Chemistry Performance Task #1 Ernest Rutherford’s Gold Foil Experiment Day 1

Chemistry

Performance Task #1

Ernest Rutherford’s Gold Foil Experiment


Day 1: Introductory video about the development of the current atomic theory.

Day 2 and 3: Students will work on the Atomic Theory Timeline project.

Day 4: Students will read the three printed articles that present Rutherford’s experiment in detail. The students will answer the following questions to guide their reading.

Day 5: Students will write a multi-paragraph essay that is explained in the prompt below. Two copies will be printed. One will be scored and one will be used to study from.
Guiding questions for readings:

1) Who is Ernest Rutherford?

2) Identify and explain the scientific understanding of the atomic structure prior to Rutherford’s Gold Foil Experiment.
3) Describe the format/set-up of the Gold Foil Experiment.
4) Describe what an alpha particle is in detail (include size, origin, behavior, and charge).
5) Describe what was observed during the Gold Foil Experiment.
6) What were Rutherford’s conclusions. (Include specific values.)
7) Make specific connections between Rutherford’s observations from the Gold Foil Experiment and his conclusions about the atomic structure.

Your Writing Assignment

Prompt: You are a graduate student of Ernest Rutherford who has just completed his findings from the Gold Foil Experiment. He has asked you to write a paper documenting the background information and findings from his experiment. This paper will be submitted to a scientific review committee who will be evaluating this research for possible publication. You must include specific background information about the understanding of the atom prior to this experiment, the format/set-up of the experiment including a detailed description of the alpha particles that were used. Include the specific observations made about the behavior of the alpha particles and tie this directly to the conclusions that Rutherford arrived at about atomic structure. You must explain his conclusions using specific observations from his experiment. Include all details!

This is a multi-paragraph writing assignment. Be thorough and detailed in your writing!
Resources
http://en.wikipedia.org/wiki/Ernest_Rutherford

http://myweb.usf.edu/~mhight/goldfoil.html

http://en.wikipedia.org/wiki/Alpha_particle

Ernest Rutherford

Ernest Rutherford, 1st Baron Rutherford of Nelson, OM FRS^[1] (30 August 1871 – 19 October 1937) was a New Zealand-born physicist and chemist who became known as the father of nuclear physics.^[2] He is considered the greatest experimentalist since Michael Faraday (1791–1867).^[2]

In early work he discovered the concept of radioactive half-life, proved that radioactivity involved the transmutation of one chemical element to another, and also differentiated and named alpha and beta radiation.^[3] This work was done at McGill University in Canada. It is the basis for the Nobel Prize in Chemistry he was awarded in 1908 "for his investigations into the disintegration of the elements, and the chemistry of radioactive substances".^[4]

Rutherford moved in 1907 to the Victoria University of Manchester (today University of Manchester) in the UK, where he and Thomas Royds proved that alpha radiation was helium ions.^[5][6] Rutherford performed his most famous work after he became a Nobel laureate.^[4] In 1911, although he could not prove that it was positive or negative,^[7] he theorized that atoms have their charge concentrated in a very small nucleus,^[8] and thereby pioneered the Rutherford model of the atom, through his discovery and interpretation of Rutherford scattering in his gold foil experiment. He is widely credited with first "splitting the atom" in 1917 in a nuclear reaction between nitrogen and alpha particles, in which he also discovered (and named) the proton.^[9]

Rutherford became Director of the Cavendish Laboratory at Cambridge University in 1919. Under his leadership the neutron was discovered by James Chadwick in 1932 and in the same year the first experiment to split the nucleus in a fully controlled manner, performed by students working under his direction, John Cockcroft and Ernest Walton. After his death in 1937, he was honoured by being interred with the greatest scientists of the United Kingdom, near Sir Isaac Newton's tomb in Westminster Abbey. The chemical element rutherfordium (element 104) was named after him in 1997.

Early life and education

Ernest Rutherford was the son of James Rutherford, a farmer, and his wife Martha Thompson, originally from Hornchurch, Essex, England.^[10]James had emigrated to New Zealand from Perth, Scotland, "to raise a little flax and a lot of children". Ernest was born at Spring Grove (now Brightwater), near Nelson, New Zealand. His first name was mistakenly spelled 'Earnest' when his birth was registered.^[11]

He studied at Havelock School and then Nelson College and won a scholarship to study at Canterbury College, University of New Zealandwhere he participated in the debating society and played rugby.^[12] After gaining his BA, MA and BSc, and doing two years of research during which he invented a new form of radio receiver, in 1895 Rutherford was awarded an "1851 Exhibition Scholarship" to travel to England for postgraduate study at the Cavendish Laboratory, University of Cambridge.^[13] He was among the first of the 'aliens' (those without a Cambridge degree) allowed to do research at the university, under the inspiring leadership of J. J. Thomson, and the newcomers aroused jealousies from the more conservative members of the Cavendish fraternity. With Thomson's encouragement, he managed to detect radio waves at half a mile and briefly held the world record for the distance over which electromagnetic waves could be detected, though when he presented his results at the British Association meeting in 1896, he discovered he had been outdone by another lecturer, by the name ofMarconi.

