Discovery of the
The Atom
Democritus
Democritus (460-370 BC) Greek natural philosopher
Democritus did not create the atomic theory; he learned it from its founder, Leucippus, the author of the Big Cosmology. He opposed the atomic theory, but in doing so he summarized its principals. Therefore he attributed to Leucippus the ideas that the atoms are "infinite in number and unnoticeable because of their small size. They move about in empty space and by joining together they produce definite objects, which are destroyed when the atoms separate." The point at which Leucippus’ expansion of the atomic theory stopped and Democritus’ contributions to it began can no longer be identified. We also now know that that theory is relatively incorrect.
Perhaps according to both of them and certainly according to Democritus, the atom was the smallest part of matter. The concept of the infinite divisibility of matter was flatly contradicted by the atomic theory, since within the interior of the atom there could be no physical parts or unoccupied space. Every atom was exactly like every other atom. But the atoms differed in shape, and since their contours showed an infinite variety and could be oriented in any direction and arranged in any order, the atoms could enter into countless combinations. In their solid interior there was no motion, while they themselves could move about in empty space. Thus, for the atomic theory, the physical universe had two basic ingredients: impenetrable atoms and penetrable space. For Democritus, space was infinite in amount, and the atoms were infinite in number.
By their very nature the atoms were endowed with a motion that was eternal and not initiated by any outside force. Since the atoms were not created at any time in the past and would never disintegrate at any time in the future, the total quantity of matter in the universe remained constant: this fundamental principle of Democritus’ atomic theory implies the conservation of matter, the sum total of which in the universe neither increases nor diminishes. Though Democritus’ formation of the atom has been modified in several essential respects in modern times, his atomic theory remains the foundation of modern science. As the founder of the atomic theory declared in his only surviving statement, "Nothing occurs at random, but everything happens for a reason and by necessity."
Experiments
Democritus had no instruments to extend the reach of his senses, so all of his experiments were just "mind experiments." The ancient Greeks gave humanity tremendous gifts despite only having their minds to work with. We humans tend to be sceptical and want proof before we believe in most things, so thousands of years passed before we could prove his theories. Also, being only "mind experiments," they really don't quantify or say much about the nature of matter. Their theories tended to be very general. So although he was correct that there was this thing that could not really be cut any further, he could not tell you all the things we know about atoms, like the mass, charge or what they were made of.
John Dalton
John Dalton (1766 – 1844) English chemist, meteorologist and physicist
The idea of atoms had been proposed much earlier. The ancient Greek philosophers had talked about atoms, but Dalton's theory was different in that it had the weight of careful chemical measurements behind it. It wasn't just a philosophical statement that there are atoms because there must be atoms. His atomic theory stated that elements consisted of tiny particles called atoms. He said that the reason an element is pure is because all atoms of an element were identical and that in particular they had the same mass. He also said that the reason elements differed from one another was that atoms of each element were different from one another; in particular, they had different masses. He also said that compounds consisted of atoms of different elements combined together. Compounds are pure substances because the atoms of different elements are bonded to one another somehow, perhaps by hooks, and are not easily separated from one another. Compounds have constant composition because they contain a fixed ratio of atoms and each atom has its own characteristic weight, thus fixing the weight ratio of one element to the other. In addition he said that chemical reactions involved the rearrangement of combinations of those atoms.
Experiment
Dalton made it a regular habit to track and record the weather in his home town of Manchester, England. Through his observations of morning fog and other weather patterns, Dalton realized that water could exist as a gas that mixed with air and occupied the same space as air. Solids could not occupy the same space as each other; for example, ice could not mix with air. So what could allow water to sometimes behave as a solid and sometimes as a gas? Dalton realized that all matter must be composed of tiny particles. In the gas state, those particles floated freely around and could mix with other gases, as Bernoulli had proposed. But Dalton extended this idea to apply to all matter – gases, solids and liquids.
Dalton's atomic theory makes the following assumptions:
· All matter consists of tiny particles.
· Atoms are indestructible and unchangeable.
· Elements are characterized by the mass of their atoms.
· When elements react, their atoms combine in simple, whole-number ratios.
* Today we know that atoms can be destroyed via nuclear reactions but not by chemical reactions. Also, there are different kinds of atoms (differing by their masses) within an element that is known as "isotopes", but isotopes of an element have the same chemical properties. Many unexplained chemical phenomena were quickly explained by Dalton with his theory. Dalton's theory quickly became the theoretical foundation in chemistry.
Michael Faraday
Michael Faraday (1791 – 1867) English chemist and physicist
Michael Faraday discovered electromagnetic induction, the principle behind the electric transformer and generator. This discovery was crucial in allowing electricity to be transformed from a curiosity into a powerful new technology. During the remainder of the decade he worked on developing his ideas about electricity. He was partly responsible for coining many familiar words including 'electrode', 'cathode' and 'ion'.
