Thomson

Model Atom JJ. Thomson Pada awal 1900an, J.J. Thomson mengusulkan model atom baru yang mengikutkan keberadaan partikel elektron dan proton. Karena eksperimen menunjukkan proton memiliki massa yang jauh lebih besar dibandingkan elektron, maka model Thomson menggambarkan atom sebagai proton tunggal yang besar. Di dalam partikel proton, Thomson memasukkan elektron yang menetralkan adanya muatan positif dari proton. Menurut Thomson, atom terdiri dari suatu bulatan bermuatan positif dengan rapat muatan yang merata. Di dalam muatan positif ini tersebar elektron dengan muatan negatif yang besarnya sama dengan muatan positif. Cara yang populer untuk menggambarkan model ini adalah dengan menganggap elektron sebagai kismis (plumb) di dalam kue puding proton, sehingga model ini diberi nama model kue kismis (plumb-pudding model). Walaupun model atom Thomson adalah yang pertama yang memasukkan konsep adanya proton dan elektron yang bermuatan, model Thomson tidak mampu melewati pengamatan pada eksperimen-eksperimen berikutnya. Sebagai catatan, proton yang digunakan dalam model Thomson ini bukanlah partikel proton yang ditemukan di model yang lebih modern. Bahkan sesungguhnya dapat dikatakan model Thomson tidak memiliki proton, namun sebuah sel bermuatan positif. Pengaruh model atom Dalton dapat dilihat dengan jelas pada model Thomson. Dalton berspekulasi bahwa atom adalah benda padat, dan Thomson mendukung gagasan ini dalam modelnya dengan mengelompokkan elektron dan proton bersama-sama. Dari hasil percobaannya, Thomson menyatakan bahwa sinar katoda merupakan partikel penyusun atom (partikel sub atom) yang bermuatan negatif yang selanjutnya dinamakan sebagai elektron. Atom merupakan partikel yang bersifat netral. Oleh karena elektron bermuatan negatif, maka untuk menghasilkan muatan total netral harus ada muatan positif. Dengan demikian, Thomson telah menyempurnakan teori atom dari Dalton dan mengemukakan teori atomnya yang dinamakan sebagai teori atom Thomson. Teori atom Thomson menyatakan bahwa: “Atom merupakan bola pejal yang bermuatan positif dan didalamya tersebar muatan negatif elektron” The electron was identified as a particle in 1897 by J. J. Thomson and his team of British physicists.[6][9] Electrons are identical particles that belong to the first generation of the lepton particle family. Electrons have quantum mechanical properties of both a particle and a wave, so they can collide with other particles and be diffracted like light. Each electron occupies a quantum state that describes its random behavior upon measuring a physical parameter, such as its energy or spin orientation. Because they are a type of fermion, no two electrons can occupy the same quantum state; a property known as the Pauli exclusion principle.[10] In 1896, British physicist J. J. Thomson, with his colleagues John S. Townsend and H. A. Wilson,[6] performed experiments indicating that cathode rays really were unique particles, rather than waves, atoms or molecules as was believed earlier. Thomson made good estimates of both the charge e and the mass m, finding that cathode ray particles,

which he called "corpuscles," had perhaps one thousandth of the mass of the least massive ion known: hydrogen. He showed that their charge to mass ratio, e/m, was independent of cathode material. He further showed that the negatively charged particles produced by radioactive materials, by heated materials and by illuminated materials were universal.[23] The name electron was again proposed for these particles by the Irish physicist George F. Fitzgerald, and it has since gained universal acceptance.[20]

