jj thomson atomic theory
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Chapter 4 Atomic Structure 41 The Nuclear Atom J J Thomson
Thomson suggested (1898) that atoms consist of positively charged lumps of matter with electrons embedded in them (\"Raisin pudding \") This model may seem silly now but as we will see soon some of the later more sophisticated models of the atom led to some very serious problems |
What did Rutherford discover about the atom?
Rutherford was interested in radio-chemistry (radioactivity) and what it might reveal about the atom. is focus was on alpha particles (from alpha rays, type of radioactivity), which he then discovered consisted of two protons and two neutrons: the equivalent of a helium nucleus (He2+)
How did Thomson explain the periodicity of chemical elements?
Very soon afterwards, Thomson began to think about how to explain the periodicity of properties of the chemical elements in terms of these negatively charged corpuscles as atomic constituents. Chemical properties would thus have to depend on the number and constellations of these corpuscles inside the atom.
Did Lockyer have a theory of a divisable atom?
Lockyer (1881) had advanced a theory of a divisable atom to explain the different stellar spectra. Interestingly, Lockyer (1897) had published his new theory in March 1897 just 1 month before Thomson proposed his corpuscle hypothesis.
What is Dalton's atomic theory?
In a previous article on Dalton's atomic theory, we discussed the following postulates: All matter is made of indivisible particles called atoms, which cannot be created or destroyed. Atoms of the same element have identical mass and physical properties. or more elements. All chemical reactions involve the rearrangement of atoms.
Overview
Thomson's cathode ray experiment and Rutherford's gold foil experiment khanacademy.org
Key points
•J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. •Thomson's plum pudding model of the atom had negatively-charged electrons embedded within a positively-charged "soup." •Rutherford's gold foil experiment showed that the atom is mostly empty space with a tiny, dense, positively-charged nucleus. •Based on these results, Rutherford proposed the nuclear model of the atom. khanacademy.org
Introduction: Building on Dalton's atomic theory
In a previous article on Dalton's atomic theory, we discussed the following postulates: •All matter is made of indivisible particles called atoms, which cannot be created or destroyed. •Atoms of the same element have identical mass and physical properties. •Compounds are combinations of atoms of 2 or more elements. •All chemical reactions involve the rearrangement of atoms. Dalton's ideas proved foundational to modern atomic theory. However, one of his underlying assumptions was later shown to be incorrect. Dalton thought that atoms were the smallest units of matter− tiny, hard spheres that could not be broken down any further. This assumption persisted until experiments in physics showed that the atom was composed of even smaller particles. In this article, we will discuss some of the key experiments that led to the discovery of the electron and the nucleus. khanacademy.org
J.J. Thomson and the discovery of the electron
In the late 19th century, physicist J.J. Thomson began experimenting with cathode ray tubes. Cathode ray tubes are sealed glass tubes from which most of the air has been evacuated. A high voltage is applied across two electrodes at one end of the tube, which causes a beam of particles to flow from the cathode (the negatively-charged electrode) to the anode (the positively-charged electrode). The tubes are called cathode ray tubes because the particle beam or "cathode ray" originates at the cathode. The ray can be detected by painting a material known as phosphors onto the far end of the tube beyond the anode. The phosphors spark, or emit light, when impacted by the cathode ray. To test the properties of the particles, Thomson placed two oppositely-charged electric plates around the cathode ray. The cathode ray was deflected away from the negatively-charged electric plate and towards the positively-charged plate. This indicated that the cathode ray was composed of negatively-charged particles. Thomson also placed two magnets on either side of the tube, and observed that this magnetic field also deflected the cathode ray. The results of these experiments helped Thomson determine the mass-to-charge ratio of the cathode ray particles, which led to a fascinating discovery− the mass of each particle was much, much smaller than that of any known atom. Thomson repeated his experiments using different metals as electrode materials, and found that the properties of the cathode ray remained constant no matter what cathode material they originated from. From this evidence, Thomson made the following conclusions: •The cathode ray is composed of negatively-charged particles. •The particles must exist as part of the atom, since the mass of each particle is only ∼ 12000 the mass of a hydrogen atom. •These subatomic particles can be found within atoms of all elements. khanacademy.org
