2. Ancient Philosophy Leucippus(490BC) and his pupil Democritus (470 – 380 BC), during one of their walks along the seashore, noted that the beach looked like one whole continuous piece from afar. A material can be broken into smaller pieces Atomos – a greek word which means indivisible
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5. Dalton’s Atomic Theory In 1808, he published a book, A New System of Chemical Philosophy in which he presented the theory in detail
6. 1. All matter is composed of indestructible atoms. “ Matter, though divisible in an extreme degree, is nevertheless not infinitely divisible. That is, there must be some point beyond which we cannot go in the division of matter. The existence of these ultimate particles of matter can scarcely be doubted, though they are probably too small ever to be exhibited by microscopic improvements. I have chosen the word atom to signify these ultimate particles…” 2. The atoms of a given element are identical. They are different from the atoms of all other elements. They are unchageable. “the atoms never can be metamorphosed one into another by any power we can control” Compounds are formed by the combination of the atoms of two or more elements forming compound-atoms (what we now call molecules). The atoms combine with each other in definite ratio of small whole numbers. (Law of Definite Composition by Proust)
7. 4. Chemical reactions involve only the separation and or union of atoms. The atoms are only rearranged; none are created nor destroyed. “Chemical analysis and synthesis go no farther that the separations of particles from another, and to their reunion. No new creation or destruction of matter is within reach of chemical agency. We might as well attempt to introduce a new planet or to manipulate one already in existence, as to create or destroy a particle of hydrogen. All the changes can produce consist in separating particles that are in state of cohesion or combination, and joining those that were previously at a distance”
10. In England, 2000 years later, Sir William Gilbert tried similar experiments and learned that many materials when rubbed together becomes electrically charged
15. Discovery of Electrons Who:J. J. Thomson When: 1897 Where: England What: Thompson discovered that electrons were smaller particles of an atom and were negatively charged. Why:Thompson knew atoms were neutrally charged, but couldn’t find the positive particle. J. J Thomson made a piece of equipment called a cathode ray tube.
16. Discovery of electron..before Thomson Sir Humphry Davy(1778-1829) – the electrical nature of matter was revealed in his discovery that when electric current is passed through molten compounds of metals resulted in the decomposition of compounds to produce the metals. Led to the discovery of Na,K,Ca,Mg,Sr,Ba. Michael Faraday (1791-1867) – showed that mass of the element formed is proportional to the quantity of electricity that was passed. Each atom of the element was interacting with a definite amount of electricity. In 1891, George J. Stoney ( 1874-1911) – Irish physicist who suggested the name electron for the particle of electricity
17. Direct observation and characterization of electron Began with the work of physicist on the discharge of electricity through a vacuum tube. Heinrich Geissler (1814-1879) a German inventor, was able to device a method of producing a good vacuum in glass tubes. Julius Plucker (1801-1868), a German mathematician and physicist, sealed two metal pieces into a Geissler tube and applied a high voltage across the electrodes and observe a greenish luminescence emanating from the negative electrode, the cathode (or the negatively charged plate). The cathode ray is drawn to the positively charged plate, called the anode. Eugene Goldstein proposed that the luminescence observed by Plucker is cathode ray (later on identified as electron by Thomson).
18. Sir William Crookes (1832-1919) – an English physicist, showed that the cathode ray travelled in a straight line and that objects placed in its path cast a shadow at the opposite side of the tube (possibly a form of an Electromagnetic Radiation). Later he confirmed Plucker’s observation that the ray was deflected by a magnet in a way that indicated it to be unlike light but rather consist of negatively charged particles. Direct observation and characterization of electron
19. A Anode Cathode B C Fluorescent screen Cathode Ray Tube – N S High voltage +
25. Anode Cathode B Fluorescent screen – N S High voltage + Effects of electric field and magnetic field cancel
26. – A Anode Cathode N B S C Fluorescent screen + High voltage
27. Cathode ray tube According to electromagnetic theory, a moving charged body behaves like a magnet and can interact with electric and magnetic fields though which it passes. Because cathode ray is attracted by the plate bearing positive charges and repelled by the plate bearing negative charges, it must consist of negatively charged particles. These negatively charged particles are electrons.
