what was robert millikans contribution to the atomic theory?

Learning Outcomes

• Outline milestones in the evolution of modernistic atomic theory
• Summarize and interpret the results of the experiments of Thomson, Millikan, and Rutherford
• Depict the 3 subatomic particles that etch atoms
• Define isotopes and give examples for several elements

In the two centuries since Dalton developed his ideas, scientists accept made meaning progress in furthering our understanding of atomic theory. Much of this came from the results of several seminal experiments that revealed the details of the internal construction of atoms. Hither, nosotros will discuss some of those key developments, with an emphasis on awarding of the scientific method, as well as understanding how the experimental show was analyzed. While the historical persons and dates backside these experiments tin exist quite interesting, information technology is nigh important to sympathise the concepts resulting from their work.

Atomic Theory after the Nineteenth Century

If affair were composed of atoms, what were atoms composed of? Were they the smallest particles, or was there something smaller? In the late 1800s, a number of scientists interested in questions like these investigated the electric discharges that could be produced in low-pressure gases, with the about significant discovery made past English language physicist J. J. Thomson using a cathode ray tube. This apparatus consisted of a sealed glass tube from which nearly all the air had been removed; the tube contained 2 metal electrodes. When loftier voltage was applied across the electrodes, a visible beam chosen a cathode ray appeared betwixt them. This beam was deflected toward the positive charge and away from the negative accuse, and was produced in the same way with identical properties when different metals were used for the electrodes. In like experiments, the ray was simultaneously deflected by an applied magnetic field, and measurements of the extent of deflection and the magnetic field strength immune Thomson to calculate the accuse-to-mass ratio of the cathode ray particles. The results of these measurements indicated that these particles were much lighter than atoms (Effigy i).

Figure 1. (a) J. J. Thomson produced a visible beam in a cathode ray tube. (b) This is an early cathode ray tube, invented in 1897 past Ferdinand Braun. (c) In the cathode ray, the axle (shown in yellow) comes from the cathode and is accelerated past the anode toward a fluorescent scale at the end of the tube. Simultaneous deflections by practical electric and magnetic fields permitted Thomson to calculate the mass-to-charge ratio of the particles composing the cathode ray. (credit a: modification of piece of work past Nobel Foundation; credit b: modification of piece of work by Eugen Nesper; credit c: modification of work by "Kurzon"/Wikimedia Commons)

Based on his observations, hither is what Thomson proposed and why: The particles are attracted by positive (+) charges and repelled past negative (-) charges, so they must be negatively charged (similar charges repel and unlike charges attract); they are less massive than atoms and indistinguishable, regardless of the source material, so they must be fundamental, subatomic constituents of all atoms. Although controversial at the time, Thomson's idea was gradually accepted, and his cathode ray particle is what we now call an electron, a negatively charged, subatomic particle with a mass more than than one thousand-times less that of an atom. The term "electron" was coined in 1891 by Irish physicist George Stoney, from "electric ion."

Click this link to "JJ Thompson Talks About the Size of the Electron" to hear Thomson describe his discovery in his own voice.

In 1909, more information about the electron was uncovered by American physicist Robert A. Millikan via his "oil drop" experiments. Millikan created microscopic oil droplets, which could be electrically charged past friction as they formed or by using X-rays. These aerosol initially cruel due to gravity, but their down progress could be slowed or fifty-fifty reversed by an electric field lower in the apparatus. Past adjusting the electric field forcefulness and making conscientious measurements and advisable calculations, Millikan was able to decide the charge on individual drops (Figure 2).

Figure 2. Millikan'southward experiment measured the charge of individual oil drops. The tabulated data are examples of a few possible values.

Looking at the charge information that Millikan gathered, you may have recognized that the charge of an oil droplet is ever a multiple of a specific charge, 1.6 × ten^-xix C. Millikan concluded that this value must therefore be a primal charge—the charge of a unmarried electron—with his measured charges due to an excess of one electron (1 times 1.6 × 10^-xix C), two electrons (2 times 1.6 × 10^-19 C), iii electrons (3 times 1.6 × 10^-19 C), and so on, on a given oil droplet. Since the charge of an electron was now known due to Millikan's enquiry, and the charge-to-mass ratio was already known due to Thomson's research (ane.759 × 10¹¹ C/kg), information technology only required a uncomplicated calculation to determine the mass of the electron as well.

