History of our Conceptualization of the Atom



History of our Conceptualization of the Atom

Ancient atomism

From the 6th century BC, Hindu, Buddhist and Jaina philosophers in ancient India developed the earliest atomic theories. The first philosopher who formulated ideas about the atom in a systematic manner was Kanada who lived in the 6th century BC. Another Indian philosopher, Pakudha Katyayana who also lived in the 6th century BC and was a contemporary of Gautama Buddha, had also propounded ideas about the atomic constitution of the material world. Indian atomists believed that an atom could be one of up to six elements, with each element having up to 24 properties. They developed detailed theories of how atoms could combine, react, vibrate, move, and perform other actions, and had particularly elaborate theories of how atoms combine, which explains how atoms first combine in pairs, and then group into trios of pairs, which are the smallest visible units of matter. This parallels with the structure of modern atomic theory, in which pairs or triplets of supposedly fundamental quarks combine to create most typical forms of matter. They had also suggested the possibility of splitting an atom which, as we know today, is the source of atomic energy.

Democrites

… was a pre-Socratic Greek philosopher (born around 450 BC; died in about 370 BC). Democritus was a student of Leucippus, and co-originator of the belief that all matter is made up of various imperishable indivisible (or perhaps undivided) elements which he called "atoma", from which we get the English word atom. It is virtually impossible to tell which of these ideas were unique to Democritus, and which are attributable to Leucippus.

Atoms and the void

Democritus agreed that everything which is must be eternal, but denied that "the void" can be equated with nothing. This makes him the first thinker on record to argue for the existence of an entirely empty "void". In order to explain the change around us from basic, unchangeable substance he argued that there are various basic elements which always existed but can be rearranged into many different forms. He argued that atoms only had several properties, particularly size, shape, and (perhaps) weight; all other properties that we attribute to matter, such as color and taste, are but the result of complex interactions between the atoms in our bodies and the atoms of the matter that we are examining. Furthermore, he believed that the real properties of atoms determine the perceived properties of matter--for example, something that tastes sharp is made of small, pointy atoms, while something sweet is made of large, round atoms; the interactions of those atoms with the atoms of the tongue give the impression of taste. Some types of matter are particularly solid because their atoms have hooks to attach to each other; some are oily because they are made of very fine, small atoms which can easily slip past each other. In Democritus' own words, "By convention sweet, by convention bitter, by convention hot, by convention cold, by convention colour: but in reality atoms and void."

Aristotle (384-322 B.C, Greece.)

Aristotelian discussions about science had only been qualitative, not quantitative. By the modern definition of the term, Aristotelian philosophy was not science, as this worldview did not attempt to probe how the world actually worked through experiment. For example, in his book The History of Animals he claimed that human males have more teeth than females. Had he only made some observations, he would have discovered that this claim is false.

Rather, based on what one's senses told one, Aristotelian philosophy then depended upon the assumption that man's mind could elucidate all the laws of the universe, based on simple observation (without experimentation) through reason alone.

Artistotle's Elements

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Aristotle added later another "element" - Ether which was a perfect substance an what the heavenly bodies are composed of) The qualities are: hot, cold, wet, dry. The qualities define the character of "elements". Fire was seen as ideal mixture of hotness & dryness. One element could be changed into another like mixing solutions. Combustion and other chemical reactions were considered a type of motion. Aristotle's elements differed from alchemical elements. He believed that Arsenic was actually a kind of sulfur, and it was used in his time to harden copper. Aristotle along with other Greek Philosophers questioned whether matter was composed of divisible or indivisible particles. To Aristotle, all material was made of atoms with the exception of the Prime Mover(God). He also argues that there would be no motion at all unless there is first a force of movement that is itself unmoved, namely God.

Some existing theories about atoms:

[pic]All existing things were atoms or empty space.

[pic]Atoms were seen as all being the same size like grains of sand, always in constant motion.

[pic]Atoms were building blocks while elements were fundamental substances.

Relation of the atom and element was never developed because of early rejection of the atomic concept. The Pythagoreans gave up atomic concept because irrational numbers were involved. Later, during the golden age of Arabic science(the 8th & 11th century), the ideas of Aristotle were modified.

John Dalton , 1766 –1844, a British chemist and physicist

The most important of all Dalton's investigations are those concerned with the atomic theory in chemistry, with which his name is inseparably associated. A study of Dalton's own laboratory notebooks concluded that so far from Dalton being led to the idea, that chemical combination consists in the interaction of atoms of definite and characteristic weight, by his search for an explanation of the law of multiple proportions, the idea of atomic structure arose in his mind as a purely physical concept, forced upon him by study of the physical properties of the atmosphere and other gases. The first published indications of this idea are to be found at the end of his paper on the absorption of gases already mentioned, which was read on October 21, 1803 though not published till 1805. Here he says:

"Why does not water admit its bulk of every kind of gas alike? This question I have duly considered, and though I am not able to satisfy myself completely I am nearly persuaded that the circumstance depends on the weight and number of the ultimate particles of the several gases."

