The Atom's Ancestry

1 The Atoms Ancestry What in the world is an atom? Or, more appropriately; what in the world is not an atom? Air, water, earth, people, robots—everything is made up of atoms. As early as 500 B.C. the Greeks speculated that matter can be split into smaller and smaller bite, but they expected a limit, beyond which it could not be further subdivided. Etymology of Atom We come to know from Aristotle that the founder of the atomic theory was Democritus and Leucippus. The word ‘atom’ is derived from atomos, ‘a’ = not and tomos = a cut; thereby meaning ‘indivisible’. The concept of atom that Western scientists accepted in broad outline from 1600s until 1900 originated with Greek philosophers in the 5th century. The atom was described as being hard; having a form, size, and weight and being in ceaseless motion. This speculation was replaced slowly by scientific theory supported by experiments and mathematical deductions. The Atomic Philosophy of Early Greeks Leucippus of Miletus is thought to have originated the atomic philosophy. His famous disciple, Democritus of Abdera, named the building blocks of matter. He believed that atoms were uniform, solid, hard, incompressible, and indestructible and keep moving in empty space in infinite numbers till stopped. Differences in atomic shape and size determined the various properties of matter. In Democritus’s philosophy, atoms existed not only for matter but also for such qualities as perception and the human soul. For example, sourness was supposed to be caused by needle-shaped atoms while the color white was believed to be composed of smooth- surfaced atoms. Very fine atoms were considered to be forming the soul. This atomic philosophy was developed as a middle ground between two opposing Greek theories about reality and illusion of change. He argued that matter was subdivided into indivisible and immutable particles that created the appearance of change when they joined and separated from others. The philosopher Epicurus of Samos (341–270 B.C.) used Democritus’s ideas to subside the fears of superstitious Greeks. According to Epicurus’s materialistic theory, the entire universe was composed of atoms and voids and so even the Gods were subjected to Laws of Nature. The great Latin poet Titus Lucretius Carus (57 B.C.) gave a faithful picture of the atom as solid seed in his long poem ‘De Rerum Natura’(‘On the Nature of Things’), 2 NUCLEAR SCIENCE as quoted partly here: “But solid seeds exist. Which fill their place; And make a difference between full and space. These, as I proved before, no active flame, No subtle cold can pierce and break their frame. Tho’ every compound yields: no powerful blow, No subtle wedge divide, or break in four. For nothing can be exposed, No part destroyed by force; Or cleft without a void, And things that hold most void, When strokes do squeeze, Or subtle wedges enter, yield with ease. If seeds then solid are, they must endure Eternally, from force, from stroke secure.” That was the explanation of an atom two thousand years ago. The Greek atomic theory is significant historically and philosophically, it has no scientific value as it was not based on observations of nature, measurements, tests, or experiments. Instead, Greeks used mathematics and reason almost exclusively when they wrote about science. They wanted an all-encompassing theory to explain the universe and not merely a detailed view of as tiny portion of it as an atom. Thus Plato and Aristotle attacked Democritus’s theory of atom on philosophical grounds rather than on scientific grounds. Plato valued abstract ideas more and so he rejected the notion that attributes qualities like beauty and goodness to “mechanical manifestations of material atoms”. Democritus believed that matter can only move through a vacuum and light was the rapid movement of particles through a void. Aristotle rejected the existence of vacuum. The Emergence of Experimental Science The poem De Rerum Natura, which was rediscovered in the 15th century helped fuel a debate between orthodox people belonging to Roman Catholics, guided by Aristotle’s philosophy and new scientific experimenters in 17th century. The poem was popularized by Pierre Gassendi, a French priest who tried to separate Epicurus’s atomism from its materialistic background by telling that God created the atoms. Today, as science has advanced so much, chemists and physicists have turned their thoughts towards the atom. Soon after Galileo Galilei expressed that vacuums can exist, scientists began studying properties of air and partial vacuum to test the relative merits of orthodox views and the atomic theory. Robert Boyle (1627–91), an Anglo-Irish chemist and Sir Isaac Newton (1642–1726) are well known scientists to the world, who were the first to suggest that