[sunting] J.J. Thomson: Elektron Pada tahun 1899, Joseph John Thomson meneliti cahaya ultraungu dalam tabung sinar katoda. Dipengaruhi oleh kerja James Clerk Maxwell, Thomson menyimpulkan bahwa sinar katoda terdiri atas partikel-partikel bermuatan negatif, yang dia sebut corpuscles (belakangan disebut "elektron"). Dalam penelitian tersebut, Thomson menempatkan pelat logam (yaitu, katoda) dalam tabung hampa, dan menyinarinya dengan radiasi frekuensi tinggi. Kelemahan Teori atom Thomson Tidak dapat menjelaskan bagaimana susunan elektron dan muatan positif di dalam atom Perkembangan Berikutnya Penemuan-penemuan baru dalam bidang fisika ternyata mampu membuka cakrawala baru pemahaman atom oleh manusia. Penemuan elektron oleh J.J. Thomson menyebabkan model atom yang dikemukakan Dalton tidak dapat diterima lagi. Dengan gugurnya model atom Dalton ini, Thomson terdorong untuk mengemukakan teori atom baru yang dikemukakannya pada tahun 1904. Thomson melukiskan bahwa atom bukanlah merupakan partikel terkecil yang tidak dapat dibagi-bagi lagi, seperti yang dipahami manusia sebelumnya. Ia melukiskan bahwa atom mempunyai bentuk seperti bola yang muatan positifnya terbagi merata ke seluruh isi atom. Muatan positif itu dinetralkan oleh elektron-elektron bermuatan negatif yang tersebar di antara muatan positif tadi. Teori atom ini diterima secara luas oleh para ilmuwan hingga akhir abad ke18. Dalam perjalanan berikutnya, teori atom Thomson inipun akhirnya gugur oleh pengujian yang dilakukan Ernest Rutherford. Pengujian itu dilakukan dengan cara menembaki lempengan emas yang sangat tipis (ketebalan 0,01 mm) dengan partikel alfa. Apabila model atom Thomson itu benar, maka gerakan partikel alfa tidak akan dibelokkan sewaktu menumbuk lempeng emas.

Elektron Dari Wikipedia bahasa Indonesia, ensiklopedia bebas Langsung ke: navigasi, cari

Estimasi teoritis dari densitas elektron untuk atom Hidrogen dengan beberapa orbit elektron Elektron adalah partikel subatomik. Memiliki muatan listrik negatif sebesar -1.6 × 10-19 coulomb, dan massanya 9.10 × 10-31 kg (0.51 MeV/c2). Elektron umumnya ditulis sebagai e-. Elektron memiliki partikel lawan yang dikenal sebagai positron yang identik dengan dirinya namun bermuatan positif. Atom tersusun dari inti berupa proton dan neutron serta elektron-elektron yang mengelilingi inti tadi. Elektron sangat ringan jika dibandingkan dengan proton dan neutron. Sebutir proton sekitar 1800 kali lebih berat daripada elektron. Elektron adalah salah satu dari sekelas partikel subatom yang dikenal dengan lepton yang dipercaya merupakan partikel dasar (yakni, mereka tak dapat dipecah lagi ke dalam bagian yang lebih kecil). Elektron memiliki spin 1/2, artinya elektron merupakan sebuah fermion, dengan kata lain, mematuhi statistik Fermi-Dirac.

Sejarah Elektron pertama kali ditemukan oleh J.J. Thomson di Laboratorium Cavendish, Universitas Cambridge, pada tahun 1897, pada saat beliau sedang mempelajari "sinar katoda".

[sunting] Rincian Teknis Penjelasan mengenai elektron dibahas di mekanika kuantum dengan Persamaan Dirac. Dalam Model Standarnya, elektron membentuk suatu doublet dalam SU(2) dengan neutrino elektron, karena ia berinteraksi lewat interaksi lemah. Elektron memiliki dua rekan massive lagi, yang muatannya sama namun berbeda massanya: muon dan tau.

[sunting] Arus Listrik Jika elektron bergerak, lepas bebas dari pengaruh inti atom, serta terdapat suatu aliran (net flow), aliran ini dikenal sebagai arus listrik. Ini dapat dibayangkan sebagai serombongan domba yang bergerak bersama-sama ke utara namun tanpa diikuti oleh penggembalanya. Muatan listrik dapat diukur secara langsung menggunakan elektrometer. Arus listrik dapat diukur secara langsung menggunakan galvanometer. Apa yang dikenal dengan "listrik statis" bukanlah aliran elektron sama sekali. Ini lebih tepat disebut sebagai sebuah "muatan statik", mengacu pada sebuah benda yang memiliki lebih banyak atau lebih sedikit elektron daripada yang dibutuhkan untuk mengimbangi muatan positif sang inti. Jika terdapat kelebihan elektron, maka benda tadi dikatakan sebagai "bermuatan negatif". Jika terdapat kekurangan elektron dibanding proton, benda tersebut dikatakan "bermuatan positif". Jika jumlah elektron dan proton adalah sama, benda tersebut dikatakan "netral".

