A Survey of Nuclear Chemistry | Teen Ink

A Survey of Nuclear Chemistry

June 15, 2018
By spinnerofyarns GOLD, Pittsburgh, Pennsylvania
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spinnerofyarns GOLD, Pittsburgh, Pennsylvania
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Favorite Quote:
"Nobody but a reader ever became a writer." -- Richard Peck


Since the late nineteenth century, scientists have been studying nuclear chemistry—radioactivity and nuclear reactions—and seeking to apply it to our lives.  Although we have known about radioactivity for just over one hundred years, we have already incorporated it into many aspects of our lives, using it to defeat enemies, fight cancers, and power our electrical devices.  But, considering the dangers of radioactivity, is it worthwhile to continue using these technologies?

Henri Becquerel, a French scientist who studied phosphorescence (which is when objects exposed to light absorb photons, retain them, and then emit them after the light source is removed), was the first to recognize radioactivity.  Having heard of Wilhelm Röntgen’s experiments with x-rays, he wondered whether any phosphorescent substance could emit this energy, or only the substance which Röntgen had used, so in 1896, not long after the discovery of x-rays, he began to experiment with different substances.  One of these was potassium uranyl sulfate, a uranium-containing salt, samples of which he set on top of photographic plates wrapped in black paper and put out in the sun, to allow the salt to absorb light (Bowersox; Gramling).  (The plates were wrapped in black paper to prevent sunlight from reaching them; the x-rays of which he hoped to find evidence, however, would easily penetrate the paper and appear on them.)  Becquerel’s experiment was successful: x-rays from the salt appeared on his photographic plates.  However, he could not repeat the process due to cloudy weather, and so put his materials together in a closed drawer, which was the key to the discovery of radioactivity.  When the plates were developed (likely because Becquerel “was just being thorough”), he saw that some type of radiation from the potassium uranyl sulfate had passed through the paper to darken the plate—and evidently, since the materials had been kept in a dark space, it was not sunlight that gave the salt the energy for this radiation (Bowersox)!  Further experimentation proved that it was not x-ray energy which had left its marks on the plates: Becquerel found that, while x-rays could not be bent by magnets, this new radiation could; depending on the source, it might be attracted to the magnet or repelled by it (“The Discovery of Radioactivity”).

The new radiation drew the interest of another scientist, Polish-born chemist Marie Curie, who was fascinated by the question of why the uranium emitted “an entirely new form of invisible ray, a narrow beam of energy,” which she called radioactive energy.  She analyzed the radioactivity of different substances, learning that a mineral called pitchblende was even more radioactive than the uranium used by Becquerel.  Her conclusion was that there must be another radioactive element, and in 1898, after a long process of separating small quantities of the radioactive substance out of pitchblende, Marie and her husband Pierre found that it was indeed a new element, which they called polonium, and which had 330 times the radioactivity of uranium (Feder; “Marie Curie the scientist”).  Not long after discovering polonium, the Curies found another new element, radium, which they isolated from a highly radioactive liquid “left behind after they had extracted polonium” from pitchblende (“Marie Curie the scientist”).  Studying these materials led Marie to hypothesize radioactive decay—she thought that the radioactive elements were emitting “particles from tiny atoms that were disintegrating inside the elements” (Feder).

Her hypothesis was confirmed by Ernest Rutherford, when he learned that radioactive elements break down by giving off particles which he called alpha and beta particles.  Rutherford made further discoveries about these particles, too: that alpha particles have less “penetrating power” than beta particles—they cannot pass through as much matter—, which he learned by studying the type and amount of radiation that could penetrate “an increasing number of layers of metal foil.”  This was not his only contribution to nuclear chemistry, however.  Because of an experiment in 1909, he was able to assert that the plum-pudding atomic model (positive and negative areas distributed throughout the atom) was incorrect, and atoms actually had positive centers, surrounded by negative particles.   (In this experiment, a piece of gold foil was bombarded with alpha particles, most of which passed through the foil; however, an intriguing few bounced off the foil, leading Rutherford to suspect that the alpha particles had been repelled by positive areas at the centers of the gold atoms—the nuclei of the atoms. (Stewart).)  Later, during 1919 and 1920, Rutherford made what was perhaps one of his most significant discoveries—that, if alpha particles collided with “nitrogen and other light elements,” the atoms of those elements would give off protons, and take in the alpha particles[1].  This gave the atoms more positively charged particles than they had had previously, thereby changing them into different elements (e.g. nitrogen, bombarded by alpha particles, transformed into oxygen) (“Atop the Physics Wave”).

