The Atomic Hypothesis
Characteristics of Atoms
Atomic Imagery
Atomic Structure
The Elements
The Periodic Table of the Elements
Isotopes
Compounds and Mixtures
Molecules
The Placebo Effect
Antimatter
Dark Matter
The treatment of atoms in this chapter is much different than the Tenth Edition. Much of the history of atomic knowledge is now in Chapter 32. Atoms provide a good background for the chapters on heat, and also provide background for Chapters 22, 24, and 30- 34.
This chapter is the most important chapter in Part two, and should not be skipped.
An excellent 10-minute oldie but goodie film that makes an excellent jump in scale from the solar system, galaxies, and the universe, discussed briefly in the preceding chapter, to the atom—comparing sizes as positive and negative powers of ten is the now classic Powers of Ten, by Charles and Ray Eames, and narrated by Philip Morrison (Pyramid Films, 1978).
There are no OHTs for this chapter, nor problems in the Problem Solving in Conceptual Physics book for this chapter.
In the Practicing Physics book:
• Atoms and Atomic Nuclei • Subatomic Particles
In the Next-Time Questions book:
• Germanium Capsules • Adding or Subtracting Protons
Two excellent experiments in the Laboratory Manual are Diameter of a BB and Oleic Acid Pancake. The first nicely leads into the second, and both may be combined.
Although only one neighbor check question is identified in the suggested lecture here, please make your own as your lecture unfolds.
SUGGESTED LECTURE PRESENTATION
Begin by posing the situation of breaking a boulder into rocks, rocks into stones, stones into pebbles, pebbles into gravel, gravel into sand, sand into powder, and so forth until you get to the fundamental building block—the atom. Relate how from the earliest days of science people wondered how far the idea of breaking boulders into stones, pebbles, sand, powder, and so on, would go. Does it ever end? Hundreds of years ago, people had no way of finding out, and they instead carried on with philosophical speculation. Not until “modern” chemistry in the late 1700s did people begin to get indirect evidence of some basic order in the combinations of things. The first real “proof” that there were atoms was given by Einstein in 1905, the same year he published his paper on relativity. He calculated what kind of motion there ought to be in Brownian motion, based on ideas we’ve considered already, like energy and momentum conservation, and the idea of heat as atomic motion. Many of the “heavies” in physics at that time didn’t believe in atoms until Einstein’s work.
Smallness of Atoms: Give examples to convey the idea of the smallness of the atom, i.e., an atom is as many orders of magnitude smaller than a person as an average star is larger than a person—so we stand between the atoms and the stars. The size of an atom is to the size of an apple as the size of an apple is to the size of the Earth. So if you want to imagine an apple full of atoms, think of the Earth, solid-packed with apples.
CHECK QUESTION: Ask what an atom would “look like” if viewed through a vertical bank of about 40 high-powered optical microscopes stacked one atop the other. [It turns out they wouldn’t have an appearance, at least not in the range of frequencies we call light. The atom is smaller than the wavelength of light.]
You might allude to the later study of Chapter 32 and state that the electron beam in the electron microscope has the properties of high-frequency light. Acknowledge the wave nature of matter—the fuzziness in the distinction between particles and waves at the atomic level—that “solid” particles seem to be congealed standing waves of energy.
The photo of individual atoms taken by Crewe and associates in Figure 11.4 on page 214 is historically significant. It was the first of many to follow.
Recycling of Atoms: You can lead into the idea of more molecules in your lungs than there are breaths of air in the world with the following: State that if you put a drop of ink in a bathtub full of water, that you (the students) know that in a short time you can sample any part of the water and find ink in it. The atoms of ink spread out. We can get an idea of how small atoms are from this fact: There are more atoms in a thimbleful of ink than there are thimblefuls of water in the Atlantic Ocean . That means if you throw a thimbleful of ink into the Atlantic Ocean and give it enough years to mix uniformly, and then dip anywhere in the ocean with a thimble, you’ll have some atoms of ink in your sample. You may want to discuss Problem 6 at this point and demonstrate its solution. (Note that the data for this problem concerns all the oceans of the world, not just the Atlantic .) By now your class is ready for the more interesting bit about breaths of air in the atmosphere. Relate this to Problem 7 and the statement made by little Andres Riveros Mendoza in the part opening photo of Part 2.
