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Chapter #4 Section Assessment #1 - 33 4.1 Section Assessment 1. How did Democritus characterize atoms? Democritus believed that atoms were indivisible (you couldn’t split them into pieces, hence the name “atomos”), and they were indestructible. Do we still believe these things to be true? No. Atoms can be split. J. J. Thomson discovered the electron in c. 1897. The “atom bomb” gets its explosive power from splitting uranium or plutonium atoms in half. Democritus 460 B.C. – 370 B.C. 4.1 Section Assessment 1. How did Democritus characterize atoms? Democritus believed that atoms were indivisible (you couldn’t split them into pieces, hence the name “atomos”), and they were indestructible. Do we still believe these things to be true? No. Atoms can be destroyed. Antimatter, discovered in 1932 by Carl D. Anderson, destroys matter when it touches it. Matter and antimatter explode with a hundred times more force than nuclear bombs as they annihilate each other. Democritus 460 B.C. – 370 B.C. 4.1 Section Assessment 2. How did Dalton advance the atomic philosophy proposed by Democritus? Dalton used experimental methods, whereas Democritus had used only imagination and reasoning. Dalton also had much better knowledge of the elements, and he studied the mass ratios in which elements combined to make compounds. Democritus 460 B.C. – 370 B.C. Ancient Greek “elements” fire Modern “States of Matter” Some of Dalton’s elements hot gas or plasma air gas water liquid earth solid hydrogen oxygen nitrogen carbon sulfur phosphorus John Dalton 1766-1844 4.1 Section Assessment 3. What instrument can be used to observe individual atoms? The scanning tunneling microscope can generate pictures where individual atoms can be distinguished. For instance, on page . . 103, . each iron atom in the picture appears as a single . . . cone. Scanning Tunneling Microscope 1981 4.1 Section Assessment 4. In your own words, state the main ideas of Dalton’s atomic theory. i) All matter is composed of tiny, indivisible particles called “atoms”. Is everything he said here still believed to be true? ii) Atoms of the same element are identical. The atoms of any one element are different from those of another element. Is everything he said here still believed to be true? iii) Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds. Is everything he said here still believed to be true? iv) Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another element as a result of chemical reaction. Is everything he said here still believed to be true? 4.1 Section Assessment 5. According to Dalton’s theory, is it possible to convert atoms of one element into atoms of another? Explain. According to Dalton, atoms can not be changed from one element to another. For example, a carbon atom will always be a carbon atom, forever. Of course, we know better than that now. We now know that in addition to chemical reactions, which merely rearrange atoms, there are also nuclear reactions, which DO change atoms from one element to another. For instance, carbon-14 atoms will spontaneously turn into nitrogen-14 atoms by the process of radioactive decay. Does that mean that Dalton was an idiot? NO WAY! Dalton was brilliant. Even Einstein was wrong about certain things. 4.1 Section Assessment 6. Describe the range of radii of most atoms in nanometers (nm). 5 x 10-2 nm to 2 x 10-1 nm That’s the short version of the answer. If you know how to get that answer, then click the red button below to skip to #7. You can also click the red button if you just don’t care about #6. If you don’t understand how to get the answer to #6 and you do want to learn how, then click the green button instead. Skip to #7 I want to learn about #6! Okay. Buckle your seatbelt. 4.1 Section Assessment 6. Describe the range of radii of most atoms in nanometers (nm). The book says, on page 103, at the beginning of the 3rd paragraph, that “The radii of most atoms fall within the range of 5 x 10-11 m to 2 x 10-10 m.” First of all, we’d better make sure we know what “radii” means. The width of a circle is called its “diameter”. Half the width of a circle is called its “radius”. The radius is the distance from the center of the circle to the edge of the circle. “Radii” is just the plural of “radius”. A yardstick is three feet long. An American football field is 100 yards long. These days, America is the only country that measures distance in yards. The rest of the world uses the metric system. In the metric system, the meter is used instead of the yard. A meter is a little bit longer than a yard. An ordinary ruler is twelve inches long. 0 1 INCHES 2 5 4 3 7 6 8 9 10 11 12 cm 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 . . . You will notice that it is about 30 centimeters long. A paperclip is about one centimeter wide. 0 If you flip it around . . . If you look closely, you will notice that there are ten tiny marks between each centimeter. These marks are one millimeter apart. If you look closely, you will notice that there are ten tiny marks between each centimeter. These marks are one millimeter apart. The wire that a paperclip is made out of is about one millimeter thick. A millimeter is pretty small, isn’t it? A one-millimeter metal wire is still much thicker than a human hair, though. According to the tables on page 74, how big is a nanometer? It says that 1 m = 109 nm 109 is 1,000,000,000, which is one billion. Therefore, a nanometer is one one-billionth of a meter. The tables also say that 1 m = 103 mm 103 is 1,000, which is one thousand. Therefore, a millimeter is one one-thousandth of a meter. If 1m = 109 nm and 1 m = 103 mm, then . . . 