In 1898 Thomson offered Rutherford the chance of a post at McGill University in Montreal, Canada. He was to replace Hugh Longbourne Callendar who held the chair of Macdonald Professor of physics and was coming to Cambridge.^[14] Rutherford was accepted, which meant that in 1900 he could marry Mary Georgina Newton (1876–1945) to whom he had become engaged before leaving New Zealand; they had one daughter, Eileen Mary (1901–1930), who married Ralph Fowler. In 1900 he gained a DSc from the University of New Zealand. In 1907 Rutherford returned to Britain to take the chair of physics at the University of Manchester.

Later years and honors

He was knighted in 1914. During World War I, he worked on the practical problems of submarine detection. In 1916 he was awarded the Hector Memorial Medal. In 1919 he returned to the Cavendish succeeding J. J. Thomson as the Cavendish professor and Director. Under him, Nobel Prizes were awarded to James Chadwick for discovering the neutron (in 1932), John Cockcroft and Ernest Walton for an experiment which was to be known as splitting the atom using a particle accelerator, and Edward Appleton for demonstrating the existence of the ionosphere. Between 1925 and 1930 he served as President of the Royal Society, and later as president of the Academic Assistance Council which helped almost 1,000 university refugees from Germany.^[15] He was admitted to the Order of Merit in 1925 and raised to the peerage as Baron Rutherford of Nelson, in 1931,^[16] a title that became extinct upon his unexpected death in 1937.

For some time beforehand, Rutherford had a small hernia, which he had neglected to have fixed, and it became strangulated, causing him to be violently ill. Despite an emergency operation in London, he died four days afterwards of what physicians termed "intestinal paralysis", at Cambridge.^[17] After cremation at Golders Green Crematorium,^[17] he was given the high honour of burial in Westminster Abbey, near Isaac Newton and other illustrious British scientists.^[18]

Scientific research

the laboratory of Rutherford, early 20th century

At Cambridge, Rutherford started working with J. J. Thomson on the conductive effects of X-rays on gases, work which led to the discovery of the electron which Thomson presented to the world in 1897. Hearing of Becquerel's experience with uranium, Rutherford started to explore its radioactivity, discovering two types that differed from X-rays in their penetrating power and continuing his research in Canada. He coined the terms alpha ray and beta ray in 1899 to describe the two distinct types of radiation. He then discovered that thorium gave off a gas which produced an emanation which was itself radioactive and would coat other substances. He found that a sample of this radioactive material of any size invariably took the same amount of time for half the sample to decay – its "half-life" (11½ minutes in this case).

From 1900 to 1903, he was joined at McGill by the young chemist Frederick Soddy (Nobel Prize in Chemistry, 1921) for whom he set the problem of identifying the thorium emanations. Once he had eliminated all the normal chemical reactions, Soddy suggested that it must be one of the inert gases, which they named thoron (later found to be an isotope of radon). They also found another type of thorium they called Thorium X, and kept on finding traces of helium. They also worked with samples of "Uranium X" from William Crookes and radium from Marie Curie.

In 1902, they produced a "Theory of Atomic Disintegration" to account for all their experiments. Up till then atoms were assumed to be the indestructable basis of all matter and although Curie had suggested that radioactivity was an atomic phenomenon, the idea of the atoms of radioactive substances breaking up was a radically new idea. Rutherford and Soddy demonstrated that radioactivity involved the spontaneous disintegration of atoms into other types of atoms (one element spontaneously being changed to another).

In 1903, Rutherford considered a type of radiation discovered (but not named) by French chemist Paul Villard in 1900, as an emission from radium, and realised that this observation must represent something different from his own alpha and beta rays, due to its very much greater penetrating power. Rutherford therefore gave this third type of radiation the name of gamma ray. All three of Rutherford's terms are in standard use today – other types of radioactive decay have since been discovered, but Rutherford's three types are among the most common.