Almost twenty years after this process of combination was discovered, a group of German scientists created a separating process, not a slow disintegration like radioactive decay, but a much more dramatic reaction.  In 1938, Otto Hahn and Fritz Strassmann (also spelled Strassman), working with Lise Meitner, bombarded uranium with neutrons, releasing energy and causing the uranium atoms to split into multiple parts, the nuclei themselves breaking down to create new nuclei with fewer protons, which were the nuclei of smaller atoms (“Otto Hahn - Biographical”; Madsen).  Hahn, Strassmann, and Meitner had produced nuclear fission, the reaction soon to be used in powerful nuclear weapons and power plants (Madsen).  Hahn and Strassmann made other contributions to nuclear chemistry (Hahn identifying an isotope of uranium, and several other “radioactive substances,” while Strassmann played a role in the development of rubidium-strontium dating), but fission, which has had an impact on many lives, seems to have been their most memorable (“Otto Hahn - Biographical”; the Editors of Encylopædia Britannica).

 



[1] See section on alpha decay for detail of alpha particle structure.

Radioactive decay, the breaking down of a radioactive element over time, can occur in three forms: alpha decay, beta decay, and gamma decay.  When a single atom of an element breaks down through alpha decay, it emits alpha particles at random, but with a greater number of decaying atoms, there is a steady rate of decay.  The alpha particles which are given off consist of two protons and two neutrons, forming a helium-4 nucleus, as can be seen in the formula for alpha decay.  Using uranium as an example:

23892U à 42He + 23490Th

A uranium atom decays into a helium nucleus and an atom of thorium.

Beta decay can occur in two forms: β- and β+.  In β-, a neutron breaks down into an antineutrino, a proton, and an electron; the proton remains in the atom, while the antineutrino and electron are expelled from it. β+ decay involves the transformation of a proton into a positron, a neutron, and a neutrino.

β- decay of a neutron: 10|n --> 11|p + e^- + v̅

β- decay of thorium into protactinium 23|490|Th --> 0|1|β + 23|491|Pa

After alpha or beta decay occurs, an atom has “excess energy,” which it reduces by giving off a gamma ray.

99m|43|Tc --> 0|0|γ + 99|43|Tc

Where m indicates the “metastable state,” in which the atom has extra energy.

Decay is measured through half-life, which is defined as “the time after which, on average, half of the original material will have decayed.”  Half-life varies by element, some decaying slowly, and others very rapidly.

Equation for half-life:

N(t) = N0*e^(-kt)

Where:

N(t) = quantity of the element at time t

t = period of time after which you will have N quantity of the element

N0 = the initial amount of the element

-k = “constant to represent growth rate”

(The constant k for any element can be found by substituting an element’s half-life for t; substituting N0/2 for N(t), because half of the initial amount will be left after the half-life period; and finally solving for k.)

Radioactive decay, the breaking down of a radioactive element over time, can occur in three forms: alpha decay, beta decay, and gamma decay.  When a single atom of an element breaks down through alpha decay, it emits alpha particles at random, but with a greater number of decaying atoms, there is a steady rate of decay.  The alpha particles which are given off consist of two protons and two neutrons, forming a helium-4 nucleus, as can be seen in the formula for alpha decay (Brain).  Using uranium as an example:

238|92|U --> 4|2|He + 234|90|Th

A uranium atom decays into a helium nucleus and an atom of thorium (Writing nuclear equations for alpha, beta, and gamma decay).