Faraday studied the magnetic field around a conductor carrying a DC electric current, and established the basis for the magnetic field concept in physics. He discovered electromagnetic induction, diamagnetism, and laws of electrolysis. He established that magnetism could affect rays of light and that there was an underlying relationship between the two phenomena. His inventions of electromagnetic rotary devices formed the foundation of electric motor technology, and it was largely due to his efforts that electricity became viable for use in technology.
As a chemist, Faraday discovered benzene, investigated the clathrate hydrate of chlorine, invented an early form of the Bunsen burner and the system of oxidation numbers, and popularized terminology such as anode, cathode, electrode, and ion. Although Faraday received little formal education and knew little of higher mathematics, such as calculus, he was one of the most influential scientists in history. Some historians of science refer to him as the best experimentalist in the history of science. The SI unit of capacitance, the farad, is named after him, as is the Faraday constant, the charge on a mole of electrons (about 96,485 coulombs). Faraday's law of induction states that a magnetic field changing in time creates a proportional electromotive force.
Experiments
Set up a pair of metal plates sealed in a glass tube. The tube was filled with a gas, and the metal plates were connected to a series of batteries.
Cathode- Metal plate connected to the negative end.
Anode- Metal plate connected to the positive end.
Conclusion- The effect of the magnetic field as evidence that whatever produced this glow was electrically charged.
J.J.Thomson
Sir Joseph John (1856 – 1940) British physicist and Nobel laureate
Thomson's notion of the electron came from his work with a nineteenth century scientific curiosity: the cathode ray tube. For years scientists had known that if an electric current was passed through a vacuum tube, a stream of glowing material could be seen; however, no one could explain why. Thomson found that the mysterious glowing stream would bend toward a positively charged electric plate. Thomson theorized, and was later proven correct, that the stream was in fact made up of small particles, pieces of atoms that carried a negative charge. These particles were later named electrons. After Eugen Goldstein’s 1886 discovery that atoms had positive charges, Thomson imagined that atoms looked like pieces of raisin bread, a structure in which clumps of small, negatively charged electrons (the "raisins") were scattered inside a smear of positive charges. In 1908, Ernest Rutherford, a former student of Thomson's, proved Thomson's raisin bread structure incorrect.
Experiment 1
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 rays.
Experiment 2
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). 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.
Experiment 3
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.
Raisin Pudding Model
Concluded that:
· Matter is electrically neutral and electrons are much lighter than atoms.
· There must be positively charged particles which also must carry the mass of the atom. Dalton's model is now incorrect because atoms are divisible.
Summary/Conclusions
· Found that cathode rays could be deflected by an electric field.
· Showed that cathode "rays" were actually particles.
· Found the charge to mass ratio of the particles to be approximately
· 108 Coulomb (C) per gram.
· Same charge to mass ratio regardless of metal used for cathode/anode or gas used to fill the tube.
· Conclusion: Particles were a universal component of matter.
· Electron-(originally called corpuscles by Thompson) particles given off by the cathode; fundamental unit of negative electricity.
Marie Curie
Marie Skłodowska Curie (1867 – 1934) was a physicist and chemist
She was born in Warsaw, Poland and lived there until she was 24. In 1891 she followed her elder sister Bronisława to study in Paris, where she obtained her higher degrees and conducted her subsequent scientific work. She founded the Curie Institutes in Paris and Warsaw. Her achievements include the creation of a theory of radioactivity (a term coined by her), techniques for isolating radioactive isotopes, and the discovery of two new elements, polonium and radium. It was also under her personal direction that the world's first studies were conducted into the treatment of neoplasm’s’ ("cancers"), using radioactive isotopes. While an actively loyal French citizen, she never lost her sense of Polish identity. She named the first new chemical element that she discovered (1898) "polonium" for her native country, and in 1932 she founded Radium Institute (now the Maria Skłodowska–Curie Institute of Oncology) in her home town Warsaw, headed by her physician-sister Bronisława.
Experiments
The ignored uranium rays appealed to Marie Curie. Since she would not have a long bibliography of published papers to read, she could begin experimental work on them immediately. The director of the Paris Municipal School of Industrial Physics and Chemistry, where Pierre was professor of physics, permitted her to use a crowded, damp storeroom there as a lab. A clever technique was her key to success.
About 15 years earlier, Pierre and his older brother, Jacques, had invented a new kind of electrometer, a device for measuring extremely low electrical currents. Marie now put the Curie electrometer to use in measuring the faint currents that can pass through air that has been bombarded with uranium rays. The moist air in the storeroom tended to dissipate the electric charge, but she managed to make reproducible measurements.