In Manchester, he continued to work with alpha radiation. In conjunction with Hans Geiger, he developed zinc sulfide scintillation screens and ionisation chambers to count alphas. By dividing the total charge they produced by the number counted, Rutherford decided that the charge on the alpha was two. In late 1907, Ernest Rutherford and Thomas Royds allowed alphas to penetrate a very thin window into an evacuated tube. As they sparked the tube into discharge, the spectrum obtained from it changed, as the alphas accumulated in the tube. Eventually, the clear spectrum of helium gas appeared, proving that alphas were at least ionised helium atoms, and probably helium nuclei.

Rutherford and the Gold Foil Experiment

Top: Expected results: alpha particles passing through the plum pudding model of the atom undisturbed.

Bottom: Observed results: a small portion of the particles were deflected, indicating a small, concentrated charge. Note that the image is not to scale; in reality the nucleus is vastly smaller than the electron shell.

Rutherford remains the only science Nobel Prize winner to have performed his most famous work after receiving the prize.^{[citation needed]} Along with Hans Geiger and Ernest Marsden in 1909, he carried out the Geiger–Marsden experiment, which demonstrated the nuclear nature of atoms. Rutherford was inspired to ask Geiger and Marsden in this experiment to look for alpha particles with very high deflection angles, of a type not expected from any theory of matter at that time. Such deflections, though rare, were found, and proved to be a smooth but high-order function of the deflection angle. It was Rutherford's interpretation of this data that led him to formulate the Rutherford model of the atom in 1911 – that a very small charged ^[7] nucleus, containing much of the atom's mass, was orbited by low-mass electrons.

Before leaving Manchester in 1919 to take over the Cavendish laboratory in Cambridge, Rutherford became, in 1919, the first person to deliberately transmute oneelement into another.^[4] In this experiment, he had discovered peculiar radiations when alphas were projected into air, and narrowed the effect down to the nitrogen, not the oxygen in the air. Using pure nitrogen, Rutherford used alpha radiation to convert nitrogen into oxygen through the nuclear reaction ¹⁴N + α →¹⁷O + proton. The proton was not then known. In the products of this reaction Rutherford simply identified hydrogen nuclei, by their similarity to the particle radiation from earlier experiments in which he had bombarded hydrogen gas with alpha particles to knock hydrogen nuclei out of hydrogen atoms. This result showed Rutherford that hydrogen nuclei were a part of nitrogen nuclei (and by inference, probably other nuclei as well). Such a construction had been suspected for many years on the basis of atomic weights which were whole numbers of that of hydrogen; see Prout's hypothesis. Hydrogen was known to be the lightest element, and its nuclei presumably the lightest nuclei. Now, because of all these considerations, Rutherford decided that a hydrogen nucleus was possibly a fundamental building block of all nuclei, and also possibly a new fundamental particle as well, since nothing was known from the nucleus that was lighter. Thus, Rutherford postulated hydrogen nuclei to be a new particle in 1920, which he dubbed the proton.

In 1921, while working with Niels Bohr (who postulated that electrons moved in specific orbits), Rutherford theorized about the existence of neutrons, (which he had christened in his 1920 Bakerian Lecture), which could somehow compensate for the repelling effect of the positive charges of protons by causing an attractive nuclear force and thus keep the nuclei from flying apart from the repulsion between protons. The only alternative to neutrons was the existence of "nuclear electrons" which would counteract some of the proton charges in the nucleus, since by then it was known that nuclei had about twice the mass that could be accounted for if they were simply assembled from hydrogen nuclei (protons). But how these nuclear electrons could be trapped in the nucleus, was a mystery.

Rutherford's theory of neutrons was proved in 1932 by his associate James Chadwick, who recognized neutrons immediately when they were produced by other scientists and later himself, in bombarding beryllium with alpha particles. In 1935, Chadwick was awarded the Nobel Prize in Physics for this discovery.

Alpha particle From Wikipedia, the free encyclopedia

Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. They are generally produced in the process of alpha decay, but may also be produced in other ways. Alpha particles are named after the first letter in the Greek alphabet, α. The symbol for the alpha particle is α or α²⁺. Because they are identical to helium nuclei, they are also sometimes written as He2+

indicating a Helium ion with a +2 charge (missing its two electrons). If the ion gains electrons from its environment, the alpha particle can be written as a normal (electrically neutral) Helium atom.

The nomenclature is not well defined, and thus not all high-velocity helium nuclei are considered by all authors as alpha particles. As with beta and gamma rays/particles, the name used for the particle carries some mild connotations about its production process and energy, but these are not rigorously applied.^[3] Some science authors may use doubly ionized helium nuclei (He2+

) and alpha particles as interchangeable terms. Thus, alpha particles may be loosely used as a term when referring to stellar helium nuclei reactions (for example the alpha processes), and even when they occur as components of cosmic rays. A higher energy version of alphas than produced in alpha decay is a common product of an uncommon nuclear fission result called ternary fission. However, helium nuclei produced by particle accelerators (cyclotrons, synchrotrons, and the like) are less likely to be referred to as "alpha particles".

