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The Atom’s 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.

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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. This ratio was known for the atoms from electrochemical studies. This e/m ratio was the same for any gas taken in the Crooke’s tube. Measuring this and comparing it with that for an atom Thomson discovered that the mass of an electron is very small, merely 1/1836 that of a hydrogen ion. When scientists realized that an electron is virtually 2000 times lighter than the smallest atom—hydrogen, they understood how cathode rays could penetrate metal sheets and how electric current flows through copper wires. In deriving the e/m ratio, Thomson had calculated the velocity of electron. It was 1/10 the speed of light, thus amounting to roughly 30,000 km per second. Thus, the first subatomic particle was identified, the smallest and the fastest bit of matter known at that time. These electrons do a lot of things for us. They constitute the electric current, they hop inside a radio valve, they sweep across a cathode tube to show us pictures in television, etc. About six billion electrons pass through the filament of an ordinary 100 watt lamp each second. THE ATOM’S ANCESTRY 5

In 1895, Wilhelm K. Rontgen observed that when a hard substance like tungsten (W) was bombarded by cathode rays (Fig. 1.2), a very penetrating radiation was obtained. He named this radiation ‘X-rays’. These rays are similar to light rays, but they have much smaller wavelength of the order of 1/5000 of the wavelength of visible light. These rays easily passed through a variety of substances, including human flesh. When Rontgen placed his hand in the path of the rays, he saw the clear image of the bones on the fluorescent screen. CHAPTER 1 CHAPTER

Fig. 1.2. An X-ray tube Robert Andrews Milliken an American scientist greatly improved the method employed by Thomson for measuring directly the charge of an electron. In Milliken’s oil- drop experiment (1909-1910), he produced microscopic droplets of an oil and observed them falling in the space between two electrically charged plates. Some of the droplets became charged and could be suspended by a delicate adjustment of the electric field. Milliken knew the weight of the droplets from their rate of falling when the electric field was turned off. From the balance of the gravitational and electrical forces, he could determine the charge on each of the droplets. All the measured charges were integral multiples of a quantity 1.60217733 × 10–19 coulomb. This experiment was the first to detect and measure the effect of an individual subatomic particle. For this work Milliken was awarded the Nobel Prize in 1923.

Discovery of Proton In 1886, Goldstein used energized discharge tube also called Crooke’s tube having perforated cathode (Fig. 1.3) containing a gas at a very low pressure and on passing high voltage found that, some rays coming from the side of anode passed through the holes in the perforated cathode and produced a green fluorescence on the opposite glass wall coated with Zinc Sulphide. These rays were called ‘anode rays’ or ‘canal rays’. These rays were produced as a result of knocking off the electron from gaseous atoms by the impact of high speed electrons of cathode rays on them. The anode rays were made up of the positively charged particles. The German scientist Wilhelm Wien analyzed these rays in 1898 and found that the particles have different e/m ratio for different gases taken in the discharge tube. Moreover, e/m ratio is about 1/1000 times that of an electron. Because the ratio of the particles is also comparable to the e/m ratio 6 NUCLEAR SCIENCE of the residual atoms in the discharge tube, the scientists thought that the rays were actually ions from the gases inside the tube. The mass of anode ray particle was found to be different for different experimental gases. Further experiments revealed that when hydrogen gas was taken inside the discharge tube, the particles present in anode rays have a minimum mass. These particles were termed protons. The mass of proton is nearly the same as that of an atom of hydrogen, i.e., 1.67 × 10–24 grams. The charge on the proton is equal in magnitude to that on the electron, i.e., 1.602 × 10–19 coulomb but opposite in nature. The attractive force between positively charged protons and negatively charged electrons keeps electrons moving in orbits around the nucleus, something like the way the gravity keeps the Earth in orbit around the sun.