[latex]\text{Mass of electron}=1.602\times {10}^{-nineteen}\text{C}\times\frac{i\text{kg}}{one.759\times {x}^{11}\text{C}}=nine.107\times {x}^{-31}\text{kg}[/latex]

Scientists had now established that the atom was not indivisible as Dalton had believed, and due to the piece of work of Thomson, Millikan, and others, the accuse and mass of the negative, subatomic particles—the electrons—were known. However, the positively charged part of an atom was not yet well understood. In 1904, Thomson proposed the "plum pudding" model of atoms, which described a positively charged mass with an equal amount of negative accuse in the form of electrons embedded in it, since all atoms are electrically neutral. A competing model had been proposed in 1903 past Hantaro Nagaoka, who postulated a Saturn-like atom, consisting of a positively charged sphere surrounded past a halo of electrons (Figure iii).

Figure 3. (a) Thomson suggested that atoms resembled plum pudding, an English dessert consisting of moist cake with embedded raisins ("plums"). (b) Nagaoka proposed that atoms resembled the planet Saturn, with a ring of electrons surrounding a positive "planet." (credit a: modification of piece of work by "Man vyi"/Wikimedia Commons; credit b: modification of piece of work past "NASA"/Wikimedia Commons)

The adjacent major development in understanding the atom came from Ernest Rutherford, a physicist from New Zealand who largely spent his scientific career in Canada and England. He performed a series of experiments using a beam of high-speed, positively charged blastoff particles (α particles) that were produced by the radioactive decay of radium; α particles consist of two protons and two neutrons (y'all will learn more about radioactive decay in the module on nuclear chemistry). Rutherford and his colleagues Hans Geiger (subsequently famous for the Geiger counter) and Ernest Marsden aimed a beam of α particles, the source of which was embedded in a lead cake to absorb virtually of the radiations, at a very thin piece of golden foil and examined the resultant scattering of the α particles using a luminescent screen that glowed briefly where hitting by an α particle.

What did they notice? Most particles passed right through the foil without being deflected at all. However, some were diverted slightly, and a very pocket-size number were deflected almost straight back toward the source (Figure 4). Rutherford described finding these results: "It was quite the most incredible event that has ever happened to me in my life. Information technology was near as incredible as if you fired a 15-inch shell at a slice of tissue paper and it came back and hit you lot."

Figure 4. Geiger and Rutherford fired α particles at a slice of gold foil and detected where those particles went, every bit shown in this schematic diagram of their experiment. Most of the particles passed direct through the foil, just a few were deflected slightly and a very small number were significantly deflected.

Hither is what Rutherford deduced: Because nigh of the fast-moving α particles passed through the gold atoms undeflected, they must accept traveled through essentially empty space inside the cantlet. Alpha particles are positively charged, so deflections arose when they encountered another positive charge (like charges repel each other). Since like charges repel one another, the few positively charged α particles that changed paths abruptly must have hitting, or closely approached, another body that also had a highly concentrated, positive charge. Since the deflections occurred a minor fraction of the time, this charge only occupied a small amount of the space in the gilded foil. Analyzing a serial of such experiments in detail, Rutherford drew two conclusions:

1. The volume occupied past an atom must consist of a large amount of empty infinite.
2. A small, relatively heavy, positively charged torso, the nucleus, must be at the center of each atom.

View this simulation of the Rutherford gilt foil experiment. Arrange the slit width to produce a narrower or broader axle of α particles to meet how that affects the handful pattern.

This assay led Rutherford to propose a model in which an atom consists of a very pocket-size, positively charged nucleus, in which nearly of the mass of the atom is concentrated, surrounded past the negatively charged electrons, then that the cantlet is electrically neutral (Figure five). Later many more experiments, Rutherford also discovered that the nuclei of other elements contain the hydrogen nucleus as a "edifice block," and he named this more than fundamental particle the proton, the positively charged, subatomic particle found in the nucleus. With ane improver, which you will acquire next, this nuclear model of the atom, proposed over a century agone, is all the same used today.

Effigy 5. The α particles are deflected just when they collide with or pass close to the much heavier, positively charged aureate nucleus. Considering the nucleus is very small compared to the size of an atom, very few α particles are deflected. Almost pass through the relatively large region occupied past electrons, which are besides light to deflect the rapidly moving particles.

The Rutherford Scattering simulation allows you lot to investigate the differences between a "plum pudding" atom and a Rutherford atom by firing α particles at each blazon of cantlet.

Another important finding was the discovery of isotopes. During the early 1900s, scientists identified several substances that appeared to be new elements, isolating them from radioactive ores. For example, a "new element" produced by the radioactive decay of thorium was initially given the proper noun mesothorium. However, a more detailed assay showed that mesothorium was chemically identical to radium (another decay product), despite having a different atomic mass. This event, along with like findings for other elements, led the English chemist Frederick Soddy to realize that an element could take types of atoms with unlike masses that were chemically indistinguishable. These unlike types are chosen isotopes—atoms of the aforementioned chemical element that differ in mass. Soddy was awarded the Nobel Prize in Chemical science in 1921 for this discovery.

