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Home , Atomic nucleus

The Atomic lindens and Radioactivity

he configuration of in an determines whether and how the atom T bonds to form compounds. It also dictates the melting and freezing temperatures and the thermal and electrical conductivity as well as the taste, texture, appearance, and color of substances. Changes in elec- Radioactivity, a tool of tron produce visible light. Larger changes produce X-rays. medicine. This chapter goes even deeper into the atom—to the .

39.1 The Atomic Nucleus

It would take 30 000 nuclei to stretch across a single carbon atom. The nucleus within the atom is as inconspicuous as a cookie crumb in the middle of the Rose Bowl football stadium. Despite the small size of the nucleus, much has been learned about its structure. The nucleus is composed of called , which when electrically charged are , and when electrically neutral are .* Neutrons and protons have close to the same , with the 's being slightly greater. Nucleons have nearly 2000 times the mass of electrons, so the mass of an atom is practically equal to the mass of its nucleus alone. The positively charged protons in the nucleus hold the nega- tively charged electrons in their orbits. Each has exactly the same magnitude of charge as the , but the opposite sign. So in an electrically neutral atom, there are as many protons in the

* Protons and neutrons are themselves composed of subnuglear particles called . Are quarks themselves made of still smaller particles? They may be, but so far there is no evidence or theoretical reason for believing so. Theoretical physicists say today that quarks are the elementary particles of which all nucleons and other strongly interacting 2 LabOrcitorii Manual 97 particles are made.

009 nucleus as there are electrons outside. The number of protons in the nucleus therefore determines the chemical properties of that atom, because the positive nuclear charge determines the possible struc- tures of electron orbits that can occur. The number of neutrons in the nucleus has no direct effect on the electron structure, and hence does not affect the of the atom. The principal role of the neutrons is to act as a sort of nuclear Figure 39.1 A cement to hold the nucleus together. Nucleons are bound together The number of electrons that by an attractive nuclear appropriately called the strong force. 11 nucleus is surround the atomic The of attraction is strong only over a very short matched by the number of pro- (Figure 39.2). Whereas the electrical force between charges tons in the nucleus. distance decreases as the inverse square of the distance, the nuclear force decreases tar more rapaly. When: two otteleunb ate just few diameters apart, the nuclear force they exert on each other -00-- is nearly zero. This means that if nucleons are to be held together by the strong force, they must be held in a very small volume. Nuclei are tiny because the nuclear force is very short-range. It is an interesting feature of quantum that particles Figure 39.2 A held close together have large kinetic and tend to fly apart. So, The nuclear strong force is a although the nuclear force is strong, it is only barely strong enough to very short-range force. For hold a pair of nucleons together. For a pair of protons, which repel nucleons very close or in con- tact, it is very strong (large force each other electrically, the nuclear force is not quite strong enough to vectors). But a few nucleon keep them together. When neutrons are present, however, the attrac- diameters away it is nearly zero tive strong force is increased relative to the repulsive electric force (small force vectors). (since neutrons have no charge). Thus, the presence of neutrons adds to the nuclear attraction and keeps protons from flying apart. The more protons there are in a nucleus, the more neutrons are needed to hold them together. For light elements, it is sufficient to have about as many neutrons as protons. For heavy elements, extra neutrons are required. The most common form of , for example, has 82 protons and 126 neutrons, or about one and a half times as many neutrons as protons. For elements with more than 83 protons, even the addition of extra neutrons cannot stabilize the nucleus.

Figure 39.3 • A strong attractive nuclear force 39.2 acts between nearby protons A and B, but not significantly One of the factors that sets a limit on how many stable nuclei can exist between A and C. The longer- is the instability of the neutron. A lone neutron will spontaneously range electric force repels pro- decay into a proton plus an electron (and also an antineutrino, a tiny tons A and C as well as A and B. we will not discuss here). Out of a bunch of lone neutrons, The mutual repulsion of all the about half of them will decay in 11 minutes. Particles that decay in this protons in a heavy nucleus tends or in similar ways are said to be radioactive. A lone neutron is to make such nuclei unstable. radioactive. The rules of radioactivity inside atomic nuclei are governed by the mass-energy equivalence. Particles decay only when their com- bined products have less mass after decay than before. The mass of a neutron is slightly greater than the total mass of a proton plus elec- tron (and the antineutrino). So when a neutron decays, there is less mass after decay than before. Decay will not spontaneously occur for reactions where more macs results. The reverse reaction, a proton decaying into a neutron, can occur only with external energy input. 610 Chapter 39 The Atomic Nucleus and Radioactivity • tit !tithe tar d ne. er Figure 39.4 A efes A neutron—proton combination is al: stable hut a neutron by itself is fe MAGNET unstable and turns into a proton aapr by emitting an electron (as well ogby as an antineutrino—not shown). !. 'Sam