Fig. 1.3. Anode rays or canal rays

Charge, Mass, Spin and Size The electron has a mass of about 9.1093897 × 10–28 grams. The mass of a proton or a neutron is about 1,836 times more than the electron. The mass of proton is 1.67 × 10–24 grams and the charge of the proton and electron is ± 1.602 × 10–19 coulomb. Neutron has no charge and it is slightly more massive than a proton. The atom as a whole is electrically neutral as it contains equal number of protons and electrons. All atoms are roughly the same size, whether they have 3 or 90 electrons. Approximately, 50 million atoms of solid matter lined up in a row would measure only 1 cm. A convenient unit of length for measuring atomic sizes is the angstrom (Å) which is equal to 10–10 meter. The radius of an atom measures 1 to 2 angstroms. Compared with the overall size of the atom the nucleus is much tinier. It is in the same proportion to the atom as the marble is to a football ground. The volume of a nucleus is about 10–14 meters. Thus, it occupies 1/100,000 part of an atom. Hence the more practical unit of length for measuring nuclear sizes is the femtometre (fm) which is equal to 10–15 meter. The diameter of the nucleus depends on the number of particles it contains and it ranges from 4 fm for light nucleus like carbon to 15 fm for the heavy nucleus like lead. In spite of the small size of the nucleus, virtually all the mass of the atom is concentrated there. The lightest nucleus, that of hydrogen, is only 1836 times heavier than electron, but some heavy nuclei are nearly 500,000 times more massive than the electron.

Life of Particles Protons, neutrons and electrons are long-lived particles. Other subatomic particles are also known now, but they are short-lived. We shall discuss about them in a separate chapter in this book. THE ATOM’S ANCESTRY 7

The electron has other intrinsic property like spin. The electron cannot spin in any arbitrarily manner, but spins only at a certain specific rates governed by Quantum Mechanics. These rates can be 1/2, 1, 3/2, 2, 5/2,—times a basic unit of rotation. Like protons and neutrons, electrons have a spin equal to 1/2.

Atomic Number

The single most important characteristic of an atom is its atomic number usually denoted 1 CHAPTER by Z. Atomic number is defined as the number of units of positive charge in the nucleus. It is equal to number of protons in the nucleus. For example, if an atom has Z equal to 11, it is sodium, while Z equal to 92 corresponds to uranium.

Model of an Atom Some five centuries before christ that is from Democritus’s time, many theories for structure of atom have been suggested, argued upon and rejected. Much later on there was rather compelling evidence that matter is composed of relatively few building blocks that we refer to as atoms and matter could be described by an atomic theory. Since then, a few of the models gain popularity in the course of time. Although atomic models have taken years and years to build, some of them are as follows: • Indivisible atom model: After the Dalton’s theory was put up in 1803, the atom was perceived as a hard sphere which is indivisible, impenetrable and indestructible.

• Plum-pudding model: In 1897, J.J.Thomson showed that electrons have negative electric charge and come from ordinary matter. For matter to be electrically neutral, there must also be an equal amount of positive charge lurking somewhere. In a prevailing model of the time of J.J.Thomson, it was proposed that negatively charged particles (electrons) were scattered like plums in the smeared- out positive charges (the pudding).The model explained the neutrality of the bulk material, yet allowed the description of flow of electric current. In this model, it would be very unlikely for an alpha particle to scatter through an angle greater than a small fraction of a degree, and a vast majority should undergo almost no scattering at all. (Refer to Rutherford’s experiment of scattering of alpha particles described on page 13). 8 NUCLEAR SCIENCE

• Cubical atom model: Some scientists suggested a cubical shape for an atom. But this did not gain any support due to lack of evidence in favour of it. • Rutherford’s model: The results from Rutherford’s experiment (see page 13) were astounding. The vast majority of alpha particles behaved as expected, and hardly scattered at all. But there were alpha particles that scattered through angles greater than 90 degrees, incredible in the light of expectations for a plum-pudding like model of an atom. It was largely the evidence from this type of experiment that led to a model of the atom as having a very tiny, compact central core(nucleus) that houses the entire positive charge and most of the mass of the atom, while the majority of the atom’s volume contains discrete electrons orbiting around the central nucleus. In Rutherford’s model the electrons moving in the orbits around the nucleus can be likened to our solar system in which the planets move in orbits around the sun. • Bohr model: Under classical electromagnetic theory, a charged body that is moving in a circular path, loses energy. However, in Rutherford’s model, there was nothing to prevent the electrons from loosing energy and finally falling into the nucleus under the electrostatic force of attraction (see also page 31).This stability problem was solved by Neil Bohr in1913, with a new model in which there are particular orbits in which electrons move without loosing energy and therefore do not spiral into the nucleus. This model was based on quantum mechanics, which successfully explained most of the behaviour of atoms. Bohr’s model is still the convenient description of the energy levels of the atom (see the details on page 31). • Wave mechanical model: This model is widely accepted today and this has left all the above models behind. This model is described in detail on pages 36 to 53. • String theory model: It is a very recent model, based on a similar idea of a stretched string and vibration governing a subatomic particle’s properties.This is being developed at Atomic Research centre at the Department of Physics UWA. A little about this theory is in the chapter of subatomic particles. 2

