what new information did robert millikan contribute to the understanding of the atom?

Learning Outcomes

• Outline milestones in the evolution of modern atomic theory
• Summarize and interpret the results of the experiments of Thomson, Millikan, and Rutherford
• Draw the three subatomic particles that compose atoms
• Define isotopes and give examples for several elements

In the two centuries since Dalton developed his ideas, scientists have 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. Here, we volition talk over some of those key developments, with an accent on awarding of the scientific method, as well every bit agreement how the experimental evidence was analyzed. While the historical persons and dates behind these experiments can exist quite interesting, information technology is virtually of import to understand the concepts resulting from their work.

Atomic Theory subsequently the Nineteenth Century

If matter 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 electrical discharges that could be produced in depression-force per unit area gases, with the almost significant discovery made by English physicist J. J. Thomson using a cathode ray tube. This appliance consisted of a sealed glass tube from which about all the air had been removed; the tube contained two metal electrodes. When high voltage was applied across the electrodes, a visible beam called a cathode ray appeared betwixt them. This beam was deflected toward the positive accuse and abroad from the negative charge, and was produced in the aforementioned 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 allowed 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 (Figure one).

Figure 1. (a) J. J. Thomson produced a visible axle in a cathode ray tube. (b) This is an early cathode ray tube, invented in 1897 by 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 calibration at the stop of the tube. Simultaneous deflections by applied electric and magnetic fields permitted Thomson to summate the mass-to-charge ratio of the particles composing the cathode ray. (credit a: modification of work past Nobel Foundation; credit b: modification of piece of work by Eugen Nesper; credit c: modification of work by "Kurzon"/Wikimedia Eatables)

Based on his observations, here is what Thomson proposed and why: The particles are attracted by positive (+) charges and repelled by 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 telephone call an electron, a negatively charged, subatomic particle with a mass more than 1 thousand-times less that of an atom. The term "electron" was coined in 1891 past Irish gaelic 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 ain voice.

In 1909, more data 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 by friction as they formed or by using X-rays. These droplets initially barbarous due to gravity, but their downward progress could be slowed or even reversed by an electric field lower in the apparatus. Past adjusting the electric field strength and making careful measurements and advisable calculations, Millikan was able to determine the charge on private 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 accuse data that Millikan gathered, y'all may have recognized that the accuse of an oil droplet is ever a multiple of a specific accuse, i.6 × 10^-19 C. Millikan concluded that this value must therefore be a key charge—the charge of a single electron—with his measured charges due to an excess of one electron (one times i.half-dozen × 10^-19 C), two electrons (ii times 1.6 × 10^-xix C), three electrons (iii times 1.6 × ten^-xix C), and and so on, on a given oil droplet. Since the accuse of an electron was at present known due to Millikan's research, and the charge-to-mass ratio was already known due to Thomson's research (1.759 × ten¹¹ C/kg), information technology merely required a elementary calculation to determine the mass of the electron also.

[latex]\text{Mass of electron}=i.602\times {10}^{-xix}\text{C}\times\frac{1\text{kg}}{1.759\times {10}^{11}\text{C}}=9.107\times {10}^{-31}\text{kg}[/latex]

Scientists had now established that the atom was not indivisible as Dalton had believed, and due to the work of Thomson, Millikan, and others, the charge and mass of the negative, subatomic particles—the electrons—were known. Even so, the positively charged part of an atom was non nonetheless well understood. In 1904, Thomson proposed the "plum pudding" model of atoms, which described a positively charged mass with an equal corporeality of negative charge in the course of electrons embedded in information technology, 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 by a halo of electrons (Effigy 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 work by "Man vyi"/Wikimedia Commons; credit b: modification of work by "NASA"/Wikimedia Eatables)

The next major evolution in agreement the atom came from Ernest Rutherford, a physicist from New Zealand who largely spent his scientific career in Canada and England. He performed a serial 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 (you will learn more about radioactive disuse in the module on nuclear chemistry). Rutherford and his colleagues Hans Geiger (afterwards famous for the Geiger counter) and Ernest Marsden aimed a beam of α particles, the source of which was embedded in a lead block to absorb near of the radiations, at a very thin piece of gold foil and examined the resultant scattering of the α particles using a luminescent screen that glowed briefly where hit by an α particle.

What did they discover? Nearly particles passed right through the foil without being deflected at all. Still, some were diverted slightly, and a very small number were deflected well-nigh straight back toward the source (Figure four). Rutherford described finding these results: "Information technology was quite the most incredible effect that has ever happened to me in my life. It was nigh equally incredible equally if you lot fired a 15-inch shell at a piece of tissue paper and it came back and hit you."

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

Here is what Rutherford deduced: Because most of the fast-moving α particles passed through the gold atoms undeflected, they must have traveled through essentially empty space inside the atom. Alpha particles are positively charged, and 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 hit, or closely approached, another body that besides had a highly concentrated, positive accuse. Since the deflections occurred a minor fraction of the time, this charge only occupied a small-scale corporeality of the space in the aureate foil. Analyzing a series of such experiments in particular, Rutherford drew 2 conclusions:

1. The volume occupied by an atom must consist of a large corporeality of empty space.
2. A small, relatively heavy, positively charged body, the nucleus, must be at the center of each cantlet.