With numerous experiments Marie confirmed Becquerel's observations that the electrical effects of uranium rays are constant, regardless of whether the uranium was solid or pulverized, pure or in a compound, wet or dry, or whether exposed to light or heat. Likewise, her study of the rays emitted by different uranium compounds validated Becquerel's conclusion that the minerals with a higher proportion of uranium emitted the most intense rays. She went beyond Becquerel's work, however, in forming a crucial hypothesis: the emission of rays by uranium compounds could be an atomic property of the element uranium--something built into the very structure of its atoms.
Ernest Rutherford
Ernest Rutherford (1871 – 1937) a New Zealand chemist and physicist
A consummate experimentalist, Ernest Rutherford was responsible for a remarkable series of discoveries in the fields of radioactivity and nuclear physics. He discovered alpha and beta rays, set forth the laws of radioactive decay, and identified alpha particles as helium nuclei. Most important, he postulated the nuclear structure of the atom: experiments done in Rutherford's laboratory showed that when alpha particles are fired into gas atoms, a few are violently deflected, which implies a dense, positively charged central region containing most of the atomic mass.
Experiment
Results
· Most alpha particles (Helium atoms minus electrons) pass right through gold fold.
· A small fraction of alphas are deflected very slightly.
· A small fraction of alphas are deflected almost
Conclusions
· The atom is mostly empty space.
· The nucleus is positively charged as is the alpha particle (we now know that it is neutrally charged).
· The nucleus carries most of the atom's mass.
Ernest Rutherford publishes his atomic theory describing the atom as having a central positive nucleus surrounded by negative orbiting electrons. This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space. Rutherford came to this conclusion following the results of his famous gold foil experiment. This experiment involved the firing of radioactive particles through minutely thin metal foils (notably gold) and detecting those using screens coated with zinc sulphide (a scintillator). Rutherford found that although the vast majority of particles passed straight through the foil approximately 1 in 8000 were deflected leading him to his theory that most of the atom was made up of 'empty space'.
He studied the following and concluded:
Studied absorption of radioactivity;
· Alpha radiation - positive charge - absorbed by a few hundredths of a cm or metal foil.
· Beta radiation - negative charge - could pass through 100x as much foil before it was absorbed.
· Gamma rays - no charge - could penetrate several cm of lead.
Studied the deflection of alpha particles as they were targeted at thin gold foil sheets;
· Most of the alpha particles penetrated straight through.
· However few were deflected at slight angles.
· Even fewer (only about 1 in 20,000) were deflected at angles over 90 degrees.
· Conclusion: The positive charge and mass of an atom were concentrated in the centre and only made up a small fraction of the total volume. He named this concentrated centre the nucleus.
· Rutherford was also able to estimate the charge of an atom by studying the deflection of alpha particles. He found that the positive charge on the atom was approximately half of the atomic weight.
James Chadwick
Sir James Chadwick (1891 – 1974) English physicist and Nobel laureate in physics
James Chadwick made a fundamental discovery in the domain of nuclear science: he discovered the particle in the nucleus of an atom that became known as the neutron because it has no electric charge. In contrast with the helium nuclei (alpha particles) which are positively charged, and therefore repelled by the considerable electrical forces present in the nuclei of heavy atoms, this new tool in atomic disintegration need not overcome any Coulomb barrier and is capable of penetrating and splitting the nuclei of even the heaviest elements. In this way, Chadwick prepared the way towards the fission of uranium 235. For this important discovery he was awarded the Hughes Medal of the Royal Society in 1932, and subsequently the Nobel Prize for Physics in 1935.
Chadwick’s discovery made it possible to create elements heavier than uranium in the laboratory. His discovery particularly inspired Enrico Fermi, Italian physicist and Nobel laureate, to discover nuclear reactions brought by slowed neutrons, and led Otto Hahn and Fritz Strassmann, German radio chemists in Berlin, to the revolutionary discovery of “nuclear fission”.
Chadwick smashed alpha particles into beryllium, a rare metallic element, and allowed the radiation that was released to hit another target: paraffin wax. When the beryllium radiation hit hydrogen atoms in the wax, the atoms were sent into a detecting chamber. In physics it is known that only a particle having almost the same mass as a hydrogen atom could affect hydrogen in that manner. The experiment results showed a collision with beryllium atoms would release massive neutral particles, which Chadwick named neutrons. This provided the answer for hidden mass in atoms.
Experiment
The search was over. Chadwick had found a new elementary particle, the third basic component of the nucleus. It increased the mass of elements without adding electrical charge. Two protons and 2 neutrons made a helium nucleus while 92 protons and 146 (or 143) neutrons made uranium, the heaviest known element. This not only changed our view of the nucleus, but also provided a new, relatively inexpensive means of probing the nucleus. Because the neutron was relatively massive but neutral, it was scarcely affected by the cloud of electrons surrounding the nucleus or by the positive electrical barrier of the nucleus itself; thus it could penetrate the nucleus of any element.