[sunting] Penemuan Sekitar periode 1870-an, Ahli kimia dan fisika Inggris, Sir William Crookes membuat tabung sinar katoda pertama untuk menghasilkan ruang hampa udara bertekanan tinggi didalamnya.[1] Dia kemudian menunjukkan bahwa sinar luminescence yang muncul dalam tabung membawa energi dan bergerak dari katoda ke anoda. Lebih jauh, dengan menerapkan sebuah medan magnet, dia dapat mengalihkan sinar tersebut, sehingga hal ini dapat memperagakan bahwa cahaya dapat dikendalikan dengan sinar negatif.[2][3] Pada tahun 1879, dia mengusulkan hal ini dapat dijelaskan secara logika dengan apa yang dia sebut sebagai persamaan 'radiant matter'. Dia menyarankan bahwa pada keadaan seperti ini, bagian cahaya ini akan mengandung molekul negatif yang dapat diarahkan dengan kecepatan tinggi dengan menggunakan katoda.[4]

J. J. Thomson From Wikipedia, the free encyclopedia Jump to: navigation, search

J. J. Thomson

Sir Joseph John Thomson (1856-1940). Portrait by Arthur Hacker.

Born Died Nationality Fields Institutions Alma mater Academic advisors

18 December 1856 Cheetham Hill, Manchester, UK 30 August 1940 (aged 83) Cambridge, UK United Kingdom Physics University of Cambridge University of Manchester University of Cambridge John Strutt (Rayleigh) Edward John Routh Charles Glover Barkla Charles T. R. Wilson Ernest Rutherford Francis William Aston John Townsend J. Robert Oppenheimer Owen Richardson

Notable students

William Henry Bragg H. Stanley Allen John Zeleny Daniel Frost Comstock Max Born T. H. Laby Paul Langevin Balthasar van der Pol

Plum pudding model Discovery of electron Discovery of isotopes Mass spectrometer invention First m/e measurement Known for

Proposed first waveguide Thomson scattering Thomson problem Coining term 'delta ray' Coining term 'epsilon radiation' Thomson (unit)

Notable awards

Nobel Prize for Physics (1906)

Religious stance

Anglican Signature

Notes Thomson is the father of Nobel laureate George Paget Thomson.

Sir Joseph John “J.J.” Thomson, OM, FRS (18 December 1856 – 30 August 1940) was a British physicist and Nobel laureate, credited for the discovery of the electron and of isotopes, and the invention of the mass spectrometer. He was awarded the 1906 Nobel Prize in Physics for the discovery of the electron and his work on the conduction of electricity in gases.

Contents [hide]

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1 Biography o 1.1 Career  1.1.1 Cathode rays  1.1.1.1 First experiment  1.1.1.2 Second experiment  1.1.1.3 Third experiment  1.1.2 Nobel Prize  1.1.3 Isotopes and mass spectrometry  1.1.4 Other work 2 Awards 3 Bibliography 4 Notes 5 References

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6 External links

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[edit] Biography Joseph J. Thomson was born in 1856 in Cheetham Hill, Manchester in England, of Scottish parentage. His father died when he was 16 years old.[1] In 1870 he studied engineering at University of Manchester known as Owens College at that time, and moved on to Trinity College, Cambridge in 1876. In 1880, he obtained his BA in mathematics (Second Wrangler and 2nd Smith's prize) and MA (with Adams Prize) in 1883. In 1884 he became Cavendish Professor of Physics. One of his students was Ernest Rutherford, who would later succeed him in the post. In 1890 he married Rose Elisabeth Paget, daughter of Sir George Edward Paget, KCB, a physician and then Regius Professor of Physic at Cambridge. He fathered one son, George Paget Thomson, and one daughter, Joan Paget Thomson, with her. One of Thomson's greatest contributions to modern science was in his role as a highly gifted teacher, as seven of his research assistants and his aforementioned son won Nobel Prizes in physics. His son won the Nobel Prize in 1937 for proving the wavelike properties of electrons. He was awarded a Nobel Prize in 1906, "in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases." He was knighted in 1908 and appointed to the Order of Merit in 1912. In 1914 he gave the Romanes Lecture in Oxford on "The atomic theory". In 1918 he became Master of Trinity College, Cambridge, where he remained until his death. He died on 30 August 1940 and was buried in Westminster Abbey, close to Sir Isaac Newton. Thomson was elected a Fellow of the Royal Society on 12 June 1884 and was subsequently President of the Royal Society from 1915 to 1920.

Sir Joseph John Thomson.

[edit] Career

[edit] Cathode rays Thomson conducted a series of experiments with cathode rays and cathode ray tubes leading him to the discovery of electrons and subatomic particles. Thomson used the cathode ray tube in three different experiments.