28. A cathode ray produced in a discharge tube travelling from the cathode (left) to the anode(right). The ray itself is invisible, but the fluorescence of a zinc sulfide coating on the glass causes it to appear green
29. The cathode ray is bent downward when the north pole of the bar magnet is brought toward it. When the polarity of the magnet is reversed, the ray bends in the opposite direction
37. Thomson’s Model Found the electron Atoms were made of small negatively charged particles. Assumed that larger part of atom is positively charged with small electrons scattered in it. Said the atom was like plum pudding
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40. An oil droplet is allowed to fall between two charged plates.
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43. The balanced state will be shown by the state of the particle; it will remain suspended in the mid-air
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45. He observed that the electrical charge was always a whole-number multiple of a smallest charge, which he called the unit charge.
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48. Each proton has a charge of +1 unit, equivalent to +1.60 x 10-19HOW ABOUT THE MASS OF AN ELECTRON?
49. How is the charge of a particle measured? Charged plate Oil droplets (+) Small hole (–) Oil droplet under observation Charged plate Atomizer Viewing microscope
50. How is the charge of a particle measured? Charged plate (+) (–) Charged plate Atomizer Small hole Viewing microscope
51. How is the charge of a particle measured? Charged plate Oil droplets (+) Small hole (–) Charged plate Atomizer Viewing microscope
52. How is the charge of a particle measured? Charged plate Oil droplets (+) Small hole (–) Oil droplet under observation Charged plate Atomizer Viewing microscope
53. Short quiz. ¼ pad paper What is a cathode ray tube? What did scientists conclude about the composition of a cathode ray? (5pts) If the mass and electrical charge were uniformly distributed throughout an atom, what would be the expected results of an alpha scattering experiment? What was the major conclusion drawn from the results of alpha particle scattering experiment?(5pts)
54. Radioactivity In 1895, German physicist Wilhelm Roentgen noticed that cathode rays caused glass and metals to emit unusual rays. These rays could not be deflected by a magnet, they could not contain charged particles as cathode rays do ---X rays. Antoine Becquerel professor of Physics in Paris began to study fluorescent properties of substances. He found that exposing thickly wrapped photographic plates to a certain uranium compounds caused them to darken, even without stimulation of cathode rays
62. Ernest Rutherford’s GoldFoil Experiment Who: Ernest Rutherford When: 1911 Where: England What: Conducted an experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core. Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged.
63. Ernest Rutherford (1871-1937), a British physicist and his associate Hans Geiger(1882-1945) a German physicist, studied the alpha particles emitted by radium which was isolated by Marie and Pierre Curie. Alpha particles are found to be helium atoms with their electrons removed, positively charged and mass of 2500 times that of the electron.
64. Together with Ernest Marsden (an undergraduate student) they studied the scattering of high speed alpha particles when passed through thin metal foils.(about 2000 atoms thick) Believed in the plum pudding model of the atom. Wanted to see how big they are
69. He Expected The alpha particles to pass through without changing direction very much Because most of the mass of the atom (positive charges) were spread. Alone they were not enough to stop the alpha particles If the Thomson model were correct, all the alpha particles, travelling at high speeds and massive, would have passed through the metal foil undeflected or only slightly deflected
74. What he got They observed that although majority of the alpha particles passed through undeflected, some were only slightly deflected, some were scattered by more than 90 degrees and a few by nearly 180 degrees or almost completely turned back
75. + How he explained it Atom consists of a very small nucleus surrounded by electrons. Rutherford estimated the radius at 10-12 to 10-13 cm compared to radius of the atom of about 10-8 cm The nucleus contains most of the mass of the atom and all of its positive charge. Alpha particles are deflected by nucleus it if they get close enough at each other
78. The Modern Reassessment of the Atomic Theory Modern Reassessment of the atomic theory 1. All matter is composed of atoms. The atom is the smallest body that retains the unique identity of the element. 2. Atoms of one element cannot be converted into atoms of another element in a chemical reaction. Elements can only be converted into other elements in nuclear reactions.