109 nm = 103 mm 103 103 10(9-3) nm = 1 mm 1 mm = 1,000,000 nm A nanometer is one one-millionth of a millimeter. That’s small. 106 nm = 1 mm If you got confused during the math on that last slide, don’t worry. I was just tyring to show you how ridiculously small a nanometer (nm) is. What metric unit of length is just a bit longer than three feet? 1 meter = 1 m How wide is a paper clip? 1 centimeter = 1 cm How thick is the wire that a paper clip is made of? 1 millimeter = 1 mm 1 nm = one one-billionth of a meter How big is a nanometer? 1 nm = one one-millionth of a millimeter GREAT! Now that you know your units, let’s do the math . . . 4.1 Section Assessment 6. Describe the range of radii of most atoms in nanometers (nm). The book says, on page 103, at the beginning of the 3rd paragraph, that “The radii of most atoms fall within the range of 5 x 10-11 m to 2 x 10-10 m.” Unfortunately, we need to give our answer in nm, not m. Page 103 gives us good information, but it’s in the wrong UNITS. We need to CONVERT meters (m) into nanometers (nm). We need to do a UNIT CONVERSION. 4.1 Section Assessment 6. Describe the range of radii of most atoms in nanometers (nm). 5 x 10-11 m 109 nm x 5x 10(-11 + 9) = 5x 10(-2) = 2x 10(-10 + 9) = 2x 10(-1) nm 1 m 1 2 x 10-10 m 109 nm x 1 = nm nm 1 m nm 5 x 10-2 nm to 2 x 10-1 nm 4.1 Section Assessment 7. A sample of copper with a mass of 63.5 grams contains Calculate the mass of a single copper atom. So what the heck do you do with these numbers? 63.5 g = 10.5 6.02 x 1023 atoms 10.54817276 6.02 ) 63.5 = 1.05 x 10-23 x 10-22 g/atom g/atom 6.02 x 1023 atoms. 4.2 Section Assessment 8. What are three types of subatomic particles? The three kinds of subatomic particles are the electron, the proton, and the neutron. All atoms are made merely of different combinations of these three fundamental building blocks. Since you are made of atoms, that means that you are made of protons, neutrons, and electrons. 4.2 Section Assessment 9. How does the Rutherford model describe the structure of atoms? From his experiments, Rutherford concluded that an atom is made of a positively-charged nucleus surrounded by a region of empty space in which electrons orbit that nucleus. Rutherford believed that an atom’s nucleus was very tiny compared to the atom as a whole, and that, in spite of this, the nucleus is where most of the atom’s mass is. Thus, according to Rutherford’s interpretation of his experimental results, atoms are made mostly of empty space. This implies that all things made of matter, anything that is solid, liquid, or gas, is actually made mostly of empty space. 4.2 Section Assessment 10. What are the charges and relative masses of the three main subatomic particles? (Table 4.1 at the top of page 106 summarizes this nicely for you.) Particle electron proton neutron Symbol Charge Relative mass ep+ (or H+!) n0 1- 1/1840 1+ 1 0 1 (actually a little more than that) 4.2 Section Assessment 11. Describe Thomson’s and Millikan’s contributions to atomic theory. Thomson, through his cathode ray tube experiments, discovered the electron. Millikan, through his oil drop experiments, determined the charge and mass of the electron. 4.2 Section Assessment 12. Compare Rutherford’s expected outcome of the gold foil experiment with the actual outcome. Rutherford expected that the alpha particles he shot at the gold foil would pass through it with little deflection. Instead, he found that, although most did shoot straight through with little or no deflection, some alpha particles were deflected at very large angles, and some alpha particles even bounced back toward the alpha particle source. He likened this to shooting a cannonball at a piece of facial tissue and having it bounce back. Rutherford had no idea that there was anything dense and heavy enough in an atom to bounce an alpha particle back to where it came from. 4.2 Section Assessment 13. What experimental evidence led Rutherford to conclude that an atom is mostly empty space? The great majority of the alpha particles went straight through the gold foil, as though nothing were there. 4.2 Section Assessment 14. How did Rutherford’s model of the atom differ from Thomson’s? Thomson thought the atom was a mass of positive charge with negative electrons embedded in its outer surface. Rutherford’s model didn’t envision the atom being a big ball of positive charge, but, rather, a tiny speck of positive charge in the middle of an almost perfectly empty region of space. Rutherford did not envision electrons as stuck in anything, but, rather, as whizzing through space, in “orbit” around the positive nucleus. He envisioned the atom as a tiny solar system, in which the nucleus was like the sun and the electrons were like planets. #’s 15 – 24 are “Practice Problems”, so the answers are in the back of the book. Look on page R84. 