pas y afy, ' en. to RADIUM SAMPLE LEAD BLOCK AT BOTTOM OF HOLE en( to he lc - c fo Figure 39.5 A The separation of alpha, , and gamma rays in a magnetic field. The rays tronlds come from a radioactive source placed at the bottom of a hole drilled in a rt. lead block. trone ieni tts, ea All the elements heavier than bismuth ( 83) exane, decay in one way or another. Thus, these elements are radioactive. mes , Their emit three distinct types of rays. The rays are named Figure 39.6 A protts, alpha, beta, and gamma, respectively, after the first three letters of the Greek alphabet, a, p, and y. Alpha rays have a positive electric An contains two leus. protons and two neutrons bound charge, beta rays are negative, and gamma rays are electrically neu- together and is identical to a tral. The three rays can be separated by putting a magnetic field nucleus. across their path (Figure 39.5). An alpha ray is a stream of particles that are made of two pro- tons and two neutrons and that are identical to the nuclei of helium atoms. These are called alpha particles (Figure 39.6). LIGHT RAY A beta ray is simply a stream of electrons. An electron is ejected from the nucleus when a neutron is transformed into a proton. It may seem that the electrons are "buried" inside the neutron, but this is not true. An electron does not exist in a neutron any more than a W AN spark exists inside a rock about to be scraped across a rough surface. The electron that pops out of the neutron, like the spark that pops out of the scraped rock, is produced during an interaction. !d by A gamma ray is massless energy Like visible light, gamma rays C0111- are simply of electromagnetic , but of much higher if ass of a frequency and energy. Visible light is emitted when electrons jump elec- - Figure 39.7 A s less from one orbit to another of lower energy Gamma rays are emitted A gamma ray is simply electro- cur for when nucleons do a similar sort of thing inside the nucleus. Some- magnetic radiation, much higher ton times there are great energy differences in nuclear energy levels, so in frequency and energy per put.. the photons (gamma rays) emitted carry a large amount of energy. than light and X-rays. 611 39.3 Radiation Penetrating Power

There is a great difference in the penetrating power of the three types R AD I OACT I V E of rays emitted by radioactive elements. Alpha rays are the easiest to SOURCE stop. They can be stopped by a reasonably heavy piece of paper or a few sheets of thin paper. Beta rays go right through paper but are stopped by several sheets of aluminum foil. Gamma rays are the stop and require lead or other heavy shielding to ot most difficult to block them. An alpha particle is easy to stop because it is relatively slow and its double-positive charge InLeiauts with thc mclocuies it encounters along its path. It slows down as it shakes many of these apart and leaves positive and negative in its wake. Even when traveling through nothing but air, an alpha particle will come to a stop after only a few centimeters. It soon grabs up a couple of stray electrons and becomes nothing more than a harmless helium atom. A normally moves at a faster speed than an alpha particle, carries only a single negative charge, and is able to travel much farther through the air. Most beta particles lose their energy during the course of a large number of glancing collisions with atomic electrons Except for rare direct hits, energy is lost in many small steps. Beta particles slow down until they reach the speeds of thermal , becoming a part of the material they are in, like any other electron. A Figure 39.8 Gamma rays are the most penetrating of the three because they Alpha particles penetrate least no electrical attraction or deflection, a gamma and can be stopped by a few have no charge. With sheets of paper; beta particles by ray photon interacts with the absorbing material only via a direct a sheet of aluminum; gamma hit with an atomic electron or a nucleus. Unlike charged particles, a rays by a thick layer of lead. gamma ray photon can be removed from its beam in a single encounter. Dense materials such as lead are good absorbers mainly because of their high electron .

• Question Pretend you are given three radioactive cookies—one alpha, one beta, and the other gamma. Pretend that you must eat one, hold one in your hand, and put the other in your pocket. Which would you eat, hold, and pocket, if you are trying to minimize your expo- sure to radiation?

• Answer

Ideally, of course, get as far from the cookies as possible. But if you must eat one, hold one, and put one in your pocket then hold the alpha; the skin on your hand will shield you. Put the beta in your pocket your clothing will likely shield you. Eat the gamma; it will pen- etrate ,your body in any of these cases, anyway. (In real life always use appropriate safe- guards when near radioactive materials.)

612 Chapter 39 The Atomic Nucleus and Radioactivity 39.4 Radioactive

It has already been stated that the number of protons in an atomic nucleus determines the number of electrons surrounding the nucleus in a neutral atom. If there is a difference in the number of electrons and protons, the atom is charged and is called an . An ionized atom is one that has a different number of electrons than nuclear protons.