I puzzle remained: The nucleus was known to contain well-nigh all of the mass of an cantlet, with the number of protons only providing one-half, or less, of that mass. Different proposals were made to explain what constituted the remaining mass, including the existence of neutral particles in the nucleus. As y'all might expect, detecting uncharged particles is very challenging, and it was not until 1932 that James Chadwick found evidence of neutrons, uncharged, subatomic particles with a mass approximately the same equally that of protons. The existence of the neutron too explained isotopes: They differ in mass because they take unlike numbers of neutrons, but they are chemically identical because they have the same number of protons. This volition exist explained in more detail later.

Central Concepts and Summary

Although no one has actually seen the inside of an cantlet, experiments have demonstrated much about diminutive structure. Thomson's cathode ray tube showed that atoms contain pocket-size, negatively charged particles called electrons. Millikan discovered that there is a fundamental electric charge—the charge of an electron. Rutherford'due south gilt foil experiment showed that atoms have a small, dense, positively charged nucleus; the positively charged particles within the nucleus are called protons. Chadwick discovered that the nucleus also contains neutral particles called neutrons. Soddy demonstrated that atoms of the same element can differ in mass; these are called isotopes.

Try It

1. The being of isotopes violates ane of the original ideas of Dalton's atomic theory. Which one?
2. How are electrons and protons similar? How are they unlike?
3. How are protons and neutrons similar? How are they dissimilar?
4. Predict and test the behavior of α particles fired at a "plum pudding" model cantlet.
   1. Predict the paths taken by α particles that are fired at atoms with a Thomson's plum pudding model structure. Explain why you expect the α particles to have these paths.
   2. If α particles of higher energy than those in (a) are fired at plum pudding atoms, predict how their paths will differ from the lower-free energy α particle paths. Explain your reasoning.
   3. Now examination your predictions from (a) and (b). Open the Rutherford Handful simulation and select the "Plum Pudding Cantlet" tab. Set "Alpha Particles Energy" to "min," and select "prove traces." Click on the gun to start firing α particles. Does this match your prediction from (a)? If non, explain why the actual path would be that shown in the simulation. Hit the pause button, or "Reset All." Set "Alpha Particles Energy" to "max," and start firing α particles. Does this match your prediction from (b)? If non, explicate the result of increased energy on the actual paths as shown in the simulation.
5. Predict and examination the beliefs of α particles fired at a Rutherford atom model.
   1. Predict the paths taken by α particles that are fired at atoms with a Rutherford atom model structure. Explain why you expect the α particles to have these paths.
   2. If α particles of higher energy than those in (a) are fired at Rutherford atoms, predict how their paths will differ from the lower-free energy α particle paths. Explain your reasoning.
   3. Predict how the paths taken by the α particles will differ if they are fired at Rutherford atoms of elements other than gilt. What factor exercise y'all expect to crusade this deviation in paths, and why?
   4. Now test your predictions from (a), (b), and (c). Open the Rutherford Scattering simulation and select the "Rutherford Atom" tab. Due to the scale of the simulation, it is best to kickoff with a small nucleus, so select "xx" for both protons and neutrons, "min" for energy, evidence traces, and then start firing α particles. Does this match your prediction from (a)? If not, explain why the actual path would be that shown in the simulation. Pause or reset, set energy to "max," and outset firing α particles. Does this lucifer your prediction from (b)? If non, explicate the result of increased energy on the actual path as shown in the simulation. Intermission or reset, select "forty" for both protons and neutrons, "min" for free energy, show traces, and burn away. Does this match your prediction from (c)? If non, explicate why the actual path would be that shown in the simulation. Repeat this with larger numbers of protons and neutrons. What generalization can you brand regarding the type of cantlet and effect on the path of α particles? Be articulate and specific.

Glossary

alpha particle (α particle):positively charged particle consisting of two protons and two neutrons

electron:negatively charged, subatomic particle of relatively depression mass located outside the nucleus

isotopes:atoms that comprise the same number of protons just different numbers of neutrons

neutron:uncharged, subatomic particle located in the nucleus

nucleus:massive, positively charged center of an atom made up of protons and neutrons

proton:positively charged, subatomic particle located in the nucleus

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Source: https://courses.lumenlearning.com/chemistryformajors/chapter/evolution-of-atomic-theory/

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