Empty Space: Discuss the Bohr model of the atom and the electrical role of the nucleus and surrounding electrons. Stress the emptiness of the atom and lead into the idea of solid matter being mostly empty space. State how our bodies are 99.999% empty spaces, and how a particle, if tiny enough and not affected by electrical forces, could be shot straight through us without even making a hole! Making a direct hit with an atomic nucleus or an electron is as improbable as making a direct hit with a planet or the Sun if you throw a gravity-free dart from outer space at the solar system. Both the solar system and an atom are mostly empty space. Walk through a beam of neutrons and very few if any will interact with your body. Still smaller neutral particles called neutrinos, the most elusive yet most numerous and fastest of all particles, pass through us every moment. But they do so without consequence, for only very rarely, perhaps once or so per year, do any make a bull’s-eye collision with any of our atomic nuclei. They freely pass through the entire Earth with rare interactions. (Interestingly enough, the neutrino flux from the 1987 supernova was so enormous that about 1 out of every 240 people on Earth absorbed one of its neutrinos. This tidbit from John Learned, University of Hawaii
Molecules: Distinguish between atoms and molecules. There are a limited number of different atoms, but there are innumerable different molecules—and more are being discovered and constructed.
CHECK QUESTIONS: What is the number of elements in a water molecule? What is the number of atoms in a water molecule? [Two elements (hydrogen and oxygen), and three atoms, two of hydrogen and one of water.]
Interestingly enough, whereas an individual atom cannot be seen by the naked eye, some molecules can. One such molecule, called a macro-molecule, is a diamond. A diamond is actually one big carbon molecule!
Electrical Forces: Discuss the role of electrical forces in preventing us from oozing into our chairs and so forth. Ask the class to imagine that the lecture table is a large magnet, and that you wear magnetic shoes that are repelled by the table you “stand” on. Ask them to imagine whether or not a sheet of paper could be passed between your shoes and the table. For there is a space there. Then state that on the submicroscopic scale that this is indeed what happens when you walk on any solid surface. Only the repelling force isn’t magnetic, it’s electric! Discuss the submicroscopic notion of things touching. Acknowledge that under very special circumstances the nucleus of one atom can physically touch the nucleus of another atom—that this is what happens in a thermonuclear reaction.
Discuss the relative distances between positive and negative charges in neighboring atoms and the role of the electric forces in molecular structure. (You’re discussing the implications of Coulomb’s law at short distances—combined with the ideas you previously discussed in your treatment of tides and tidal forces, namely the importance of relative distances.)
Atomic Number and Periodic Table: Schematically show the hydrogen atom, and add a proton and neutrons to build a helium atom, and then a lithium atom, and so on. Discuss atomic number, and the role that the number of protons play in the nucleus in dictating the surrounding electron configuration. Call attention to and briefly discuss the periodic table. Point out that the atomic configurations depicted in Figure 11.6 are simply models not be taken seriously. For example, if the nuclei were drawn to scale they would be scarcely visible specks. And the electrons don’t really “orbit,” as the drawings suggest—such terms don’t seem to have much meaning at the atomic level. It would be more precise to say they “swarm,” or are “smeared,” around the central nuclei. You might state that the configuration of electrons and their interactions with each other is basically what the field of chemistry is about.
Antimatter: Discuss antimatter, and the speculations that other galaxies may be composed of antimatter. There are even antiquarks. Our knowledge of quarks is relatively new, newer than the first edition of this text. Until recent times it was a fact that the fundamental building block of matter was the proton, neutrons, and electrons discussed in this chapter. Now it is a fact that the proton and neutron are not the fundamental particles, but are composed of quarks. This change of view or advancement in our knowledge, like others, is often cited as a weakness by people who do not understand what science is about. Science is not a bag of answers to all the questions of the world, but is a process for finding answers to many questions about the world. We continue to refine our models and add new layers to our understanding—sometimes building onto layers and other times replacing layers. It is unfortunate that some people see this as a weakness. This is remindful of Bertrand Russell, who publicly changed his mind about certain ideas in the course of his life—changes that were part of his growth, but were looked upon by some as a sign of weakness (as discussed in Chapter 1). Likewise with physics. Our knowledge grows and that’s nice!
Dark Matter: Lest anyone feel that physics is near its end insofar as what there is still to be known in this realm, consider dark matter—today’s major physics mystery. Whatever it is, there is very little chance it will occupy any place on the periodic table of the elements. How intriguing—most of the stuff of the universe isn’t on the periodic table. And it is “out there?” Bear in mind, that we are “out there.” Dark matter is likely infused in matter as we know it. Interesting point: There is likely dark matter in the platinum cylinder that defines the kilogram, locked in a glass case in France
Phases of Matter: Briefly discuss the phases of matter, and how changes in molecular motion (temperature) are responsible for changes from the solid to liquid to gaseous to plasma phases. In earlier editions of Conceptual Physics, “states” of matter is spoken of. Either may be used. One ambiguity is that states also refers to the energy states of atoms—a confusion to avoid.
