Space, time & Cosmos Lecture 7: Atom, nucleus and quantum theory

1. Space, time & CosmosLecture 7: Atom, nucleus and quantum theory Dr. Ken Tsang

2. Ancient (Philosophical) Atomism • The earliest known theories were developed in ancient India in the 6th century BCE by Kanada, a Hindu philosopher. • Leucippus and Democritus, Greek philosophers in the 5th century BCE, presented their own theory of atoms. • Little is known about Leucippus, while the ideas of his student Democritus—who is said to have taken over and systematized his teacher's theory—are known from a large number of reports.

3. Greek Atomism • These ancient atomists theorized that the two fundamental and oppositely characterized constituents of the natural world are indivisible bodies—atoms—and void. • The latter is described simply as nothing, or emptiness. • Atoms are solid and impenetrable bodies, and intrinsically unchangeable; they can only move about in the void and combine into different clusters. • Since the atoms are separated by void, they cannot fuse, but must rather bounce off one another when they collide. • All macroscopic objects are in fact combinations of atoms. Everything in the macroscopic world is subject to change, as their constituent atoms shift or move away. Thus, while the atoms themselves persist through all time, everything in the world of our experience is transitory and subject to dissolution.

4. Plato and Platonists • Plato, a Greek philosopher, presented a different kind of physical theory based on indivisibles. • In this theory, it is the elemental triangles composing the solids that are regarded as indivisible, not the solids themselves. • The term elements (stoicheia) was first used by Plato in about 360 BC, in his dialogue Timaeus, which includes a discussion of the composition of inorganic and organic bodies and is a rudimentary treatise on chemistry.

5. Plato’s 4 elements Plato's Timaeus conjectures on the composition of the four elements which the ancient Greeks thought made up the universe: earth, water, air, and fire. Plato assumed that the minute particle of each element had a special geometric shape: tetrahedron (fire), octahedron (air), icosahedron (water), and cube (earth).

6. Islamic Atomism During the 11th century (in the Islamic Golden Age), Islamic atomists developed atomic theories that represent a synthesis of both Greek and Indian atomism. The most successful form of Islamic atomism was in the Asharite school of philosophy, most notably in the work of the philosopher al-Ghazali (1058-1111). In Asharite atomism, atoms are the only perpetual, material things in existence, and all else in the world is “accidental” meaning something that lasts for only an instant.

7. Modern atomic theory • In the early years of the 19th century, John Dalton developed the first useful atomic theory of matter around 1803 in which he proposed that each chemical element is composed of atoms of a single, unique type, and that though they are both immutable and indestructible, they can combine to form more complex structures (chemical compounds).

8. John Dalton(1766-1844)

9. Background of Dalton's Atomic Theory • Less than twenty years earlier, in the 1780's, AntoineLavoisier ushered in a new chemical era by making careful quantitative measurements which allowed the compositions of compounds to be determined with accuracy. He formulated the Law of conservation of mass in 1789, which states that the total mass in a chemical reaction remains constant (that is, the reactants have the same mass as the products). This law suggested to Dalton that matter is fundamentally indestructible. • By 1799 enough data had been accumulated for Proust to establish the Law of Constant Composition ( also called the Law of Definite Proportions). This law states that if a compound is broken down into its constituent elements, then the masses of the constituents will always have the same proportions, regardless of the quantity or source of the original substance. He had synthesized copper carbonate through numerous methods and found that in each case the ingredients combined in the same proportions as they were produced when he broke down natural copper carbonate.

10. Background of Dalton's Atomic Theory • In 1803 Dalton noted that oxygen and carbon combined to make two compounds.  Of course, each had its own particular weight ratio of oxygen to carbon (1.33:1 and 2.66:1), but also, for the same amount of carbon, one had exactly twice as much oxygen as the other. This led him to propose the Law of Simple Multiple Proportions, which was later verified by the Swedish chemist Berzelius. • In an attempt to explain how and why elements would combine with one another in fixed ratios and sometimes also in multiples of those ratios, Dalton formulated his atomic theory.

11. Five main points of Dalton's Atomic Theory • Chemical 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, nor destroyed in the chemical process. A chemical reaction simply changes the way atoms are grouped together.

12. Additional work of Dalton • In 1803 Dalton published his first list of relative atomic weights for a number of substances (though he did not publicly discuss how he obtained these figures until 1808). • Dalton estimated the atomic weights according to the mass ratios in which they combined, with hydrogen being the basic unit.