Alpha particles, like helium nuclei, have a net spin of zero. Due to the mechanism of their production in standard alpha radioactive decay, alpha particles generally have a kinetic energy of about 5 MeV, and a velocity in the vicinity of 5% the speed of light (see discussion below for the limits of these figures in alpha decay). They are a highly ionizing form of particle radiation, and (when resulting from radioactive alpha decay) have lowpenetration depth. They are able to be stopped by a few centimeters of air, or by the skin. However, so-called long range alpha particles from ternary fission are three times as energetic, and penetrate three times as far. As noted, the helium nuclei that form 10-12% of cosmic rays are also usually of much higher energy than those produced by nuclear decay processes, and are thus capable of being highly penetrating and able to traverse the human body and also many meters of dense solid shielding, depending on their energy. To a lesser extent, this is also true of very high-energy helium nuclei produced by particle accelerators.

When alpha particle emitting isotopes are ingested, they are far more dangerous than their half-life or decay rate would suggest, due to the high relative biological effectiveness of alpha radiation to cause biological damage, after alpha-emitting radioisotopes enter living cells. Ingested alpha emitter radioisotopes (such as transuranics or actinides) are an average of about 20 times more dangerous, and in some experiments up to 1000 times more dangerous, than an equivalent activity of beta emitting or gamma emitting radioisotopes.

History of discovery and use

Alpha radiation consists of helium-4nucleus and is readily stopped by a sheet of paper. Beta radiation, consisting of electrons, is halted by an aluminium plate. Gamma radiation is eventually absorbed as it penetrates a dense material. Lead is good at absorbing gamma radiation, due to its density.



An alpha particle is deflected by a magnetic field

In the years 1899 and 1900, physicists Ernest Rutherford (working in McGill University in Montreal, Canada) and Paul Villard (working in Paris) separated radiation into three types: eventually named alpha, beta, and gamma by Rutherford, based on penetration of objects and deflection by a magnetic field.^[7] Alpha rays were defined by Rutherford as those having the lowest penetration of ordinary objects.

Rutherford's work also included measurements of the ratio of an alpha particle's mass to its charge, which led him to the hypothesis that alpha particles were doubly charged helium ions (later shown to be bare helium nuclei).^[8] In 1907, Ernest Rutherford and Thomas Royds finally proved that alpha particles were indeed helium ions.^[9] To do this they allowed alpha particles to penetrate a very thin glass wall of an evacuated tube, thus capturing a large number of the hypothesized helium ions inside the tube. They then caused an electric spark inside the tube, which provided a shower of electrons that were taken up by the ions to form neutral atoms of a gas. Subsequent study of the spectra of the resulting gas showed that it was helium and that the alpha particles were indeed the hypothesized helium ions.

Because alpha particles occur naturally, but can have energy high enough to participate in a nuclear reaction, study of them led to much early knowledge of nuclear physics. Rutherford used alpha particles emitted by radium bromide to infer that J. J. Thomson's Plum pudding model of the atom was fundamentally flawed. In Rutherford's gold foil experiment conducted by his students Hans Geiger and Ernest Marsden, a narrow beam of alpha particles was established, passing through very thin (a few hundred atoms thick) gold foil. The alpha particles were detected by a zinc sulfide screen, which emits a flash of light upon an alpha particle collision. Rutherford hypothesized that, assuming the "plum pudding" model of the atom was correct, the positively charged alpha particles would be only slightly deflected, if at all, by the dispersed positive charge predicted.

It was found that some of the alpha particles were deflected at much larger angles than expected (at a suggestion by Rutherford to check it) and some even bounced almost directly back. Although most of the alpha particles went straight through as expected, Rutherford commented that the few particles that were deflected was akin to shooting a fifteen-inch shell at tissue paper only to have it bounce off, again assuming the "plum pudding" theory was correct. It was determined that the atom's positive charge was concentrated in a small area in its center, making the positive charge dense enough to deflect any positively charged alpha particles that came close to what was later termed the nucleus.

Note: Prior to this discovery, it was not known that alpha particles are themselves atomic nuclei, nor was the existence of protons or neutrons known. After this discovery J.J. Thomson's "plum pudding" model was abandoned, and Rutherford's experiment led to the Bohr model (named for Niels Bohr) and later the modern wave-mechanical model of the atom.