4 Figure 39.9 Which of these diagrams shows en inn?

The number of neutrons in the nucleus, however, has no bearing on the number of electrons the atom may have. This means that the number of neutrons has no direct bearing on the chemistry of an atom. Let's consider a atom. The common form of hydro- gen has a bare proton as its nucleus. Any nuclear configuration that has only one proton in its nucleus is hydrogen—by definition. There can be different kinds, or isotopes, of hydrogen, however. In one iso- tope, the nucleus consists of only a single proton. In a second iso- tope, the proton is accompanied by a neutron. In a third , there are two neutrons. All the isotopes of a particular element are chemically identical. The orbital electrons are affected only by the positive charge in the nucleus, not by its neutrons. We distinguish between the different by 2IH, ly 2H , and where the lower number is the atomic number (the num- ber of protons) and the upper number is the number (the total number of nucleons). 1 Figure 39.10 The atomic number is equal to the number of protons in the ATOMIC nucleus, and the atomic mass number is equal to the number ) 4He --- CHEMICAL SYMBOL FOR HELIUM of nucleons in the nucleus (both protons and neutrons). _ 4-\-- ATOMIC NUMBER

The common isotope of hydrogen, H, is a stable element. So is the isotope H, called . "Heavy water" is the name usually given to H20 in which the H's are deuterium atoms. The triple- weight hydrogen isotope H, called , however, is unstable and undergoes . This is the radioactive isotope of hydrogen. All elements have isotopes. Some are radioactive.and some are not. All the isotopes of elements above atomic number 83, however, are radioactive.

613 Figure 3911 Ir. Three isotopes of hydrogen. Each nucleus has a single proton that holds a single orbital elec- tron, which, in turn, determines the chemical properties of the atom. The varying number of 1 neutrons changes the mass of the atom, but not its chemical properties. The common isotope of is 95v, or U-238 for short. It is radioactive, but with a smaller decay rate than 2KU, or U-235. Any nucleus with 92 protons is uranium, by definition. Nuclei with 92 protons but different numbers of neutrons are simply different iso- topes of uranium.

Figure 39.12 All isotopes of uranium are unstable and undergo radio- active decay.

• Questions 1. When the tritium nucleus, IH, undergoes beta decay, one of its neutrons is converted to a proton. What is the resulting nucleus? 2. The nucleus of -8, 84Be, undergoes a special kind of radioactive decay: it splits into two equal halves. What nuclei are the products of this decay? Why is this a form of ? 3. The electric force of repulsion between the protons in a heavy nucleus acts over a greater distance than the attractive among the neutrons and protons in the nucleus. Given this fact, explain why all of the very heavy elements are radioactive.

• Answers 1. The resulting nucleus contains two protons and one neutron, This is an isotope of helium, the second element in the . It is helium-3, or 31-Ie. 2. When beryllium-8, which contains 4 protons and 4 neutrons, splits into equal halves, a pair of nuclei with 2 protons and 2 neutrons are created. These are nuclei of helium-4, tile, also called alpha particles. So this reaction is a form of alpha decay. 3. Each proton in an atomic nucleus is repelled by every other proton in the nucleus, but it is attracted only by the nucleons closest to it. In a large nucleus, where protons such as those on-opposite sides are far apart, electrical repulsion can exceed nuclear attrac- tion. This instability makes all the heaviest atoms radioactive.

614 Chapter 39 The Atomic Nucleus and Radioactivity I LINK TO TECHNOLOGY„ 39.5 Radioactive Half-Life etirojk Radioactive isotopes decay at different rates. The radioactive decay •11.4- n: rate is measured in terms of a characteristic time, the half-life. The half-life of a radioactive material is the time needed for half of the radioactive atoms to decay. Radium-226, for example, has a half-life of 1620 years. This means that half of any given specimen of Ra-226 will have undergone radioactive decay by the end of 1620 years. In the next 1620 years, half of the remaining radium will decay, leaving only one-fourth the original number of radium atoms. The other -four Lli.> are cosiveiled, by a succession or disiniegralions, Lo lead. After 20 half-lives, an initial quantity of radioactive atoms will be diminished to about one-millionth of the original quantity cj les p the glo$941$ r0 crealingironpAkia t.„ Rate of Decay of Radium eOglit:9,1.P.PY,1.9, chaffl- 'he'',ber, the ions Aretlietoihp and the current dimin- ishes. Electronic sensors',.. in the circuit detect this - reduced current and sound the alarm. Radio- activity used for this pur- pose saves many lives.

4 kg 1/8 kg

NOW 1620 3240 4860 YEARS

Figure 39.13 • Every 1620 years the amount of radium decreases by half.

The isotopes of some elements have a half-life of less than a mil- lionth of a second, while U-238, for example, has a half-life of 4.5 bil- lion years. The isotopes of each radioactive element have their own characteristic half-lives. Rates of radioactive decay appear to be absolutely constant, unaffected by any external conditions, however drastic. High or low pressures, high or low temperatures, strong magnetic or electric fields, and even violent chemical reactions have no detectable effect 1 Explore 2 Develop 3 Apply on the rate of decay of an element. Any of these ;,but stresses, however -> ; such severe by ordinary standards, is far too mild to affect the nucleus 2 Labora ory Niehuli1"9 deep ittrac. in the interior of the atom. 2 Concept-Development How do physicists measure radioactive half-lives? They cannot Practice Book 39-1 always do it by observing a specimen and waiting until the quantity

615 reduces to half. This is often much longer than a human life span! One can measure, however, the rate at which a substance decays. There are various radiation detectors for doing this (Figure 39.14). The half-life of an isotope is related to its rate of disintegration. In general, the shorter the half-life of a substance, the faster it disintegrates, and the more active is the substance. The half-life can be computed from the rate of disintegration, which can be measured in the laboratory.