matter was composed of tiny particles which could not be split up into smaller particles arranged into molecules to give material its different properties were at man’s disposal. In the early 18th century, Sir Isaac Newton expressed his views of the atom that was similar to that of Democritus Gassendi and Boyle. THE ATOM’S ANCESTRY 3 Modern Atomic Theory The English chemist and physicist John Dalton converted the atomic philosophy of the Greeks into a scientific theory between 1803 and 1808. His book A New System of Chemical Philosophy (part I in 1808 and part II in 1810) was the first application of atomic theory to chemists. CHAPTER 1 CHAPTER 1.1 DALTON’S ATOMIC THEORY As a result of his experiments on gases, John Dalton (1766–1844) gave the famous theory (1803), which in simple language is as follows: (i) All elements are made up of minute particles (he named them atoms) which are indestructible and impenetrable. (ii) The atoms are identical in weight, size and all other physical and chemical properties for one given element, but differ from those of the other elements. (iii) Chemical combination occurs through the union of these atoms in simple numerical ratio. The above theory provided a physical picture of how elements combine to form compounds and a phenomenological reason for believing that the atoms exist. Since no other scientist of the above period presented any conflicting evidence, the theory was accepted until the close of 19th century. It is not surprising that the theory provided the foundation for the remarkable progress made in this new branch of science, dealing with Nuclear Energy. Size of the Atom The first estimate of the size of atoms and the number of atoms in a given volume were made by the German chemist Joseph Loschmidt in 1865. He used the results of Kinetic theory and some rough estimates to do his calculations. The size of the atoms and the distance between them in the gaseous state are related both to the contraction of the gas upon liquefaction and to the mean free path travelled by the molecules in the gas. Loschmidt found the size of the atom and spacing between the atoms by finding a common solution to these relationships. His result for the number of atoms in 12 grams of the carbon was 6.022 × 1023 which was remarkably close to the presently accepted value of Avogadro’s number. His result for the diameter of an atom was approximately 10–8 cm. 1.2 INTRODUCING THE ATOM How was any one going to be able to pry into anything so small as an atom? We are told that if these tiny particles are placed side by side, 250,000,000 hydrogen atoms would make a queue one inch long. (The diameter of a Hydrogen atom is found to be 10–9 cm, and that of the nucleus is 10–14 cm.) Up to the last century scientists believed that the atom was impenetrable and indivisible. But the accumulating experimental evidence suggested that this view was incorrect. 4 NUCLEAR SCIENCE Discovery of Electron In 1858, Julius Plucker, on passing an electric discharge through a gas at low pressure in a vacuum tube (Fig. 1.1), observed phosphorescence near the anode. It was a strange looking glass tube with two electrodes. These electrodes were metal plates called the cathode and anode, which were connected to a source of high voltage. The tube also had an arrangement for evacuating air, in order to obtain a high vacuum or very low pressure inside the tube. This visible discharge was named ‘cathode rays’ as it originated from cathode. The English physicist and chemist William Crookes investigated cathode rays in 1879 and found that they were bent by a magnetic and electrical fields. J.J. Thomson found in 1897 that the deflection was proportional to the difference in potential between the aluminium plates serving as electrodes. His discovery also established the particulate nature of cathode rays. The direction of the deflection suggested that they were negatively charged particles. Accordingly, he named the particles electrons. The cathode rays consisted of a stream of small particles like miniature bullets shot from the cathode at terrific velocity, in a straight line and have high penetrating power. Each of these particles was observed to have a negative charge which was equal but opposite to that of the hydrogen ion, and a mass which was approximately 1/1840 of that of the hydrogen atom; i.e., put a decimal point before 27 zeros and then the numbers 9. This is the mass in grams. Each particle has a diameter 1/40,000 of an atom. These particles are now called electrons, and they are the universal constituent of all matter. – Fig. 1.1. Electric discharge tube From the magnitude of the electrical and magnetic deflections, Thomson could calculate the ratio of charge to mass (e/m) for the electrons.

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