Beta decay can occur in two forms: β- and β+.  In β-, a neutron breaks down into an antineutrino, a proton, and an electron; the proton remains in the atom, while the antineutrino and electron are expelled from it (Brain). β+ decay involves the transformation of a proton into a positron, a neutron, and a neutrino (“What is radioactivity?”).

β- decay of a neutron: 1|0|n --> 1|1|p + 0|-1|e + v̅

β- decay of thorium into protactinium 234|90|Th --> 0|-1|β + 234|91|Pa

(Writing nuclear equations for alpha, beta, and gamma decay).

After alpha or beta decay occurs, an atom has “excess energy,” which it reduces by giving off a gamma ray (“What is radioactivity?).

99m|43|Tc --> 0|0|γ + 99|43|Tc

Where m indicates the “metastable state,” in which the atom has extra energy (Writing nuclear equations for alpha, beta, and gamma decay).

Decay is measured through half-life, which is defined as “the time after which, on average, half of the original material will have decayed.”  Half-life varies by element, some decaying slowly, and other very rapidly (“What is radioactivity?).

Equation for half-life:

N(t) = N0*e^(-kt)

Where:

N(t) = quantity of the element at time t

t = period of time after which you will have N quantity of the element

N0 = the initial amount of the element

-k = “constant to represent growth rate” (Seward)

(The constant k for any element can be found by substituting an element’s half-life for t; substituting N0/2 for N(t), because half of the initial amount will be left after the half-life period; and finally solving for k.)

(Introduction to exponential decay)

Fission and fusion are opposing nuclear reactions (atoms are split in fission and combined in fusion), both of which release enormous amounts of energy and therefore hold attractive potential as energy sources.

Nuclear fission occurs when a neutron collides with an atom, which causes the atom to break apart, giving off “heat and radiation,” as well as two to three fission products and several neutrons (“Nuclear Explained”; “Nuclear fission”).  During the reaction, a small amount of matter is converted into a large amount of energy, per Einstein’s formula E = mc^2, where energy is equal to mass times the speed of light squared, the last being a large number which accounts for the high level of energy from the small mass (“Nuclear Fission Basics).

Uranium-235 can undergo this transformation during fission:

235|92|U + 1|0|n --> 142|56|Ba + 91|36|Kr + 3(1|0|n)

Here, uranium-235 breaks into barium-142 and krypton-91 and three neutrons, when struck by a neutron (“Nuclear Fission Basics”).

In fission, the neutrons which are a product of the reaction collide with other atoms, causing them to disintegrate, as well, in a nuclear chain reaction (“Nuclear Explained”).  This chain reaction allows us to use continuous fission reactions to generate power, either in nuclear reactors in power plants and submarines (where the reactions are restricted) or in nuclear weapons (where the reactions are not restricted, but continue at an ever-increasing rate due to the increase of loose neutrons with every fission) (“Nuclear Chain Reactions”; Steinberg).  Fission products, too, can be useful for applications such as “power[ing] batteries” (Steinberg).

While nuclear fission releases energy by splitting atoms, nuclear fusion releases it by combining, or fusing, small atoms into larger ones—for example, hydrogen atoms that undergo fusion combine to form helium (“Nuclear Fusion”).  Atoms must be moving extremely quickly in order to come together, in spite of the repelling of their positively charged nuclei, which means that they must be in an extremely hot environment; and when scientists create fusion on Earth (using certain isotopes of hydrogen), the necessary temperature is 150,000,000 °C (“Fusion”).  The preferred hydrogen isotopes for fusion on Earth are deuterium (H-2, one neutron) and tritium (H-3, two neutrons), used in this fusion reaction:

2|1|H + 3|1|H --> 4|2|He + 1|0|n

(“Why materials are radioactive”)

Some of the mass from the two original atoms is not used in the formation of the helium and the neutron, and this mass is converted into a great amount of energy, based on E = mc^2 (“Fusion”).

Currently, nuclear fusion occurs in stars and scientific experimentation, but if certain difficulties can be overcome (i.e. the extraordinarily high temperature required), fusion may be a promising source of energy, without the risk of radioactive accidents that comes with fission (“Nuclear Fusion”; “What is ITER?”).  Fusion has also been used in hydrogen bombs, where a fission reaction triggers a fusion reaction, which in turn triggers another fission reaction to produce a force of several tons megatons[1] of TNT (as opposed to a fission bomb’s force of only tens of kilotons[2] of TNT) (Nave).