Figure 39.14 IP Radiation detection. A Geiger counter detects incoming radia- tion by its ionizing effect on enclosed gas in the tube. A scin- tillation counter (not shown) detects incoming radiation by flashes of light that are produced when charged particles or gamma rays pass through it.

1-

• Questions 1. If a sample of a radioactive isotope has a half-life of 1 year, how much of the original sample will be left at the end of the second year? 2. If you have equal amounts of radioactive materials, one that has a short half-life and another that has a long half-life, which will give a higher reading on a radiation detector?

• Answers

1. One-quarter of the original sample will be left. The three-quarters that underwent decay become one or more different elements altogether. 2. The material with the shorter half-life is more active and will give a higher reading on a radiation detector.

616 Chapter 39 The Atomic Nucleus and Radioactivity ie re 39.6 Natural Transmutation of Elements

When a nucleus emits an alpha or a beta particle, a different ele- a ment is formed. The changing of one element to another is called transmutation. Consider common uranium, for example. Uranium has 92 protons. When an alpha particle is ejected, the nucleus is reduced by two protons and two neutrons. This is because they make up the alpha particle that leaves. The 90 protons and 144 neu- trons left behind are then the nucleus of a new element. This ele- ment is . This reaction is expressed as

238TT 4y_Ta 92`-' —7 9±"0 -r 211c An arrow is used here to show that the 24U changes into the other elements. When this happens, energy is released in three forms: gamma radiation, the kinetic energy of the alpha particle (He), and the kinetic energy of the thorium atom. Be sure to notice in the nuclear equation that the mass numbers at the top balance (238 = 234 + 4) and that the atomic numbers at the bottom also balance (92 = 90 + 2). Thorium-234, the product of this reaction, is also radioactive. When it decays, it emits a beta particle. Recall that a beta particle is an electron ejected from the nucleus. Once ejected, a beta particle is indistinguishable from an orbital electron or any other electron. When a beta particle is ejected, a neutron changes into a proton. In the case of thorium, which has 90 protons, beta emission leaves it with one fewer neutron and one more proton. The new nucleus then has 91 protons and is no longer thorium. It is the element protactinium. The reaction is"

, how +• acond 98Th 91pa _?e lat uhich

Beta emission is accompanied by the ejection of an antineutrino, not shown here. Antineutrinos are the of , and are extremely swift (moving at or close to the ) and extremely plentiful. Whether they have mass is still questionable. If they do, it is thousands of times less than the mass of the electron. 1 Explore 2 Develop , 3 Apply Neutrinos have no charge and seldom interact with . As you are reading this sentence, a thousand billion neutrinos emanating from the sun pierce through your 2 Concept-Development reading on a body. This is true day or night, since at nighttime the aolar neutrinos travel through Practice Book 39-2 Earth and pierce you from below. A cause for concern? No, that's just in action!

617 Note that although the atomic number has increased by 1 in this process, the mass number (number of nucleons) remains the same. Also note that the beta particle (electron) is written as _7e. The —1 is the charge of the electron. The 0 indicates that its mass is insignificant when compared with the mass of the protons and neutrons that alone contribute to the mass number. Beta emission has hardly any effect on the mass of the nucleus; only the charge (atomic number) changes. As the example of uranium-238 decay shows, when an atom ejects an alpha particle from its nucleus, the mass number of the resulting atom decreases by 4, and its atomic number decreases by 2. The resulting atom belongs to an element two spaces back in the periodic tahla.*Whan an atom elects a beta narticle from its nucleus., it loses no nucleons, so there is no change in mass number but its atomic number increases by 1.** The resulting atom belongs to an element one place forward in the periodic table. Thus, radioactive elements decay backward or forward in the periodic table. A radioac- tive nucleus may emit gamma radiation along with an alpha particle or a beta particle. Gamma emission has no effect on the mass num- ber or the atomic number. The radioactive decay of 2D.J. to an isotope of lead, 2,1913b, is shown on the next page in Figure 39.15. The steps in the decay process are shown in the diagram, where each nucleus that plays a part in the series is shown by a burst. The vertical column that con- tains the burst shows the atomic number of the nucleus, and the horizontal row shows its mass number. Each arrow that slants down- ward toward the left shows an alpha decay. Each arrow that points to the right shows a beta decay. Notice that some of the nuclei in the series can decay either way. This is one of several similar radioactive series that occur in nature.

