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Chapter 2
Atoms and the Atomic Theory

 
 
 
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 The law of multiple proportions is illustrated for carbon monoxide and carbon dioxide.
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 Animation: Multiple Proportions (*) The oxidation of carbon is illustrated. In the animation the red molecules represent oxygen and the grey atoms represent an inert gas. At low concentrations of oxygen, CO is formed. At high concentrations of oxygen, CO2 is formed. The oxygen to carbon mass ratios for both compounds are given and used to illustrate the law of multiple proportions.
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 This animation demonstrates the Millikan oil drop experiment.
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 Animation: Millikan Oil Drop Experiment (**) In Millikan's experiment, oil droplets are given a negative charge by irradiation with x-rays. When the negatively charged droplets are between electrically charged plates, they may fall, remain stationary, or rise. The effect of voltage on the rate at which the charged droplets move is used to calculate the charge of an electron.
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 Separation of alpha, beta, and gamma rays by an electric field
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 Animation: Separation of Alpha, Beta, and Gamma Rays (**) Passing radiation through an electric field shows that there are three types of radiation. Beta rays, which are high-speed electrons, are bent toward the positive electrode. Uncharged gamma rays are not deflected by the electric field. Alpha rays, which are positively charged helium nuclei, are bent toward the negative electrode.
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 Rutherford's experiment from an experimental and a molecular perspective
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 Animation: Rutherford Experiment (***) Rutherford's experiment was a major breakthrough in understanding the structure of an atom. Alpha particles were directed toward a thin layer of gold foil and the angles at which they were deflected were observed. The majority of the particles were not deflected, indicating that they did not undergo a collision. A few particles were deflected by large angles, indicating that they had collided with a very small, massive nucleus.
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 The reactions of metallic sodium and potassium with water are demonstrated.
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 Demonstration: Sodium and Potassium in Water (***) When a small piece of Na is added to a solution containing an indicator, evidence of the reaction can be observed by the change in the color of the solution as NaOH is formed, by the melting of the Na and by the movement of the Na caused by formation of hydrogen gas. K is more reactive than Na as demonstrated by its reaction with water. This reaction produces enough heat to ignite the H2 produced.
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 This animation illustrates the arrangement of elements in the periodic table.
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 Animation: Periodic Properties (*) Elements are arranged in the periodic table in order of increasing atomic number (number of protons). The horizontal rows are called periods and the columns are called groups. Elements in groups have similar chemical and physical properties. The metallic character of the elements increases to the left and down the periodic table. Nonmetallic character increases to the right and up the periodic table.
2.3
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 02.03
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 Ball & stick and space-filling models of CO and CO2 illustrating law of multiple proportions
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2.5
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 02.05
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 Effect of a magnetic field on charged particles. When charged particles travel through a magnetic field in such a way that their path is perpendicular to the field, they are deflected by the field. Negatively charged particles are deflected in one direction, and positively charged particles are deflected in the opposite direction. Several of the phenomena described in this section depend on this behavior.
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2.6
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 02.06
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 Cathode-ray
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2.7a-c
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 02.07
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 3 different cathode ray diagrams
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2.8
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 02.08
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 Millikan’s oil drop experiment. Ions (charged atoms or molecules) are produced by energetic radiation such as X-rays (X). Some of these ions become attached to oil droplets, giving them a net charge. The fall of a droplet in the electric field between the condenser plates is speeded up or slowed down, depending on the magnitude and sign of the charge on the droplet. By analyzing data from a large number of droplets, Millikan concluded that the magnitude of the charge, q, on a droplet is an integral multiple of the electronic charge, e. That is, q 5 ne (where n 5 1, 2, 3, . . . ).
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2.9
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 02.09
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 The “plum pudding” atomic model. According to this model, a hydrogen atom consists of a “cloud” of positive charge (11) and one electron (21). Helium would have a 1 2 cloud and two electrons (22). If a helium atom loses one electron, the atom becomes charged and is called an ion. This ion, referred to as He1, has a net charge of 11. If the helium atom loses both electrons, the He21 ion forms. Ions are discussed further on page 44.