Radioactivity Natural and Artificial Radioactivity

In 1901, on 3rd April, Henri Becquerel put a sealed test tube, containing very little Barium chloride, which had been previously exposed to X-rays, in his regular laboratory apron pocket and forgot about it for a few days, although he was using the same apron every day, on 13th April, he observed a red patch in the shape of the test tube on his skin. Thus, he discovered that X-rays and radiation caused skin damage. He then observed some minerals, which absorb sunlight in the daytime and glow at night. He discovered that the luminescence of these materials fogged his photographic plates. Some substances, like potassium uranyl sulphate, after exposure to sunlight, emit very powerful radiation, which affects photographic plates packed in a thin aluminium box. Further Becquerel observed that uranium salts, without being exposed to sunlight, fogged the films, suggesting that these substances emitted invisible, penetrating rays themselves. Within the next ten years investigations showed that these rays consisted of three different types of radiation. They were then named Alpha (α), Beta (β), and Gamma (γ) rays. 238 → 234 4 Examples: 92U 90Th + Alpha particles (2He ) 234 → 234 0 90Th 91Pa + Beta particles ( –1e ) 226 → 222 4 88Ra 86Rn + Alpha particles (2He ) After emitting α or β particle most of the Radioactive compounds emit gamma rays. This phenomenon is since then known as radioactivity and is exhibited only by elements having greater atomic weights than Bismuth (see the periodic table at the end of the book). α 4 The rays are bi-positively charged helium ions, 2He moving at a velocity of 1/10 of light, i.e., about 10,000 M.P.S. Note: Each particle has two positive charges and the mass number is 4. β 0 The rays are electrons ( –1e ) with a velocity up to 93% of light. Note: Each particle has one negative charge and negligible weight. The γ rays are high frequency, electromagnetic, super X-rays, of very short wavelengths. The spontaneous emission of radioactive rays from the nucleus of atoms is called radioactive disintegration or Natural Radioactivity (Fig. 2.1.) 101010 NUCLEAR SCIENCE

Fig. 2.1.2.1.Fig. Radioactive rays

Fig. 2.2.2.2.Fig. Relative penetrating power

2.1 DISCOVERY OF RADIUM AND NUCLEUS

At this very time (1901), Madame Marie Curie and her husband Pierre Curie obtained a new element, radium (Ra), which was found to be 1.5 million times more radioactive than uranium (U). Miss Marie Sklodowska (Mrs. P. Curie) was a student of physicist professor Becquerel. Prof. Becquerel knew about the invisible penetrating rays, but could not explain why and what they were. But Marie, who possessed imagination, which is one essential quality of a good research worker, suspected the presence of an element, which gave off that strange radiation. She resigned her post as the professor’s assistant and she and her husband started a research project on their own. They obtained a large quantity of minerals, Pitchblend and Chalcolite from Bohemia and worked on the ores, day and night, week after week, month after month, to extract the mysterious element. From ten thousand kilograms of the ore they obtained the last concentrate, just a small test tube containing a whitish liquid. The liquid was left to crystallize overnight; but Marie did not sleep that night. At 3 a.m. she opened the door of the garage, which was their laboratory, and she stood frozen at the entrance; for in the dark room she saw a mysterious blue light, a fantastic and almost frightening sight. The shining substance, the first grains of chloride of new element, which was named by the Curies as Radium in 1902.