• Questions 1. Complete the following nuclear reactions. a. 2nRa ??? + b. 24Po —> 25Pb + ??? 2. What finally becomes of all the uranium-238 that undergoes radioactive decay?

• Answers

1. a. 28Ra 21Mo + b. 28l4Po -4 22Pb + The 2. All the uranium-238 will ultimately become lead. On the way to becoming lead, it will exist as a series of other elements, as indicated in Figure 39.15.

* For a periodic table, see Figure 17.11. Look in the periodic table for the elements men honed in this section.

** Sometimes a nucleus emits a , which is the of an electron. A posi- tron has a charge of +1 and the same mass as the electron. In this case, a proton in the nucleus becomes a neutron, and the atomic number is decreased by 1 with no change in mass number.

618 Chapter 39 The Atomic Nucleus and Radioactivity U-238 Radioactive Decay Series

Xe 238 on r 2. 234 us,

230 ac- cle n- cc 1-1-1 226 co a LX- wn- 222 s to Lr) tn ive 218

214

210

twill 206 81 82 83 84 85 86 87 88 89 90 91 92 men-

posi- ATOMIC NUMBER t in the .hange Figure 39.15 A U-238 decays to Pb-206 through a series of alpha and beta decays.

619 39.7 Artificial Transmutation of Elements

The British physicist , in 1919, was the first of many investigators to succeed in artificially transmuting a . In a sealed container he bombarded nitrogen nuclei with alpha particles from a radioactive piece of ore and then found traces of oxygen and hydrogen that were not there before. Rutherford accounted for tho presence, cf the oxygen -rad hydrogen with the nuclear equation ii 14m , 4 Linn 17n 1 711 1r 2"c + 1/ After Rutherford's experiment there followed many such nuclear reactions—first with natural bombarding particles from radioactive elements, and then with more energetic particles (protons, deuterons, and alpha particles) hurled by giant atom-smashing particle accel- erators. Artificial transmutation is an everyday fact of life to the researchers of today. The elements beyond uranium in the periodic table—the transuranic elements—have been produced through artificial trans- mutation. All of these elements have half-lives that are much less than the age of Earth. Whatever transuranic elements might have existed naturally when Earth was formed have long since decayed.

NITROGEN GAS IN SEALED CONTAINER

LPHA TTI NG OURCE

Figure 39.16 A Artificial transmutation can be accomplished by simple means or by elabo- rate means.

620 Chapter 39 The Atomic Nucleus and Radioactivity 39.8 Carbon Dating

Earth's atmosphere is continuously bombarded from above by cos- mic rays—mainly high-energy protons—from beyond Earth. This results in the transmutation of many atoms in the upper atmos- phere. Protons, neutrons, and other particles are scattered through- out the atmosphere. Most of the protons quickly capture stray electrons and become hydrogen atoms in the upper atmosphere, but the neutrons keep going for long distances because they have no charge and do not interact electrically with matter. Sooner or later many of them collide with the nuclei of atoms in the lower atmos- phere. If they are captured by the nucleus of a nitrogen atom, the fol- lowing reaction can take place: re ifefae.liktin , bygone„tirnes.cfo fipd Out how ofd an artrfactl.. is, archeologists use car-, bon-14 dating. An arche- ologist must understand In this reaction, when nitrogen-14 is hit by a neutron (in), carbon-14 that this procedure and hydrogen are produced. works only for artifacts Most of the carbon that exists on Earth is the stable 'SC, carbon- that were once living 12. In the air, it appears mainly in the compound carbon dioxide. and that are less than Because of the cosmic bombardment, less than one-millionth of 1% 50,000 years old. of the carbon in the atmosphere is carbon-14. Like carbon-12, it Archeologists generally joins with oxygen to form carbon dioxide, which is taken in by work for government plants. This means that all plants have a tiny bit of radioactive car- and university research bon-14 in them. All animals eat plants (or eat plant-eating animals), facilities as well as for and therefore have a little carbon-14 in them. All living things con- museums and privately tain some carbon-14. funded organizations. Carbon-14 is a beta emitter and decays back into nitrogen by the following reaction:*

••01.

-) IN +le

In a living plant, which continues to take in carbon dioxide, a radioactive equilibrium is reached where there is a fixed ratio of carbon-14 to carbon-12. When a plant or animal dies, replenishment of the radioactive isotope stops. Then the percentage of carbon-14 decreases—at a known rate. The longer an organism is dead, the less carbon-14 remains. The half-life of carbon-14 is 5730 years. This means that half of the carbon-14 atoms that are now present in the remains of a body,

* The accompanying antineutrino is not shown.

621 plant, or tree will decay in the next 5730 years. Half the remaining carbon-14 atoms will then decay in the following 5730 years, and so forth. The radioactivity of once-living things therefore gradually decreases at a predictable rate (Figure 39.17).

15 188 B.C. 9458 &C. 3728 &C. 2002 AD.