79. 3. All atoms of an element have the same number of protons , which determines the chemical behaviour of the element. Isotopes of an element differ in the number of neutrons, and thus in mass number. A sample of the element is treated as though its atoms have an average mass. 4. Compounds are formed by the chemical combination of two or more elements in specific ratios.
81. FLAME TEST Objectives: To observe the colors emitted by various metal ions when heated in an open flame. To identify the elements by their emitted colors.
84. PROCEDURE Few crystals of each substance Hollows of pot plate + 1-2 drops HCl Dip toothpick into samples Heat toothpick over flame Record the color imparted to the flame
88. Bohr’s Model Electrons move in circular orbits around the nucleus Adopted Planck’s idea that energies are quantized
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90. Three postulates Only orbits of certain radii, corresponding to certain energies, are permitted for electrons in an atom. An electron in a permitted orbit has a specific energy and is in an “allowed” energy state. Electron will not radiate energy.
91. Energy is only emitted or absorbed by an electron as it changes from one allowed energy state to another. This energy is emitted or absorbed as a photon.
92. Energy states of a Hydrogen Atom Ground state – lowest energy level (n = 1) Excited state - higher energy level (n= 2…)
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94. An electron could jump from one allowed energy state to another by emitting or absorbing photons whose energy corresponds exactly to the energy difference between the two states. ΔE = Ef - Ei
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96. Spectrum Lasers emit radiation which is composed of a single wavelength. However, most common sources of emitted radiation (i.e. the sun, a lightbulb) produce radiation containing many different wavelengths. When the different wavelengths of radiation are separated from such a source a spectrum is produced. A rainbow represents the spectrum of wavelengths of light contained in the light emitted by the sun
97. Sun light passing through a prism (or raindrops) is separated into its component wavelengths Sunlight is made up of a continuous spectrum of wavelengths (from red to violet) - there are no gaps Not all radiation sources emit a continuous spectrum of wavelengths of light
100. The Nature of Light Electromagnetic radiation Electromagnetic energy or radiant energy
101. The WAVE Nature of Light WAVELENGTH (lambda) the distance between any point on a wave and the corresponding point on the next wave. FREQUENCY (ѵ ,nu) The number of cycles that pass a given point per second Tells how fast the wave oscillates
104. ELECTROMAGNETIC SPECTRUM The waves in the spectrum all travel at same speed through a vacuum but differ in the frequency and, therefore, wavelength.
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106. Check-up Which wave has the higher frequency? If one wave represents visible light and the other represents infrared radiation, which wave is which? If one is blue light and the other is red light, which would be which?
107. A photon has a frequency of 6.0 x 104 Hz. Convert this frequency into wavelength (nm). Does this frequency fall in the visible region? l n Radio wave l x n = c l = c/n l = 3.00 x 108 m/s / 6.0 x 104 Hz l = 5.0 x 103 m l = 5.0 x 1012 nm 7.1
109. The PARTICLE Nature of Light Blackbody Radiation Light given off by hot objects Wavelength distribution of the radiation depends on temperature “red-hot” object being cooler than a “white-hot” one
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111. MAX PLANCK ( 1858-1947) Energy can be released or absorbed by atoms only in discrete “chunks” of some minimum size. Quantum – “fixed amount”, smallest amount of energy that can be emitted or absorbed as electromagnetic radiation.
112. Hot glowing object could emit (or absorb) only certain quantities of energy E = hv E = energy of radiation v= frequency h= Planck’s constant ( 6.63 x 10-34 joule-seconds)
113. Hot object’s radiation is emitted by the atoms contained within it. The atom itself can have only certain quantities of energy. The energy is quantized- values are restricted only in certain quantities
120. ALBERT EINSTEIN (1905) Used Planck’s quantum theory to explain the photoelectric effect
121. Radiant energy striking the metal surface is a stream of energy packets PHOTON Behaves like a particle Has an energy proportional to Energy of photon= E = hv
122. A photon transfers its energy to an electron in the metal. A certain amount of energy is required to overcome the attractive forces that hold it within the metal.