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2.10
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 02.10
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 Three types of radiation from radioactive materials. The radioactive material is enclosed in a lead block. All the radiation except that passing through the narrow opening is absorbed by the lead. When this radiation is passed through an electric field, it splits into three beams. One beam is undeflected-these are gamma (g) rays. A second beam is attracted to the negatively charged plate. These are the positively charged alpha (a) particles. The third beam, of negatively charged beta (b) particles, is deflected toward the positive plate.
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2.11
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 02.11
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 The scattering of a-particles by metal foil. The telescope travels in a circular track around an evacuated chamber containing the metal foil. Most a-particles pass through the metal foil undeflected, but some are deflected through large angles.
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2.12a,b
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 02.12 a,b
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 Explaining the results of a-particle scattering experiments. (a) Rutherford’s expectation. Small, positively charged a-particles should pass through the nebulous, positively charged cloud of the Thomson atomic model largely undeflected. Some would be slightly deflected by passing near electrons (present to neutralize the positive charge of the cloud). (b) Rutherford’s explanation. With an atomic model based on a small, dense, positively charged nucleus and extranuclear electrons, one would expect the four different types of paths actually observed: 1. undeflected straight-line paths exhibited by most of the a-particles. 2. slight deflections of a-particles passing close to electrons. 3. severe deflections of a-particles passing close to a nucleus. 4. reflections from the foil of a-particles approaching a nucleus “head-on.”
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2.13
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 02.13
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 The nuclear atom-illustrated by the helium atom. In this drawing electrons are shown much closer to the nucleus than is the case. The actual situation is more like this: If the entire atom were represented by a room, 5 m x 5 m x 5 m, the nucleus would only occupy about as much space as the period at the end of this sentence.
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2.14
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 02.14
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 A mass spectrometer. A gaseous sample is ionized by bombardment with electrons in the lower part of the apparatus (not shown). The positive ions thus formed are subjected to an electrical force by the electrically charged velocity selector plates and a magnetic force by a magnetic field perpendicular to the page. Only ions with a particular velocity pass through and are deflected into circular paths by the magnetic field. Ions with different masses strike the detector (here a photographic plate) in different regions. The more ions of a given type, the greater the response of the detector (intensity of line on the photographic plate).
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2.15
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 02.15
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 Mass spectrum for mercury. The response of the ion detector in Figure 2-13 (intensity of lines on photographic plate) has been converted to a scale of relative numbers of atoms. The percent natural abundances of the mercury isotopes are 196Hg, 0.146%; 198Hg, 10.02%; 199Hg, 16.84%; 200Hg, 23.13%; 201Hg, 13.22%; 202Hg, 29.80%; 204Hg, 6.85%.
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2.16
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 Periodic Table of the Elements. Atomic masses are relative to carbon-12. For certain radioactive elements the numbers listed (in parentheses) are the mass numbers of the most stable isotopes. Metals are shown in orange, nonmetals in blue, and metalloids in green. The noble gases (also nonmetals) are shown in purple.
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2.17
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 An attempt to picture one mole of atoms. One mole comprises an enormously large number of atoms. (a)ÊThere is only one type of fluorine atom, 19F (shown in red). (b)ÊIn chlorine, 75.77% of the atoms are 35Cl (red) and the remainder are 37Cl (blue). (c)ÊMagnesium has one principal isotope, 24Mg (red), and two minor ones, 25Mg (gray) and 26Mg (blue). (d)ÊLead has four naturally occurring isotopes: 1.4% 204Pb (yellow), 24.1% 206Pb (blue), 22.1% 207Pb (gray), and 52.4% 208Pb (red).
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2.20
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 02.20
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 Outlining a calculation-Example 2-9 visualized.
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Table 2.1
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 Table 2.1
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Table 2.2
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 Table 2.2
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