Figure 39.17 A The radioactive carbon isotopes in the skeleton diminish by one-half every 5730 years.

Archeologists use the carbon-14 dating technique to establish the dates of wooden artifacts and skeletons. Because of fluctuations in the production of carbon-14 through the centuries (due partly to changes in Earth's magnetic field and the consequent changes in the intensity), this technique gives an uncertainty of about 15%. This means, for example, that a mastodon bone that is dated to be 10 000 years old may really be only 8500 years old on the low side, or 11 500 years old on the high side. For many purposes this is an acceptable level of uncertainty If greater accuracy is desired, then other techniques must be employed.

IN Questions 1. An archeologist extracts a gram of carbon NEW from an ancient bone and measures SAMPLE AT between 7 and 8 beta emissions per 5 COUNTS/MIN minute from the sample. A gram of car- bon extracted from a fresh piece of bone gives off 15 betas per minute. Estimate the age of the ancient bone. 2. Suppose the carbon sample from the ancient bone were found to be only OLD SAMPLE M one-fourth as radioactive as a gram COUNTS/MI of carbon from new bone. Estimate the age of the ancient bone.

MI Answers

1. Since beta emission for the old sample is one-half that of the fresh sample, about one half-life has passed, 5730 years. 2. The ancient bone is two half-lives of carbon-14 or about 11 460 years old.

622 Chapter 39 The Atomic Nucleus and Radioactivity 39.9 Uranium Dating

The dating of older, but nonliving, things is accomplished with radioactive minerals, such as uranium. The naturally occurring iso- topes U-238 and U-235 decay very slowly and ultimately become —but not the common lead isotope Pb-208. For example, U-238 decays through several stages to finally become Pb-206, whereas U-235 finally becomes the isotope Pb-207. Most of the lead isotopes 206 and 207 that exist were at one time uranium. The older the uranium-bearing rock, the higher the percentage of these lead isotopes. From the half-lives of the uranium isotopes and the percentage of lead isotopes in uranium-bearing rock, a calculation can be made of the date when the rock was formed. Rocks dated in this way have been found to be as much as 3.7 billion years old. Samples from the moon, where there has been less obliteration of early rocks than on Earth, have been dated at 4.2 billion years. This is "only" 400 million years short of the well-established 4.6-billion-year age of Earth and the solar system.

39.10 Radioactive Tracers

Radioactive isotopes of all the elements have been produced by bom- barding the elements with neutrons and other particles. These iso- topes are inexpensive, quite available, and very useful in scientific research and industry. Agricultural researchers mix a small amount of radioactive iso- topes with fertilizer before applying it to growing plants. Once the plants are growing, the amount of fertilizer taken up by the plant can be easily measured with radiation detectors. From such measure- ments, researchers can tell farmers the proper amount of fertilizer to use. When used in this way, radioactive isotopes are called tracers (Figure 39.18). Figure 39.18 Tracers are used in medicine to study the process of digestion Radioactive isotopes are used to and the way in which chemicals move about in the body. Food con- check the action of fertilizers in taining a tiny amount of radioactive isotopes is fed to a patient. The plants and the progress of food paths of the tracers in the food are then followed through the body in digestion. with a radiation detector. The same method is used to study the cir- culation of the blood. Engineers study how parts of an automobile test engine wear away by making the cylinder walls in the engine radioactive. While the engine is running, the piston rings rub against the cylinder walls. The tiny particles of radioactive metal that are worn away fall into the lubricating oil, where they can be measured with a radiation detector. This test is repeated with different oils. In this way the engi- neer can determine which oil gives the least wear and longest life to the engine.

623 NO RADIOACTIVITY Explore 2 Develop 3 Apply RADIOACTIVITY RADIOISOTOPE 3 Problem-Solving AlI Exercises in 18-4

Figure 39.19 A Tracking pipe leaks with radioactive isotopes.

There are Mftaitecis more examples of the use of radioactive iso- topes. The important thing is that this technique provides a way to detect and count atoms in quantities too small to be seen with a microscope and too small to be hazardous.*

39.11 Radiation and You

Radioactivity has been around longer than humans have. It is as much a part of our environment as the sun and the rain. It is what warms the interior of Earth and makes it molten. In fact, radioactive decay inside Earth is what heats the water that spurts from a geyser or that wells up from a natural hot spring. Even the helium in a child's balloon is the result of radioactivity. Its nuclei are nothing more than alpha particles that were once shot out of radioactive nuclei. As Figure 39.20 shows, most radiation you encounter originates in nature. It is in the ground you stand on, and in the bricks and stones of surrounding buildings. Even the cleanest air we breathe is slightly radioactive. This natural was present before humans emerged in the world. If our bodies couldn't tolerate it, we wouldn't be here. Much of the radiation we are exposed to is cosmic radiation streaming down through the atmosphere. Most of the protons and other atomic nuclei that fly toward Earth from are deflected away. The atmosphere, acting as a protective shield, stops most of the rest. But some cosmic rays penetrate the atmosphere, mostly in the form of secondary particles such as . At higher altitudes, radiation is more intense. In Denver, the "mile-high city" you receive more than twice the cosmic radiation you receive at sea level. A couple of round-trip flights between New York and San Francisco exposes you to as much radiation as in a normal chest X-ray. The air time of airline personnel is limited because of this extra radiation.