· Proved that neutrons, neutral particles in the nucleus that made up approximately half the mass of an atom, did exist.
Niels Bohr
Niels Henrik David Bohr (1885 – 1962) a Danish physicist
Niels Henrik David Bohr was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. Bohr mentored and collaborated with many of the top physicists of the century at his institute in Copenhagen. He was also part of the team of physicists working on the Manhattan Project.
Contributions to physics
· The Bohr model of the atom, the theory that electrons travel in discrete orbits around the atom's nucleus.
· The shell model of the atom, where the chemical properties of an element are determined by the electrons in the outermost orbit.
· The correspondence principle, the basic tool of Old quantum theory.
· The liquid drop model of the atomic nucleus.
· Identified the isotope of uranium that was responsible for slow-neutron fission.
· Much work on the Copenhagen interpretation of quantum mechanics.
· The principle of complementarity: Those items could be separately analyzed as having several contradictory properties.
Bohr model of the atom
Niels Bohr proposed, in 1913, what is now called the Bohr model of the atom. He suggested that electrons could only have certain classical motions:
· The electrons can only travel in special orbits: at a certain discrete set of distances from the nucleus with specific energies.
· The electrons do not continuously lose energy as they travel. They can only gain and lose energy by jumping from one allowed orbit to another, absorbing or emitting electromagnetic radiation.
· The frequency of the radiation emitted at an orbit with period T is as it would be in classical
mechanics--- it is the reciprocal of the classical orbit period:
The significance of the Bohr model is that the laws of classical mechanics apply to the motion of the electron about the nucleus only when restricted by a quantum rule.
He made the following rules for atoms:
1. Electrons can orbit only at certain allowed distances from the nucleus.
2. Atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low-energy orbit to a high-energy orbit.
Other points:
· Like Einstein's theory of the Photoelectric effect, Bohr's formula assumes that during a quantum jump a discrete amount of energy is radiated. However, unlike Einstein, Bohr stuck to the classical Maxwell theory of the electromagnetic field. Quantization of the electromagnetic field was explained by the discreteness of the atomic energy levels; Bohr did not believe in the existence of photons.
· According to the Maxwell theory the frequency ν of classical radiation is equal to the rotation frequency νrot of the electron in its orbit, with harmonics at integer multiples of this frequency. This result is obtained from the Bohr model for jumps between energy levels En and En − k when k is much smaller than n. These jumps reproduce the frequency of the k-th harmonic of orbit n. For sufficiently large values of n (so-called Rydberg states), the two orbits involved in the emission process have nearly the same rotation frequency, so that the classical orbital frequency is not ambiguous. But for small n or large k, the radiation frequency has no unambiguous classical interpretation. This marks the birth of the correspondence principle, requiring quantum theory to agree with the classical theory only in the limit of large quantum numbers.
· The Bohr-Kramers-Slater (BKS) theory is a failed attempt to extend the Bohr model which violates the conservation of energy and momentum in quantum jumps, with the conservation laws only holding on average.
The Bohr model gives almost exact results only for a system where two charged points orbit each other at speeds much less than that of light. This not only includes one-electron systems such as the hydrogen atom, singly-ionized helium, doubly ionized lithium, but it includes positronium and Rydberg states of any atom where one electron is far away from everything else.
In 1925 a new kind of mechanics was proposed, quantum mechanics in which Bohr's model of electrons travelling in quantized orbits was extended into a more accurate model of electron motion. The new theory was proposed by Werner Heisenberg. Another form of the same theory, modern quantum mechanics, was discovered by the Austrian physicist Erwin Schrödinger independently and by different reasoning.
The unearthing behind the mysteries of the atom took countless instances and men to discern...
Robin Hyder 2009
References/Bibliography
· http://www.bookrags.com/biography/democritus/
· http://dl.clackamas.edu/ch104-04/dalton's.htm
· http://www.iun.edu/~cpanhd/C101webnotes/composition/dalton.html
· http://library.thinkquest.org/C006669/data/Chem/atomic/development.html
· http://www.nobeliefs.com/atom.htm
· http://en.wikipedia.org/wiki/J._J._Thomson
· http://www.aip.org/history/curie/resbr1.htm
· http://en.wikipedia.org/wiki/Marie_Curie
· http://atropos.as.arizona.edu/aiz/teaching/nats102/mario/matterenergy.html
· http://www.rsc.org/chemsoc/timeline//pages/1911.html
· http://en.wikipedia.org/wiki/James_Chadwick
· http://answers.yahoo.com/question/index?qid=20071021081503AAIeljt
· http://chemcases.com/nuclear/nc-01.htm
· http://en.wikipedia.org/wiki/Niels_Bohr
· http://en.wikipedia.org/wiki/Bohr_model