He proceeds to give what has been quoted as his first table of atomic weights, but in his laboratory notebooks[2] there is an earlier one dated 1803 in which he sets out the relative weights of the atoms of a number of substances, derived from analysis of water, ammonia, carbon dioxide, etc. by chemists of the time.

Many of Dalton's ideas were acquired from other chemists at the time, such as Antoine Lavoisier and William Higgins. However, he was the first to put the ideas into a universal atomic theory, which was undoubtedly his greatest achievement.

Five main points of Dalton's Atomic Theory

• Elements are made of tiny particles called atoms

• All atoms of a given element are identical

• The atoms of a given element are different from those of any other element

• Atoms of one element can combine with atoms of other elements to form compounds. A given compound always has the same relative numbers of types of atoms.

• Atoms cannot be created, divided into smaller particles, or destroyed in the chemical process. A chemical reaction simply changes the way atoms are grouped together.

Unfortunately, Dalton had an additional statement that prevented his theory from being accepted for many years.

When atoms combine in only one ratio, "..it must be presumed to be a binary one, unless some cause appear to the contrary"

Dalton had no evidence to support this statement from his theory and it caused him to wrongly assume that the formula for water was OH and ammonia was NH. Because of this Daltons experimental data did not support most of the conclusions he drew from it.

Amazingly, all but two of the statements in Dalton's Atomic Theory are still believed to be true by scientists today. The statement "Atoms cannot be created, divided into smaller particles, or destroyed" is inconsistent with the existence of nuclear fusion and fission, although such processes are nuclear reactions, not chemical reactions. In addition, the statement "all atoms of a given element are identical" is not precisely true, as the different isotopes of an element have varying numbers of neutrons in their nuclei, though the number of protons remains consistent.

Joseph John Thomson , 1856 – 1940, often known as J. J. Thomson, was an English physicist, the discoverer of the electron.

One hundred years ago, amidst glowing glass tubes and the hum of electricity, the British physicist J.J. Thomson was venturing into the interior of the atom. At the Cavendish Laboratory at Cambridge University, Thomson was experimenting with currents of electricity inside empty glass tubes. He was investigating a long-standing puzzle known as "cathode rays." His experiments prompted him to make a bold proposal: these mysterious rays are streams of particles much smaller than atoms, they are in fact minuscule pieces of atoms. He called these particles "corpuscles," and suggested that they might make up all of the matter in atoms. It was startling to imagine a particle residing inside the atom--most people thought that the atom was indivisible, the most fundamental unit of matter.

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Ernest Rutherford, 1871 – 1937, was a nuclear physicist from New Zealand. He was known as the "father" of nuclear physics, pioneered the orbital theory of the atom, notably in his discovery of Rutherford scattering off the nucleus with the gold foil experiment.

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Rutherford's find came from a very strange experience. Everyone at that time imagined the atom as a "plum pudding." That is, it was roughly the same consistency throughout, with negatively-charged electrons scattered about in it like raisins in a pudding. As part of an experiment with x-rays in 1909, Rutherford was shooting a beam of alpha particles (or alpha rays, emitted by the radioactive element radium) at a sheet of gold foil only 1/3000 of an inch thick, and tracing the particles' paths. Most of the particles went right through the foil, which would be expected if the atoms in the gold were like a plum pudding. But every now and then, a particle bounced back as though it had hit something solid. After tracing many particles and examining the patterns, Rutherford deduced that the atom must have nearly all its mass, and positive charge, in a central nucleus about 10,000 times smaller than the atom itself. All of the negative charge was held in the electrons, which must orbit the dense nucleus like planets around the sun.

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He was fiercely anti-Nazi, and in 1933 he served as president of the Academic Assistance Council, established to help German refugees. He would not personally help chemist Fritz Haber, however, who had been instrumental in creating chemical weapons in World War I. Rutherford died two years before the discovery of atomic fission.

"All science is either physics or stamp collecting."

Niels Bohr, 1885- 1962, was a Danish physicist who made essential contributions to understanding atomic structure and quantum mechanics.