* The use of intense radiation in treating cancer is different The quantity of radioactive material is then far greater than in research using radioactive tracers but the benefit is - reckoned to outweigh the risk.

624 Chapter 39 The Atomic Nucleus and Radioactivity NATURAL BACKGROUND 152 (COSMIC RAYS. EARTH MIN RADON IN AIR)

CONSUMER PRODUCTS 21 (TV. MONITORS. SMOKE DETECTORS)

COAL AND PLANTS. 3' k FOOD AND WATER 82 WEAPONS TESTS FALLOUT <0.003°L (E.G.. POTASSIUM)

Figure 39.20 A Chrgins of radint!on exposure for an average individual in the United States.

We are bombarded most by what harms us least—neutrinos. Neutrinos are the most weakly interacting particles. They have near- zero mass, no charge, and are produced frequently in radioactive decays. They are the most common high-speed particles known, zapping the , and passing unhindered through our bodies by the billions every second. They pass completely through Earth with only occasional encounters. It would take a "piece" of lead 6 light-years in thickness to absorb half the neutrinos incident upon it. About once per year on the average, a triggers a in your body. We don't hear much about neutrinos because they ignore us. Of the types of radiation we have focused upon in this chapter, gamma radiation is by far the most dangerous. It emanates from radioactive materials and makes up a substantial part of the normal background radiation. Exposure to gamma radiation should be min- imized. The cells of living tissue are composed of intricately struc- tured molecules in a watery, ion-rich brine. When gamma radiation encounters this highly ordered soup, it produces damage on the atomic scale Less damage is done by a beta particle because it does not penetrate as deeply into living matter. Regardless of whether damage is by gamma, beta, or some other kind of radiation, these altered molecules are often more harmful than useful to life processes. Altered DNA molecules, for example, can produce' harm- ful genetic mutations. Cells can repair most kinds of molecular damage if the radiation they are exposed to is not too intense. This is how we are able to tol- erate small radiation doses. On the other hand, people who work around high concentrations of radioactive materials must be specially trained and protected to avoid an increased risk of cancer. This applies to medical people, workers in nuclear power plants, and per- sonnel on nuclear-powered ships. People who receive high doses of radiation (on the order of 1000 times natural background or more) run a greater risk of cancer and have a shorter life expectancy than people who are not so exposed. Figure 39.21 A Whenever possible, exposure to radiation should be avoided. This is the internationally used Unavoidable, however, is the natural radiation that all living beings symbol to indicate an area where have always absorbed. radioactive material is being han- dled or produced.

625 Chapter Assessment

Transmutation is the changing of one element For: Study and Review into another that occurs when a radioactive Go online nucleus emits an alpha or beta particle. nischeolsom Visit: PHSchool.com Web Code: csd-6390 si The transuranic elements have been created by artificial transmutation. Radioactive isotopes have several important uses. Concept Summary • Carbon-14 is used to date wooden a diacts and the remains of plants and animals. The atomic nucleus is composed of nucleons that consist of positively charged protons and electri- • Radioactive minerals, such as uranium-238 or cally neutral neutrons. uranium-235, are used to date older, nonliving • The number of protons determines the num- materials. ber of electrons and the chemical properties • Radioactive tracers are used in agriculture of an atom. and medicine. • Nucleons are bound together by the strong Natural environmental radiation constantly force. bombards us. • As the number of protons increases, more ▪ Additional exposure to radiation should be neutrons are needed to stabilize the nucleus. avoided whenever possible, because it is Radioactive elements have unstable nuclei and damaging to living molecules and cells. emit various nuclear particles. • Alpha particles consist of two protons and two neutrons and can be stopped by skin or Key Terms heavy paper. atomic mass number (39.4) • Beta particles are electrons ejected from the atomic number (39.4) nucleus. They can be stopped by clothing or half-fife (39.5) aluminum foil. isotope (39.4) • Gamma rays are high-energy photons. nucleon (39.1) Heavy shielding, such as lead, is required radioactive (39.2) for protection. strong force (39.1) Isotopes of an element are chemically identical transmutation (39.6) but differ in their numbers of neutrons. • They have the same atomic number but different atomic mass numbers. • Some isotopes are radioactive. • Half-life is a measure of the decay rate of radioactive isotopes.