13. Distinction of Atoms and Molecules • In 1811, Avogadro published an article in Journal de physique that clearly drew the distinction between the molecule and the atom. He pointed out that Dalton had confused the concepts of atoms and molecules. That was why Dalton wrongly concluded water as HO, not H2O. • Avogadro suggested that: equal volumes of all gases at the same temperature and pressure contain the same number of molecules which is now known as Avogadro's Principle. In other words, the volume of a gas at a given pressure and temperature is proportional to the number of atoms or molecules regardless of the nature of the gas,and the mass of a gas's particles does not affect its volume.

14. Avogadro's number • Avogadro's Principle allowed him to deduce the diatomic nature of numerous gases by studying the volumes at which they reacted. • For instance: since two litres of hydrogen will react with just one litre of oxygen to produce two litres of water vapor (at constant pressure and temperature). Thus two molecules of hydrogen can combine with one molecule of oxygen to produce two molecules of water. • It meant a single oxygen molecule splits in two in order to form two particles of water. Thus, Avogadro was able to offer more accurate estimates of the atomic mass of oxygen and various other elements, and firmly established the distinction between molecules and atoms. • Avogadro's number is the number of "elementary entities" (usually atoms or molecules) in one mole. For example. the number of atoms in exactly 12 grams of carbon-12 is 6.022X10^23.

15. Brownian motion: molecules in motion • In 1827, the British botanist Robert Brown observed that dust particles floating in water constantly jiggled about for no apparent reason. • In 1905, Albert Einstein theorized that this Brownian motion was caused by the water molecules continuously knocking the grains about, and developed a hypothetical mathematical model to describe it. • This model was validated experimentally in 1911 by French physicist Jean Perrin, thus providing additional validation for particle theory (and by extension atomic theory).

16. Mendeleev's Periodic table of Elements • SCIENTISTS HAD IDENTIFIED over 60 elements by Mendeleev's time (Today over 110 elements are known). • In Mendeleev's day (1834-1907). the atom was considered the most basic particle of matter. The building blocks of atoms (electrons, protons, and neutrons) were discovered only later. What Mendeleev and chemists of his time could determine, however, was the atomic weight of each element: how heavy its atoms were in comparison to an atom of hydrogen, the lightest element.

17. Mendeleev first trained as a teacher in the Pedagogic Institute of St. Petersburg before earning an advanced degree in chemistry in 1856.

18. Table from Mendeleev's 1869 paper

19. Mendeleev’s work AN OVERALL UNDERSTANDING of how the elements are related to each other and why they exhibit their particular chemical and physical properties was slow in coming. Between 1868 and 1870, in the process of writing his book, The Principles of Chemistry, Mendeleev created a table or chart that listed the known elements according to increasing order of atomic weights. When he organized the table into horizontal rows, a pattern became apparent--but only if he left blanks in the table. If he did so, elements with similar chemical properties appeared at regular intervals--periodically--in vertical columns on the table.

20. Mendeleev’s contribution • Mendeleev was bold enough to suggest that new elements not yet discovered would be found to fill the blank places. He even went so far as to predict the properties of the missing elements. • Although many scientists greeted Mendeleev's first table with skepticism, its predictive value soon became clear. • The discovery of gallium in 1875, of scandium in 1879, and of germanium in 1886 supported the idea underlying Mendeleev's table. Each of the new elements displayed properties that accorded with those Mendeleev had predicted, based on his realization that elements in the same column have similar chemical properties.

21. Mendeleev said: “I began to look about and write down the elements with their atomic weights and typical properties, analogous elements and like atomic weights on separate cards, and this soon convinced me that the properties of elements are in periodic dependence upon their atomic weights.”--Mendeleev, Principles of Chemistry, 1905, Vol. II

23. WHAT MADE Mendeleev’sTABLE PERIODIC? • The value of the table gradually became clear, but not its meaning. Scientists soon recognized that the table's arrangement of elements in order of atomic weight was problematic. • The atomic weight of the gas argon, which does not react readily with other elements, would place it in the same group as the chemically very active solids lithium and sodium. • In 1913 British physicist Henry Moseley confirmed earlier suggestions that an element's chemical properties are only roughly related to its atomic weight (now known to be roughly equal to the number of protons plus neutrons in the nucleus). • What really matters is the element's atomic number-the number of electrons its atom carries, which Moseley could measure with X-rays. Ever since, elements have been arranged on the periodic table according to their atomic numbers. • The structure of the table reflects the particular arrangement of the electrons in each type of atom. Only with the development of quantummechanics in the 1920s did scientists work out how the electrons arrange themselves to give the element its properties.