In 1912 Bohr joined Rutherford. He realized that Rutherford's model wasn't quite right. By all rules of classical physics, it should be very unstable. For one thing, the orbiting electrons should give off energy and eventually spiral down into the nucleus, making the atom collapse. Or the electrons could be knocked out of position if a charged particle passed by. Bohr turned to Planck's quantum theory to explain the stability of most atoms. Bohr suggested the revolutionary idea that electrons "jump" between energy levels (orbits) in a quantum fashion, that is, without ever existing in an in-between state. Thus when an atom absorbs or gives off energy (as in light or heat), the electron jumps to higher or lower orbits. Bohr published these ideas in 1913 to mixed reaction. Many people still hadn't accepted the idea of quanta, or they found other flaws in the theory because Bohr had based it on very simple atoms. But there was good evidence he was right: the electrons in his model lined up with the regular patterns (spectral series) of light emitted by real hydrogen atoms.

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Bohr's theory that electrons existed in set orbits around the nucleus was the key to the periodic repetition of properties of the elements. The shells in which electrons orbit have different quantum numbers and hold only certain numbers of electrons -- the first shell holds no more than 2, the second shell up to 8, the third 10, the fourth 14. Atoms with less than the maximum number in their outer shells are less stable than those with "full" outer shells. Elements that have the same number of electrons in their outermost shells appear in the same column in the periodic table of elements and tend to have similar chemical properties.

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Over the years other investigators refined Bohr's theory, but his bold application of new ideas paved the way for the development of quantum mechanics. Bohr went on to make enormous contributions to physics and, like Rutherford, to train a new generation of physicists. But his atomic model remains the best known work of a very long career.

Quantum Mechanics: Max Planck, Albert Einstein, Niels Bohr, Werner Heisenberg, Erwin Schrödinger, Max Born, John von Neumann, Paul Dirac, Wolfgang Pauli, first half of 20th century

Quantum mechanics is a more fundamental theory than Newtonian mechanics and classical electromagnetism, in the sense that it provides accurate and precise descriptions for many phenomena that these "classical" theories simply cannot explain on the atomic and subatomic level. It is necessary to use quantum mechanics to understand the behavior of systems at atomic length scales and smaller. For example, if Newtonian mechanics governed the workings of an atom, electrons would rapidly travel towards and collide with the nucleus. However, in the natural world the electron normally remains in a stable orbit around a nucleus -- seemingly defying classical electromagnetism.

Quantum mechanics was initially developed to explain the atom, especially the spectra of light emitted by different atomic species. The quantum theory of the atom developed as an explanation for the electron's staying in its orbital, which could not be explained by Newton's laws of motion and by classical electromagnetism.

Broadly speaking, quantum mechanics incorporates four classes of phenomena that classical physics cannot account for: (i) the quantization (discretization) of certain physical quantities, (ii) wave-particle duality, (iii) the uncertainty principle, and (iv) quantum entanglement. Each of these phenomena will be described in greater detail in subsequent sections.

String Theory

String theory was originally invented to explain peculiarities of hadron (subatomic particle which experiences the strong nuclear force) behavior. In particle-accelerator experiments, physicists observed that the spin of a hadron is never larger than a certain multiple of the square of its energy. No simple model of the hadron, such as picturing it as a set of smaller particles held together by spring-like forces, was able to explain these relationships. In 1968, theoretical physicist Gabriele Veneziano was trying to understand the strong nuclear force when he made a startling discovery. Veneziano found that a 200-year-old formula created by Swiss mathematician Leonhard Euler (the Euler beta function) matched the data then current on the strong force. Veneziano applied the Euler beta function to the strong force, but no one could explain why it fit well.

In 1970, Yoichiro Nambu, Holger Bech Nielsen, and Leonard Susskind presented a physical explanation for Euler's strictly mathematical formula. By representing nuclear forces as vibrating, one-dimensional strings, these physicists showed how Euler's function accurately described those forces. But even after physicists understood the physical explanation for Veneziano's insight, the string description of the strong force made many predictions that directly contradicted experimental findings. The scientific community soon lost interest in string theory, and the standard model, with its particles and fields, remained the main focus of theoretical research.

Roughly between 1984 and 1986, physicists realized that string theory could describe all elementary particles and interactions between them, and hundreds of them started to work on string theory as the most promising idea to unify theories of physics. This first superstring revolution was started by a discovery of anomaly cancellation in type I string theory by Michael Green and John Schwarz in 1984. The anomaly is cancelled due to the Green-Schwarz mechanism. Several other ground-breaking discoveries, such as the heterotic string, were made in 1985.

In the 1990s, Edward Witten and others found strong evidence that the different superstring theories were different limits of a new 11-dimensional theory called M-theory. These discoveries sparked the second superstring revolution. When Witten named it M-theory, he did not specify what the "M" stood for, presumably because he did not feel he had the right to name a theory which he had not been able to fully describe. Guessing what the "M" stands for has become a kind of game among theoretical physicists. The "M" sometimes is said to stand for Mystery, or Magic, or Mother.

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