4.3 Section Assessment 25. What distinguishes the atoms of one element from the atoms of another? The number of protons in an atom determines what element it belongs to. Number of protons is called “atomic number”. For instance, all gold atoms have exactly 79 protons in them. Any atom that has 26 protons in it is, by definition, an iron atom. 4.3 Section Assessment 26. What equation tells you how to calculate the number of neutrons in an atom? number of neutrons = mass number – atomic number This is true because mass number = number of protons + number of neutrons . . . and atomic number = number of protons 4.3 Section Assessment 27. How do the isotopes of a given element differ from one another? Isotopes of a given element vary only in the number of neutrons in each atom. For instance, most carbon atoms are from the isotope carbon-12, but a few are from the isotope carbon-14. Carbon-12 atoms and carbon-14 atoms all have 6 protons, but carbon-12 atoms have only 6 neutrons each, whereas carbon-14 atoms have 8 neutrons each. Incidentally, the two extra neutrons in carbon-14 make that isotope radioactive . . . But we’ll discuss radioactivity later . . . 4.3 Section Assessment 28. How is atomic mass calculated? Atomic mass is calculated by what is called a “weighted average” method (no pun intended). The mass of each known isotope of the element is multiplied by the % abundance of that element. Because of this, rare isotopes tend to have a very small effect upon the average atomic mass of an element. Also, the average atomic mass of the element tends to be very close to the atomic mass of the most common isotope of that element. 4.3 Section Assessment 29. What makes the periodic table such a useful tool? By looking at the periodic table, one can predict the chemical and physical properties of an element by its location on the table. Here are a few examples of this: *Elements in the same vertical column tend to have the same number of “valence electrons”, and, therefore, similar bonding properties. *Elements to the left of the “staircase” are almost all metals, elements to the right almost all nonmetals, and elements touching the staircase mostly metalloids. *Elements with high electronegativies and ionization energies tend to be clustered in the upper right hand corner of the periodic table. [Don’t burden your brain too much with these examples just yet. We’ll be looking at them more closely in chapter 6.] 4.3 Section Assessment 30. What does the number represent in the isotope name “platinum-194”? Write the symbol for this atom using superscripts and subscripts. A platinum-194 atom has a total of 194 protons and neutrons, combined, in its nucleus. (194 is the “mass number” of platinum-194.) Since all platinum atoms have 78 protons, then the symbol for a platinum-194 atoms would be . .. 194 78 Pt 4.3 Section Assessment 31. The atomic masses of elements are generally not whole numbers. Explain why. The average atomic mass of an element is typically not a whole number precisely because it is an average atomic mass. Averages tend to have decimal fractions hanging of the ends of them. An average atomic mass is the weighted average of the atomic masses of all the naturallyoccurring isotopes of the element. Even if the atomic masses of the isotopes were whole numbers (which they aren’t), the weighted average of those atomic masses would almost certainly never be a whole number. [Don’t worry about this question too much. If you get it, great. If you don’t, no biggie. There are more important concepts for you to master in this class than this one.] 4.3 Section Assessment 32. List the number of protons, neutrons, and electrons in each pair of isotopes. a. 6 3 Li 7 3 Li If an atom does have an electric charge, then it will be written where the flashing blue squares are. Protons: 3 3 (Lithium’s atomic number is 3.) Neutrons: 3 4 (# of neutrons = mass number – atomic number) 3 (no electrical charge listed, so # of electrons = # of protons = atomic number) Electrons: 3 4.3 Section Assessment 32. List the number of protons, neutrons, and electrons in each pair of isotopes. b. 42 20 Protons: Neutrons: Electrons: Ca 20 22 20 44 20 Ca 20 24 20 4.3 Section Assessment 32. List the number of protons, neutrons, and electrons in each pair of isotopes. c. 78 34 Protons: Neutrons: Electrons: Se 34 44 34 80 34 Se 34 46 34 4.3 Section Assessment 33. Name two elements that have properties similar to those of the element calcium (Ca). Be (beryllium) Mg (magnesium) Ba (barium) Sr (strontium) Ra (radium) If two elements are in the same vertical column of the periodic table, they probably have similar properties. Because of this, columns are also known as “families”. The above-named elements comprise the family known as the . . . “alkaline earth metals”.