View this simulation of the Rutherford gold foil experiment. Adapt the slit width to produce a narrower or broader beam of α particles to see how that affects the scattering blueprint.

This analysis led Rutherford to propose a model in which an atom consists of a very small, positively charged nucleus, in which nigh of the mass of the atom is full-bodied, surrounded by the negatively charged electrons, and so that the atom is electrically neutral (Effigy five). After many more experiments, Rutherford as well discovered that the nuclei of other elements contain the hydrogen nucleus as a "building block," and he named this more fundamental particle the proton, the positively charged, subatomic particle constitute in the nucleus. With 1 addition, which y'all will acquire next, this nuclear model of the atom, proposed over a century ago, is nonetheless used today.

Figure 5. The α particles are deflected only when they collide with or pass shut to the much heavier, positively charged gold nucleus. Because the nucleus is very small compared to the size of an atom, very few α particles are deflected. About pass through the relatively large region occupied by electrons, which are too light to deflect the rapidly moving particles.

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

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 chemical element" produced by the radioactive decay of thorium was initially given the proper noun mesothorium. All the same, a more detailed analysis showed that mesothorium was chemically identical to radium (another decay product), despite having a different diminutive mass. This issue, forth with like findings for other elements, led the English language chemist Frederick Soddy to realize that an element could have types of atoms with different masses that were chemically indistinguishable. These different types are called isotopes—atoms of the aforementioned element that differ in mass. Soddy was awarded the Nobel Prize in Chemistry in 1921 for this discovery.

One puzzle remained: The nucleus was known to incorporate almost all of the mass of an atom, with the number of protons only providing 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 you might look, detecting uncharged particles is very challenging, and it was non until 1932 that James Chadwick institute evidence of neutrons, uncharged, subatomic particles with a mass approximately the aforementioned as that of protons. The existence of the neutron besides explained isotopes: They differ in mass considering they have different numbers of neutrons, simply they are chemically identical because they accept the same number of protons. This volition be explained in more detail later.

Key Concepts and Summary

Although no i has actually seen the inside of an atom, experiments have demonstrated much about atomic construction. Thomson's cathode ray tube showed that atoms contain small, negatively charged particles called electrons. Millikan discovered that at that place is a central electric charge—the accuse of an electron. Rutherford'due south gilt foil experiment showed that atoms have a minor, dense, positively charged nucleus; the positively charged particles inside the nucleus are chosen 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 chosen isotopes.

Try It

1. The existence of isotopes violates one of the original ideas of Dalton's atomic theory. Which one?
2. How are electrons and protons similar? How are they different?
3. How are protons and neutrons like? How are they different?
4. Predict and exam the behavior of α particles fired at a "plum pudding" model atom.
   1. Predict the paths taken by α particles that are fired at atoms with a Thomson's plum pudding model structure. Explain why y'all expect the α particles to take these paths.
   2. If α particles of college energy than those in (a) are fired at plum pudding atoms, predict how their paths volition differ from the lower-free energy α particle paths. Explain your reasoning.
   3. At present test your predictions from (a) and (b). Open up the Rutherford Handful simulation and select the "Plum Pudding Atom" tab. Set "Alpha Particles Energy" to "min," and select "bear witness traces." Click on the gun to first firing α particles. Does this friction match your prediction from (a)? If not, explain why the actual path would be that shown in the simulation. Hit the pause button, or "Reset All." Gear up "Alpha Particles Energy" to "max," and start firing α particles. Does this friction match your prediction from (b)? If not, explain the outcome of increased energy on the actual paths as shown in the simulation.
5. Predict and exam the behavior 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 look the α particles to take these paths.
   2. If α particles of college energy than those in (a) are fired at Rutherford atoms, predict how their paths will differ from the lower-energy α particle paths. Explain your reasoning.
   3. Predict how the paths taken by the α particles volition differ if they are fired at Rutherford atoms of elements other than gold. What cistron do you expect to cause this difference in paths, and why?
   4. At present exam your predictions from (a), (b), and (c). Open the Rutherford Handful simulation and select the "Rutherford Atom" tab. Due to the scale of the simulation, information technology is best to start with a pocket-sized nucleus, so select "20" for both protons and neutrons, "min" for energy, show 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. Intermission or reset, set up energy to "max," and start firing α particles. Does this match your prediction from (b)? If not, explicate the effect of increased energy on the bodily path as shown in the simulation. Pause or reset, select "xl" for both protons and neutrons, "min" for energy, show traces, and burn down away. Does this match your prediction from (c)? If not, explicate why the bodily path would be that shown in the simulation. Repeat this with larger numbers of protons and neutrons. What generalization can you make regarding the type of atom and event on the path of α particles? Be clear and specific.

Glossary

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

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

isotopes:atoms that comprise the same number of protons but 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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