number 4) the beryllium emitted a neutral radiation that could penetrate 200 millimeters of lead. In contrast, it takes less than one millimeter of lead to stop a proton. Bothe and Becker assumed the neutral radiation was high-energy gamma rays. Marie Curie's daughter, Irene Joliot-Curie, and Irene's husband, Frederic, put a block of paraffin wax in front of the beryllium rays. They observed high-speed protons coming from the paraffin. They knew that gamma rays could eject electrons from metals. They thought the same thing was happening to the protons in the paraffin. Chadwick said the radiation could not be gamma rays. To eject protons at such a high velocity, the rays must have an energy of 50 million electron volts. The alpha particles colliding with beryllium nuclei could produce only 14 million electron volts. Chadwick had another explanation for the beryllium rays. He thought they were neutrons. He set up an experiment to test his hypothesis.

59 Chadwick’s Experiment Chadwick collided the radiation emerging from the (Po-Be) source not only with proton (paraffin), but also with helium and nitrogen. Comparing the results of these experiments with each other, Chadwick concluded that this mysterious radiation from the (Po-Be) source cannot be interpreted by assuming it to be a gamma ray. He finally concluded that all were able to be understood without any contradiction by assuming that the mysterious radiation is electrically neutral particles with almost the same mass as a proton. This is the confirmation of the existence of the "neutral proton" predicted by Rutherford. Chadwick named this particle "neutron" (1932).

60 In his own words The properties of the penetrating radiation emitted from beryllium (and boron) when bombarded by the a-particles of polonium, have been examined. It is concluded that the radiation consists, not of quanta as hitherto supposed, but of neutrons, particles of mass 1, and charge 0. Evidence is given to show that the mass of the neutron is probably between 1·005 and 1·008.

61 Thanks for watching! Now it’s your turn! Get to work!

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