24. Discovery of subatomic particles • Electron -J. J. Thomson 1896 • Radioactivity - Henri Becquerel 1896 • Alpha & beta particles -Ernest Rutherford1899 • Nucleus - Ernest Rutherford1907 • Isotopes - J. J. Thomson 1913 • Proton - Ernest Rutherford1918 • Neutron - James Chadwick 1932

25. During the 1870s, English chemist and physicist Sir William Crookes developed the first cathode ray tube to have a high vacuum inside.[19] He then showed that the luminescence rays appearing within the tube carried energy and moved from the cathode to the anode. Furthermore, by applying a magnetic field, he was able to deflect the rays, thereby demonstrating that the beam behaved as though it were negatively charged. Experiments with Crookes tube first demonstrated the particle nature of electrons. In this illustration, the profile of the cross-shaped target is projected against the tube face at right by a beam of electrons.

26. Joseph John Thomson, (1856 –1940) His discoveryof electron was made known in 1897, resulting in him being awarded a Nobel Prize in Physics in 1906. In 1896, British physicist J. J. Thomson, with his colleagues John S. Townsend and H. A. Wilson, performed experiments indicating that cathode rays really were unique particles, rather than waves, atoms or molecules as was believed earlier. Thomson made good estimates of both the charge e and the mass m, finding that cathode ray particles, which he called "corpuscles," had perhaps one thousandth of the mass of the least massive ion known: hydrogen.

27. Contribution of J. J. Thomson • He showed that atoms could be further subdivided into negative (which he named electrons) and positive components. • He postulated a "Plum Pudding" model for atoms. He calculated the charge to mass ratio (e/m) for the electron by careful observations of the curvature of an electron beam in cathode ray tubes in a magnetic field.

28. Measurement of Electronic charge • Millikan calculated the charge on the electron with his famous oil drop experiment. He measured the static electrical charge on microscopic oil droplets by balancing droplets between charged plates. • He was awarded the Nobel Prize in Physics (1923)

29. Discovery of the nucleus In a classic experiment by Hans Geiger and Ernest Marsden in 1907, under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester, a thin sheet of gold foil was bombarded with alpha particles (He nuclei: 2 protons + 2 neutrons). • They discovered that the particles bounced off of something dense in the foil. • From this experiment Rutherford postulated that atoms are formed of a small dense positively charged nucleus "orbited" by negatively charged electrons. This led him to his theory that most of the atom was made up of 'empty space'. • Ernest Rutherford (1871-1937) was Nobel Prize winner in 1908.

30. Rutherford's scattering experiment

31. Rutherford’s gold foil experimentTop: Expected results: alpha particles passing through the plum pudding model of the atom with negligible deflection.Bottom: Observed results: a small portion of the particles were deflected, indicating a small, concentrated positive charge.

32. Discovery of isotopes • In 1913, J. J. Thomson channeled a stream of neon ions through magnetic and electric fields, striking a photographic plate on the other side. He observed two glowing patches on the plate, which suggested two different deflection trajectories. • Thomson concluded this was because some of the neon ions had a different mass; thus did he discover the existence of isotopes.

33. J. J. Thomson had shown in 1897 that charged particles could be deflected by magnetic and electric fields and that the degree of the deflection depends upon the masses and electric charges of the particles. In the mass spectrometer, gas of an element enter the device and are ionized. The ions are then accelerated through a magnetic field which bends the ion paths into a semicircular shape. The radius of this path is dependent upon the mass of the particle. Thus isotopes of different masses can be separated.

34. Radioactivity • In 1896, Henri Becquerel discovered that a sample of uranium was able to expose a photographic plate even when the sample and plate were separated by black paper. He also discovered that the exposure of the plate did not depend on the chemical state of the uranium (what uranium compound was used) and therefore must be due to some property of the uranium atom itself.

35. After Becquerel abandoned this work, it was continued by Pierre and Marie Curie who went on to discover other radioactive elements including polonium, radium and thorium. In 1903, Marie and Pierre Curie were awarded half the Nobel Prize in Physics. Henri Becquerel was awarded the other half for his discovery of spontaneous radioactivity. She (Pierre was hit by a truck and killed in the middle of this work on 19 April 1906 ) further suggested that the uranium, and the new elements, were somehow disintegrating over time and emitting radiation that exposed the plate. She called this phenomenon "radioactivity". For the first time it became apparent that atoms might be composed of even smaller particles and might have a structure that could be analyzed. Marie Curie was the first woman to win a Nobel prize and the first person to win two Nobel Prizes (Nobel Prize in Chemistry 1911).

36. After determining that the radiation emitted from uranium was composed of two different components, eventually Ernest Rutherford in 1899 , using two oppositely charged plates, he identified the components as positive particles (alpha particles) and lighter mass negative particles (beta particles). Paul Villard in 1900 identified a third primary type of radioactivity, gamma rays, from a radium sample. Gamma rays have no mass and possess no charge. The behavior of the three types of particles as they pass through the electric field between two charged plates is shown below. While alpha particles were determined to have a larger charge than the beta particles (+2 vs. -1), they also have over 7000 times the mass of the beta particle. Therefore, their path is bent much less than that of the beta particle.