The plum pudding model
Thomson knew that atoms had an overall neutral charge. Therefore, he reasoned that there must be a source of positive charge within the atom to counterbalance the negative charge on the electrons. This led Thomson to propose that atoms could be described as negative particles floating within a soup of diffuse positive charge. This model is often called the plum pudding model of the atom, due to the fact that its description is very similar to plum pudding, a popular English dessert (see image below). Given what we know now about the actual structure of atoms, this model might sound a little far-fetched. Luckily, scientists continued to investigate the structure of the atom, including testing the validity of Thomson's plum pudding model. Concept check: Thomson proposed an atomic model with distinct negative charges floating within a "sea" of positive charge. Can you think of another model of the atom that would explain Thomson's experimental results? [Show the answer] khanacademy.org
Ernest Rutherford and the gold foil experiment
The next groundbreaking experiment in the history of the atom was performed by Ernest Rutherford, a physicist from New Zealand who spent most of his career in England and Canada. In his famous gold foil experiment, Rutherford fired a thin beam of α particles (pronounced alpha particles) at a very thin sheet of pure gold. Alpha particles are helium nuclei (24He2+) , and they are given off in various radioactive decay processes. In this case, Rutherford placed a sample of radium (a radioactive metal) inside a lead box with a small pinhole in it. Most of the radiation was absorbed by the lead, but a thin beam of α particles escaped out of the pinhole in the direction of the gold foil. The gold foil was surrounded by a detector screen that would flash when hit with an α particle. [Why was the foil made out of gold? Couldn't he have saved a buck and used nickel?] Based on Thomson's plum pudding model, Rutherford predicted that most of the α particles would pass straight through the gold foil. This is because the positive charge in the plum pudding model was assumed to be spread out throughout the entire volume of the atom. Therefore, the electric field from the positively charged "soup" would be too weak to significantly affect the path of the relatively massive and fast-moving α particles. The results of the experiment, however, were striking. While almost all of the α particles passed straight through the gold foil, a few α particles (about 1 in 20 ,000 ) were deflected more than 90∘ from their path Rutherford himself described the results with the following analogy: "It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15 -inch shell at a piece of tissue paper and it came back and hit you." khanacademy.org
The nuclear model of the atom
Based on his experimental results, Rutherford made the following conclusions about the structure of the atom: •The positive charge must be localized over a very tiny volume of the atom, which also contains most of the atom's mass. This explained how a very small fraction of the α particles were deflected drastically, presumably due to the rare collision with a gold nucleus. •Since most of the α particles passed straight through the gold foil, the atom must be made up of mostly empty space This led Rutherford to propose the nuclear model, in which an atom consists of a very small, positively charged nucleus surrounded by the negatively charged electrons. Based on the number of α particles deflected in his experiment, Rutherford calculated that the nucleus took up a tiny fraction of the volume of the atom. khanacademy.org
Summary
•J.J. Thomson's experiments with cathode ray tubes showed that all atoms contain tiny negatively charged subatomic particles or electrons. •Thomson proposed the plum pudding model of the atom, which had negatively-charged electrons embedded within a positively-charged "soup." •Rutherford's gold foil experiment showed that the atom is mostly empty space with a tiny, dense, positively-charged nucleus. •Based on these results, Rutherford proposed the nuclear model of the atom. khanacademy.org
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Atomic Theory Timeline
Atomic Theory Timeline. Democritus. ~450 BC. John Dalton. 1803. Michael Faraday. 1839. J. J. Thomson. 1896. Robert Millikan. 1909. Ernest Rutherford. 1909. |
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CHAP 4.pmd
For explaining this many scientists proposed various atomic models. J.J. Thomson was the first one to propose a model for the structure of an atom. 4.2.1 |
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Iesc104.pdf
For explaining this many scientists proposed various atomic models. J.J. Thomson was the first one to propose a model for the structure of an atom. 4.2.1 |
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StrUctUre of atom objectives
Thus we can conclude that electrons are basic constituent of all the atoms. 2.1.2 charge to mass ratio of electron. In 1897 |
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STRUCTURE OF ATOM