[edit] First experiment In his first experiment, he investigated whether or not the negative charge could be separated from the cathode rays by means of magnetism. He constructed a cathode ray tube ending in a pair of cylinders with slits in them. These slits were in turn connected to an electrometer. Thomson found that if the rays were magnetically bent such that they could not enter the slit, the electrometer registered little charge. Thomson concluded that the negative charge was inseparable from the cathode rays.

[edit] Second experiment Thomson's second experiment. In his second experiment, he investigated whether or not the rays could be deflected by an electric field (something that is characteristic of charged particles).[2] Previous experimenters had failed to observe this, but Thomson believed their experiments were flawed because they contained trace amounts of gas. Thomson constructed a cathode ray tube with a practically perfect vacuum, and coated one end with phosphorescent paint. Thomson found that the rays did indeed bend under the influence of an electric field, in a direction indicating a negative charge.

[edit] Third experiment

Thomson's third experiment. In his third experiment, Thomson measured the mass-to-charge ratio of the cathode rays by measuring how much they were deflected by a magnetic field and how much energy they carried. He found that the mass to charge ratio was over a thousand times lower than that of a hydrogen ion (H+), suggesting either that the particles were very light or very highly charged. Thomson's conclusions were bold: cathode rays were indeed made of particles which he called "corpuscles", and these corpuscles came from within the atoms of the electrodes themselves, meaning that atoms are in fact divisible. The "corpuscles" discovered by Thomson are identified with the electrons which had been proposed by G. Johnstone Stoney. He conducted this experiment in 1897. Thomson imagined the atom as being made up of these corpuscles swarming in a sea of positive charge; this was his plum pudding model. This model was later proved incorrect when Ernest Rutherford showed that the positive charge is concentrated in the nucleus of the atom.

[edit] Nobel Prize Thomson's discovery was made known in 1897, and caused a sensation in scientific circles, eventually resulting in him being awarded a Nobel Prize in Physics in 1906.[3] He notes that prior to his work: (1) the (negatively charged) cathode was known to be the source of the cathode rays; (2) the cathode rays were known to have the particle-like property of charge; (3) were deflected by a magnetic field like a negatively charged particle; (4) had the wave-like property of being able to penetrate thin metal foils; (5) had not yet been subject to deflection by an electric field. Thomson succeeded in causing electric deflection because his cathode ray tubes were sufficiently evacuated that they developed only a low density of ions (produced by collisions of the cathode rays with the gas remaining in the tube). Their ion densities were low enough that the gas was a poor conductor, unlike the tubes of previous workers, where the ion density was high enough that the ions could screen out the electric field. He found that the cathode rays (which he called corpuscles) were deflected by an electric field in the same direction as negatively charged particles would deflect. With the electrons moving along, say, the x-direction, the electric field E pointing along the ydirection, and the magnetic field B pointing along the z-direction, by adjusting the ratio of the magnetic field B to the electric field E he found that the cathode rays moved in a nearly straight line, an indication of a nearly uniform velocity v=E/B for the cathode rays emitted by the cathode. He then removed the magnetic field and measured the deflection of the cathode rays, and from this determined the charge-to-mass ratio e/m for the cathode rays. He writes: "however the cathode rays are produced, we always get the same value of e/m for all the particles in the rays. We may...produce great changes in the velocity of the particles, but unless the velocity of the particles becomes so great that they

are moving nearly as fast as light, when other considerations have to be taken into account, the value of e/m is constant. The value of e/m is not merely independent of the velocity...it is independent of the kind of electrodes we use and also of the kind of gas in the tube." Thomson notes that "corpuscles" are emitted by hot metals and "Corpuscles are also given out by metals and other bodies, but especially by the alkali metals, when these are exposed to light. They are being continually given out in large quantities and with very great velocities by radioactive substances such as uranium and radium; they are produced in large quantities when salts are put into flames, and there is good reason to suppose that corpuscles reach us from the sun." Thomson also describes water drop experiments that enabled him to obtain a value for e that is about twice the modern value, and close to the then current value for the charge on a hydrogen ion in an electrolyte.

[edit] Isotopes and mass spectrometry In the bottom right corner of this photographic plate are markings for the two isotopes of neon: neon-20 and neon-22. In 1913, as part of his exploration into the composition of canal rays, Thomson channelled a stream of ionized neon through a magnetic and an electric field and measured its deflection by placing a photographic plate in its path. Thomson observed two patches of light on the photographic plate (see image on right), which suggested two different parabolas of deflection. Thomson concluded that neon is composed of atoms of two different atomic masses (neon-20 and neon-22), that is to say of two isotopes. This was the first evidence for isotopes of a stable element; Frederick Soddy had previously proposed the existence of isotopes to explain the decay of certain radioactive elements. Thomson's separation of neon isotopes by their mass was the first example of mass spectrometry, which was subsequently improved and developed into a general method by Thomson's student F. W. Aston and by A. J. Dempster.