Solutions to Chapter 11 Exercises
1. One.
2. In a water molecule, H2O, there are three atoms, two hydrogen and one oxygen.
3. The average speed of molecules increases.
4. The speed at which the scent of a fragrance travels is much less than the speed of the individual molecules that make it up because of the many collisions among molecules. Although the molecular speed between collisions is great, the rate of migration in a particular direction through obstructing molecules is very much less.
5. The cat leaves a trail of molecules and atoms on the grass. These in turn leave the grass and mix with the air, where they enter the dog’s nose, activating its sense of smell.
6. A body would have no odor if all its molecules remained within it. A body has odor only if some of its molecules enter a nose.
7. The atoms that make up a newborn baby or anything else in this world originated in the explosions of ancient stars. (See Figure 11.8, my daughter Leslie.) The molecules that make up the baby, however, were formed from atoms ingested by the mother and transferred to her womb.
8. Water is not an element. It is a compound. Its molecules are made of the atoms of elements hydrogen and oxygen.
9. Agree partially. It’s better to say an element is defined by the number of protons in the nucleus. The number of protons and electrons are equal only when the element is not ionized.
10. No way! The number of protons in an atomic nucleus defines the element. Nuclei of an element can have different numbers of neutrons, but only one number of protons. If the number of protons changes, the element changes.
11. Brownian motion is the result of more atoms or molecules bumping against one side of a tiny particle than the other. This produces a net force on the particle, which it is set in motion. Such doesn’t occur for larger particles because the numbers of bumps on opposite sides is more likely equal, producing no net force. The number of bumps on a baseball is practically the same on all sides, with no net force and no change in the baseball’s motion.
12. Brownian motion is apparent only for microscopic particles because of their small mass (meaning that they also have small size). Against a large particle, the random bumps exert nearly steady forces on each side that average to zero, but for a small particle there are moments when appreciably more hits occur on one side than the other, producing motion visible in a microscope.
13. Individual Ping-Pong balls are less massive than individual golf balls, so equal masses of each means more Ping-Pong balls than golf balls.
14. Individual carbon atoms have less mass than individual oxygen atoms, so equal masses of each means more carbons than oxygens.
15. Since aluminum atoms are less massive than lead atoms, more aluminum atoms than lead atoms compose a 1-kg sample.
16. Of the substances listed, H2, He, Na, and U are pure elements. H2O and NaCl are compounds made of two elements, and three different elements contribute to H2SO4.
17. Nine.
18. (a) In both there are 27 protons (see periodic table). There are 32 neutrons in Co-59 and 33 neutrons in Co-60. (b) The number of orbiting electrons matches the atomic number, 27.
19. The element is copper, atomic number 29. Any atom having 29 protons is by definition copper.
20. The source of oxygen and nitrogen is the air, which is needed for the burning (combustion) of gasoline.
21. Carbon in trees is extracted from carbon dioxide in the air. In a loose sense, we can view a tree as solidified air!
22. The chemical properties of an element depend on the electrons in the atomic shells. But the number of electrons, in turn, is dictated by the number of protons in the nucleus. So in this indirect way, the number of protons in the atomic nucleus dictates the chemical properties of the element.
23. Check the periodic table and see that gold is atomic number 79. Taking a proton from the nucleus leaves the atomic number 78, platinum—much more valuable than adding a proton to get mercury, atomic number 80.
24. Carbon. (See the periodic table.)
25. Lead.
26. Radon.
27. In an electrically neutral atom the number of protons in the nucleus equals the number of orbiting electrons. In an ion, the number of electrons differs from the number of nuclear protons.
28. An atom gains an electron to become a negative ion. Then it has more electrons than protons.
29. An atom loses an electron to become a positive ion. Then it has more protons than electrons.
30. Boiled water expels oxygen molecules and is deficient in the oxygen that fish need to survive.
31. The capsule would be arsenic.
32. An oxygen molecule is H2; ozone is H3.
33. Sodium and chlorine atoms combine to form a molecule with completely different characteristics—the molecules of common table salt.
34. Hydrogen and oxygen.
35. Neon, argon, krypton, xenon, and radon (the noble gases).
36. Oxygen.
37. Germanium has properties most like silicon, as it is in the same column, Group XIV, as silicon in the periodic table.