626 Chapter 39 The Atomic Nucleus and Radioactivity 17. a. What is a transuranic element? Review Questions Check Concepts b. VVhy are there no ore deposits of transuranic elements on Earth? (39.7) 1. Which of the following are nucleons-protons, neutrons, or electrons? (39.1) 18. Which is radioactive, C-12 or C-14? (39.8) 2. Do electrical forces tend to hold a nucleus 19. Why is more C-14 found in new bones than in together or push it apart? (39.1) ancient bones of the same mass? (39.8) 3. Between what kinds of particles does the 20. Why would the carbon dating method be use- nuclear strong force act? (39.1) less in dating old coins but not old pieces of adobe bricks? (39.8) ;es. 4. Which force has a longer range, the electric force or the ctrnng force? f dzi . Why ara there darealta of lead in a!l dopoaitF of uranium ore? (39.9) 5. When is a neutron unstable? 22. What isotopes accumulate in old, uranium- or 6. Distinguish among alpha, beta, and gamma bearing rock? (39.9) ing rays. (39.2) 23. What is a ? (39.10) 7. How do the penetrating powers of the three types of radiation compare? (39.3) 24. From where does most of the radiation you encounter originate? (39.11) 8. Distinguish between an ion and an isotope. (39.4) 25. Why is radiation more intense at high altitudes and near Earth's poles? (39.11) 9. How does the number of electrons in a normal atom compare with the number of protons in its nucleus? (39.4) 10. Which isotope has the greater number of Think and Explain Think Critically neutrons, U-235 or U-238? (39.4) 26. What experimental evidence indicates that 11. What is meant by radioactive half-life? (39.5) radioactivity is a process that occurs in the atomic nucleus? 12. If the radioactive half-life of a certain isotope is 1620 years, how much of that substance 27. Does your body contain more neutrons than will be left at the end of 1620 years? After protons? More protons than electrons? 3240 years? (39.5) Discuss. 13. When an atom undergoes radioactive decay, 28. Why are the atomic of many elements does it become a completely different element? in the periodic table not whole numbers? (39.6) 29. How do the atomic number and atomic mass 14. a. What happens to the atomic number of an of an atom change when a proton is added to atom when it ejects an alpha particle? its nucleus? When a neutron is added? Which determines the chemical nature of the element? b. What happens to its atomic mass number? (39.6) 30. What do different isotopes of a given element have in common? How are they different? 15. a. What happens to the atomic number of an atom when it ejects a beta particle? 31. Why are alpha and beta rays deflected in opposite directions in a magnetic field? b. What happens to its atomic mass number? Would they be deflected in opposite direc- (39.6) tions in an ? Why are gamma rays 16. a. What element does thorium become if it undeflected in either field? emits an alpha particle? 32. When an alpha particle leaves the nucleus, b. What if it emits a beta particle? (39.6) would you expect it to speed up? Defend your answer.

627 33. Why does an alpha particle deflect less than a 42. a. State the numbers of neutrons and protons beta particle in a magnetic field? in each of the following nuclei: SLi, 3Fe, 2210-Hg, and 2NPu. 34. Exactly what is a positively charged hydrogen atom? b. How many electrons will typically surround each of these nuclei? 35. Why is a sample of radioactive material always a little warmer than its surroundings? 43. A radioisotope is placed near a radiation Why is the center of Earth so hot? detector, which registers 80 counts per sec- ond. Eight hours later, the detector registers 36. Why do different isotopes of the same ele- five counts per second. What is the isotope's ment have the same chemical properties? half-life? 57. If a sample of radioactive material has a h "If- 44. Radiation from a point source follows an life of one week, how much of the original sample will be left at the end of the second inverse-square law. If a Geiger counter that is week? The third week? The fourth week? 1 m away from a small source reads 100 counts per minute, what will be its reading 2 m from 38. A product of nuclear power plants is the the source? 3 m from it? isotope cesium-137, which has a half-life of 30 years. How long will it take for this isotope 45. What element results when radium-226 to decay to one-sixteenth its original amount? decays by alpha emission? What is the atomic mass of this element? 39. Coal contains minute quantities of radioac- tive materials, and in fact there is more total 46. How is it possible for an element to decay radiation outside a coal-fired power plant "forward in the periodic table"—that is, to an than outside a fission power plant. What does element of higher atomic number? this tell you about the shielding that typically 47. People working around radioactivity wear film surrounds these power plants? badges to monitor their radiation exposure. 40. When we speak of dangerous radiation expo- These badges are small pieces of photographic sure, are we generally speaking of alpha radi- film enclosed in a lightproof wrapper. What ation, beta radiation, or gamma radiation? kind of radiation do these devices monitor? Discuss. 48. The age of the Dead Sea Scrolls was found by 41. When the isotope bismuth-213 emits an carbon dating. Could this technique work if alpha particle, it becomes a new element. they were instead stone tablets? Explain. a. What are the atomic number and atomic mass number of the new element? More Probleei7Solviegf_raFticel b. VVhat element results if bismuth-213 emits Appendi 'E a beta particle instead?

628 Chapter 39 The Atomic Nucleus and Radioactivity