[edit] Other work In 1905 Thomson discovered the natural radioactivity of potassium.[4] In 1906 Thomson demonstrated that hydrogen had only a single electron per atom. Previous theories allowed various numbers of electrons.[5][6]

[edit] Awards • • • •

Royal Medal (1894) Hughes Medal (1902) Nobel Prize for Physics (1906) Copley Medal (1914)

[edit] Bibliography •

• •

• • •

1883. A Treatise on the Motion of Vortex Rings: An essay to which the Adams Prize was adjudged in 1882, in the University of Cambridge. London: Macmillan and Co., pp. 146. Recent reprint: ISBN 0-5439-5696-2. 1888. Applications of Dynamics to Physics and Chemistry. London: Macmillan and Co., pp.326. Recent reprint: ISBN 1-4021-8397-6. 1893. Notes on recent researches in electricity and magnetism: intended as a sequel to Professor Clerk-Maxwell's 'Treatise on Electricity and Magnetism'. Oxford Univ. Press, pp.xvi and 578. 1991, Cornell University Monograph: ISBN 1-4297-4053-1. 1921 (1895). Elements Of The Mathematical Theory Of Electricity And Magnetism. London: Macmillan and Co. Scan of 1895 edition. (with J.H. Poynting). A Text book of Physics in Five Volumes: Properties of Matter, Sound, Heat, Light, and Magnetism & Electricity. Navarro, Jaume, 2005, "Thomson on the Nature of Matter: Corpuscles and the Continuum," Centaurus 47(4): 259-82.

Plum pudding model From Wikipedia, the free encyclopedia (Redirected from Thomson Atomic Model) Jump to: navigation, search A schematic representation of the plum pudding model of the atom. In Thomson's mathematical model the "corpuscles" (or modern electrons) were arranged non-randomly, in rotating rings. The plum pudding model of the atom by J.J. Thomson, who discovered the electron in 1897, was proposed in 1904 before the discovery of the atomic nucleus. In this model, the atom is composed of electrons (which Thomson still called "corpuscles," though G.J. Stoney had proposed that atoms of electricity be called electrons in 1894) [1] , surrounded by a soup of positive charge to balance the electron's negative charge, like negativelycharged "plums" surrounded by positively-charged "pudding". The electrons (as we know them today) were thought to be positioned throughout the atom, but with many structures possible for positioning multiple electrons, particularly rotating rings of electrons (see below). Instead of a soup, the atom was also sometimes said to have had a cloud of positive charge. The model was disproved by the 1909 gold foil experiment, which was interpreted by Ernest Rutherford in 1911[2] to imply a very small nucleus of the atom containing a very high positive charge (enough to balance about 100 electrons in gold), thus leading to the Rutherford model of the atom, and finally (after Henry Moseley's work showed in 1913 that the nuclear charge was very close to the atomic number) to the Antonius Van den

Broek suggestion that atomic number is nuclear charge. Eventually, by 1913, this work had culminated in the solar-system-like (but quantum-limited) Bohr model of the atom, in which a nucleus containing an atomic number of positive charge is surrounded by an equal number of electrons in orbital shells. Thomson's model was compared (though not by Thomson) to a British treat called plum pudding, hence the name. It has also been called the chocolate chip cookie model or blueberry muffin model, but these mental pictures assume the particles as static, which they were not for Thomson. Thomson's paper was published in the March 1904 edition of the Philosophical Magazine, the leading British science journal of the day. In Thompson's view: ... the atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification, ... [3] In this model, the electrons were free to rotate within the blob or cloud of positive substance. These orbits were stabilized in the model by the fact that when an electron moved farther from the center of the positive cloud, it felt a larger net positive inward force, because there was more material of opposite charge, inside its orbit (see Gauss's law). In Thomson's model, electrons were free to rotate in rings which were further stabilized by interactions between the electrons, and spectra were to be accounted for by energy differences of different ring orbits. Thomson attempted to make his model account for some of the major spectral lines known for some elements, but was not notably successful at this. Still, Thomson's model (along with a similar Saturnian ring model for atomic electrons, put forward also in 1904 by Nagaoka after the Maxwell model of Saturn's rings), were earlier harbingers of the later and more successful solarsystem-like Bohr model of the atom.