Thus we can conclude that electrons are basic constituent of all the atoms. 2.1.2 Charge to Mass Ratio of Electron. In 1897 |
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ATOMIC STRUCTURE
•There was no Atomic Theory before Democritus - he 3. * Some alpha particles reflected. = nucleus is positively charged. JJ Thomson's Plum pudding model. |
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Niels Bohrs Second Atomic Theory
the atomic theory. From 1897 when J. J. Thomson proposed a con nection between atomic structure and the periodic system |
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Chapter Twelve ATOMS
In other words what is the structure of an atom? The first model of atom was proposed by J. J. Thomson in 1898. According to this model |
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Niels Bohr and the Atomic Structure
Atomic structure Bohr's model. Niels Bohr developed his model of the atomic structure in 1913 that succeeded in explaining the spectral features of hydrogen. |
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41 Chapter 4 Atomic Structure 4.1 The Nuclear Atom J. J. Thomson
Alpha particles are helium atoms minus their electrons so they have a charge of +2e. In the Thomson model |
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The History of the Atom
atom. He called this the theory of the universe: All matter consists of atoms J.J Thomson was a physicist who is credited for discovering the electron. |
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Atomic Theory Timeline
Atomic Theory Timeline. Democritus. ~450 BC. John Dalton. 1803. Michael Faraday. 1839. J. J. Thomson. 1896. Robert Millikan. 1909. Ernest Rutherford. |
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Atomic Theory Timeline
The atomic theory has changed over time as new technologies Thomson. 1897. Information. Atomic Model. Analogy. In 1897 the English scientist named J.J.. |
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Asymptotic Freedom Atomic Model Atomic Models J.J. Thomsons
Atomic Model. See also: Bohr's Atomic Model; Rutherford Atom. Atomic Models J.J. Thomson's. “Plum Pudding” Model. Klaus Hentschel. |
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Niels Bohrs Second Atomic Theory
the atomic theory. From 1897 when J. J. Thomson proposed a con nection between atomic structure and the periodic system |
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J. J. Thomson and the Bohr atom
of Niels Bohr's revolutionary quantum theory of the atom. John L. Heilbron. In 1911 Niels Bohr went to Cambridge hoping to talk physics with J. J. Thomson;. |
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Atomic Theory of Matter
Neutrons were discovered by James. Chadwick in 1932. Subatomic Particles. 1 amu is defined as 1/12 the mass of an unbound carbon atom carbon-12 at its |
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41 Chapter 4 Atomic Structure 4.1 The Nuclear Atom J. J. Thomson
Alpha particles are helium atoms minus their electrons so they have a charge of +2e. In the Thomson model |
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Thomsons Model of an Atom
Thomson discovered negatively charged particles by cathode ray tube experiment in the year 1897. The particles were named electrons. J.J Thomson believed |
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ATOMIC STRUCTURE
JJ Thomson. 3 Points to his theory. 1. * All gases had the same mass: charge ratio. 2. * All atoms contain small charged particles called electrons. |
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Foundations of Modern Atomic Theory: Thomson, Rutherford
2009 — These students had already studied in their course that J J Thomson (1897) had performed experiments to |
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Atomic theories - Junta de Andalucia
ng to these hypotheses Thomson could explain the electrostatic experiments results The explanation was easy: Electrons move from one to another, as a result, the atom that has lost the electron is a positively charged atom, while the atom that has received the electron is a negatively charged atom |
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The History of the Atom
lton was the first to adapt Democritus' theory into the first modern atomic model J J Thomson was |
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Atomic Theory & The Periodic Table
omson – Bri sh scien st in the 1800's • Showed that Dalton's theory had a flaw – atoms have parts |
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Atomic Theory Information Book
Theory Timeline Scientific knowledge builds on Thomson 1897 Information Atomic Model Analogy In 1897, the English scientist named J J Thomson provided the first hint |
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41 Chapter 4 Atomic Structure 41 The Nuclear Atom J J
suggested (1898) that atoms consist of positively charged lumps of matter with electrons |
/sir-joseph-john-thomson-physicist-and-inventor-1900-463924223-58924a5c5f9b5874eee83183.jpg "22 Evolution of Atomic Theory – Chemistry")