38. The element below carbon in the periodic table, silicon, has similar properties and could conceivably be the basis of organic molecules elsewhere in the universe.
39. Protons contribute more to an atom’s mass, and electrons more to an atom’s size.
40. Letting the formula KE = 1/2 mv2 guide your thinking, for the same speed the atom with greater mass has greater KE. Greater-mass carbon therefore has greater KE than hydrogen for the same speed.
41. The hydrogen molecules, having less mass, move faster than the heavier oxygen molecules.
42. Electrical repulsion. Electrons speeding around within an atom create an electrified cloud that repels the similar clouds of other electrons, preventing the atoms from coalescing and keeping us from falling through our chairs. (For the record, quantum effects play a large role as well.)
43. The water and alcohol molecules actually fit into one another and occupy less space when combined than they do individually. Hence, when water and alcohol are mixed, their combined volume is less than the sum of their volumes separately.
44. (a) Heating gives the molecules more kinetic energy so they can shake loose from the bonds holding them in a solid, creating a liquid. (b) The solid must have stronger interatomic forces.
45. You really are a part of every person around you in the sense that you are composed of atoms not only from every person around you, but from every person who ever lived on Earth! The child’s statement in the Part 2 photo opener is indisputable. And the atoms that now compose you will make up the atomic pool that others will draw upon.
46. With every breath of air you take, it is highly likely that you inhale one of the atoms exhaled during your very first breath. This is because the number of atoms of air in your lungs is about the same as the number of breaths of air in the atmosphere of the world.
47. They assumed hydrogen and oxygen were single-atom molecules with water’s formula being H8O.
48. There would be a 100% conversion to radiant energy.
49. The amount of matter that a given amount of antimatter would annihilate is the same as the amount of antimatter, a pair of particles at a time. The whole world could not be annihilated by antimatter unless the mass of antimatter were at least equal to the mass of the world.
50. Open-ended.
Chapter 11 Problem Solutions
1. There are 16 grams of oxygen in 18 grams of water. We can see from the formula for water, H2O, there are twice as many hydrogen atoms (each of atomic mass 1) as oxygen atoms (each of atomic mass 16). So the molecular mass of H2O is 18, with 16 parts oxygen by mass.
2. A carbon atom is 12 times as massive as a hydrogen atom, or 3 times as massive as four hydrogen atoms. A bit of reasoning will show that for every 4 grams of hydrogen there will be 3 ´ 4 = 12 grams of carbon, which when totaled gives 16 grams. So there are 4 grams of hydrogen in 16 grams of methane.
3. The atomic mass of element A is 3/2 the mass of element B. Why? Gas A has three times the mass of Gas B. If the equal number of molecules in A and B had equal numbers of atoms, then the atoms in Gas A would simply be three times as massive. But there are twice as many atoms in A, so the mass of each atom must be half of three times as much—3/2.
4. The volume of the oil is like the volume of a very large but very thin pancake, and equals its area multiplied by its thickness. V = Ah, where V is the volume (known) and A is the area (known from measurement) and h is the thickness, or diameter of the oil molecule. Solving for the thickness we get h = V/A, = (10-9 m3)/(1.0 m2) = 10-9 m (which is about ten atomic diameters). (This makes a good lab exercise with diluted oleic acid.)
5. (a) 104 atoms (length 10-6 m divided by size 10-10 m).
(b) 108 atoms (104 x 104).
(c) 1012 atoms (104 ´ 104 ´ 104).
(d) $10,000 buys a good used car, for instance. $100 million buys a few jet aircraft and an airport on which to store them, for instance. $1 trillion buys a medium-sized country, for instance. (Answers limited only by the imagination of the student.)
6. From the hint:
=
= ; x = = 1
7. There are 1022 breaths of air in the world’s atmosphere, which is the same number of atoms in a single breath. So for any one breath evenly mixed in the atmosphere, we sample one of Julius Caesar’s atoms at any place or any time in the atmosphere.
8. The total number of people who ever lived (6 x 109 ´ 20 = 120 ´ 109 which is roughly 1011 people altogether) is enormously smaller than 1022. How does 1022 compare to 1011? 1022 is (1011)2! Multiply the number of people who ever lived by the same number, and you’ll get 1022, the number of air molecules in a breath of air. Suppose each person on Earth journeyed to a different planet in the galaxy and every one of those planets contained as many people as the Earth now contains. The total number of people on all these planets would still be less than the number of molecules in a breath of air. Atoms are indeed small—and numerous!
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