37. Discovery of proton Rutherford's discovery of the nucleus demonstrated that these positive charges were concentrated in a very small fraction of the atoms' volume. In 1919 Rutherford discovered that he could change one element into another by striking it with energetic alpha particles (which we now know are just helium nuclei). In the early 1920's Rutherford and other physicists made a number experiments, transmuting one atom into another. In every case, hydrogen nuclei were emitted in the process. It was apparent that the hydrogen nucleus played a fundamental role in atomic structure, and by comparing nuclear masses to charges, it was realized that the positive charge of any nucleus could be accounted for by an integer number of hydrogen nuclei. He thus suggested that the hydrogen nucleus, which was known to have an atomic number of 1, was an elementary particle. By the late 1920's physicists were regularly referring to hydrogen nuclei as 'protons'. The term proton itself seems to have been coined by Rutherford, and first appears in print in 1920.

38. A schematic picture of the hydrogen atom. There is a single particle, proton, in the nucleus. electron proton

39. Discovery of neutron As of 1930, only two known elementary particles had been identified, the proton and the electron. Protons were known to have a mass of 1 and a charge of +1, while electrons had essentially no mass and a charge of -1. Moseley had shown convincingly that the charge on the nucleus increases in steps of +1 as one traverses the periodic table. To account for this it was apparent that the nucleus of each atom contained a number of protons equal to its atomic number. In order to remain electrically neutral, it also contained an equivalent number of electrons. The problem of the extra nuclear mass was solved in 1932 when James Chadwick identified the neutron. While studying the radiation resulting from the bombardment of beryllium with alpha particles, Chadwick noted a particle with approximately the same mass as a proton being released. He determined that, as the particle was not bent by electrical fields and was highly penetrating, it was electrically neutral.

40. After the discovery of neutron, scientist know there are three smaller particles that make up individual atoms. These are called subatomic particles as they are below the level of the atom in size.

41. The protons and neutrons are clumped together in the middle of an atom to form the nucleus and the electrons orbit around the outside. While this seems to contradict the idea that like charges repel, scientists have established that though protons (+) do indeed repel each other, once they are very close to each other another force, called the Strong Force, takes over and glues them together. As an example, for a Helium atom the structure is like this:

42. Isotopes – same atomic number but different mass number (same element, with different nuclei)

43. Isotopes of elements Carbon-12: 6 protons + 6 neutrons

44. Structure of atomic nuclei The atomic nuclei of all chemical elements consist of protons (p) and of neutrons (n). These two fundamental particles, which are summarised by the term nucleons, have almost the same mass (p: 1.00727 amu; n: 1.00866 amu), but only the protons are electrically charged (+1 e). In an atom, the number of protons indicates the atomic number (symbolised Z) of the corresponding element, while its mass number (symbolised A) is equal to the sum of protons and neutrons. 1 atomic mass unit = 1.66053886 × 10^(-27) kilograms

46. Graph of the number of neutrons versus the number of protons for all stable naturally occurring nuclei. Nuclei that lie to the right of this band of stability are neutron poor; nuclei to the left of the band are neutron-rich. The solid line represents a neutron to proton ratio of 1:1.

47. Properties of stable nuclides • The stable nuclides lie in a very narrow band of neutron-to-proton ratios. • The ratio of neutrons to protons in stable nuclides gradually increases as the number of protons in the nucleus increases. • Light nuclides, such as 12C, contain about the same number of neutrons and protons. Heavy nuclides, such as 238U, contain up to 1.6 times as many neutrons as protons. • There are no stable nuclides with atomic numbers larger than 83. • This narrow band of stable nuclei is surrounded by a sea of instability. • Nuclei that lie above this line have too many neutrons and are therefore neutron-rich. • Nuclei that lie below this line don't have enough neutrons and are therefore neutron-poor.

48. Larger nucleus need more neutrons to maintain stability. However, there is no stable nucleus beyond Z>83, no matter how many neutrons are inside the nucleus.

49. The origin of radioactivity Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting particles and radiation to reach a more stable nuclear configuration. This decay results in an atom of one type, called the parent nuclide transforming to an atom of a different type, called the daughter nuclide. The alpha particles discovered by Rutherford are identified to be just the nucleus of helium. The betaparticles are proved to be electrons.

50. Three Types of Radioactive Decay: Alpha Decay usually restricted to the heavier elements in the periodic table