 



[1] 1 megaton = 1 million tons
[2] 1 kiloton = 1 thousand tons

Nuclear chemistry has many applications in our modern world, from medicine to warfare, from agriculture to generating electricity.  Some of our uses for radioactive materials are relatively harmless, while others carry significant risks, and we must consider carefully whether those risks are justified by the benefits which we derive from using the materials.

Medical diagnoses and treatments can both be accomplished with the use of nuclear chemistry.  Gamma decay is used in imaging procedures such as PET scans, for which radioactive positron-emitters are mixed into “chemical compounds that selectively migrate to specific organs in the body,” where the emitted positrons encounter electrons, and the two types of particles “annihilate each other,” in the process giving off gamma rays, which can be tracked and mapped by computers to create images of the organ (“Nuclear Medicine: Radioisotopes for diagnosis and treatment”).  These radioisotope diagnostic procedures, effective for imaging of both soft and hard tissues, are a less invasive alternative to exploratory surgery, and although exposure to radiation may increase a patient’s risk of developing cancer, the level of exposure is no higher for radioisotope imaging than it is for CT scans and x-rays (“Radioisotopes in Medicine”; “Medical Applications”; “Nuclear Medicine”).  Nuclear medical treatments, too, have their advantages over surgery; they require no anesthetic, and they are “painless” (“Medical Applications”).  Radionucleide therapies often use beta-decaying isotopes, whose medium penetrating radiation “causes the destruction of… damaged cells” in cancer patients.  One such treatment is boron capture therapy, in which boron-10 “concentrates in [a] tumor,” and then, when the patient is “irradiated with neutrons,” the boron breaks down by alpha decay, and the tumor is destroyed by the alpha particles, which being short-range radiation do not harm the “surrounding healthy tissue” (“Radioisotopes in Medicine”).

Boron capture therapy uses this reaction: 10|5|B + 1|0|n --> 11|5|B --> 7|3|Li + 4|2|He

(“Nuclear Medicine: Cancer Therapy”)

Agriculture also benefits from nuclear chemistry, which can be used to create new varieties of plants, to help farmers to regulate fertilizer use, and to control pest populations.  Exposure to radiation (both neutron and gamma) can cause plant cells to mutate, which creates more options for breeding, leading to hardier, higher-yielding, and pest-resistant crops.  The fertilizer use of these crops can then be monitored with radioisotopes, so that farmers know how much fertilizer they need to give their plants, which means less frequent overuse of fertilizers, thus less water and ground pollution (“The Many Uses of Nuclear Technology”;  “Food and Agriculture”).  Nuclear radiation even allows for the control of harmful insects, when male flies are sterilized with gamma rays so that they fail to create offspring upon mating.  This method has proven successful in several instances—against the Mediterranean fruit fly in Central and South America, and against screwworms in Central and North America (“The Many Uses of Nuclear Technology”).  Even though all of these processes use radiation, the Canadian Nuclear Association states that they are not dangerous, either to the environment or to people (“Crop Improvement”).

In addition to radiation-treated crops, we also have irradiated food, which undergoes a process that “exposes food to gamma rays from cobalt-60, a radioisotope of cobalt.  The gamma rays kill bacteria in the food, lowering consumers’ risk of foodborne illness, as well as foreign insects on imported produce, and they extend foods’ shelf lives, without significantly decreasing their nutrient content (“Food and Agriculture”; “Food Irradiation: What You Need to Know”).  Irradiated foods are safe for consumption as well; the FDA states that “irradiation does not make foods radioactive,” and the Canadian Nuclear Association reports that scientists studying irradiation have deemed these foods safe to eat, based on “data accumulated from about 50 years of research” (“Food Irradiation: What You Need to Know”; “Food Irradiation”).  However, although the foods themselves may be perfectly harmless, there is some risk of accidents with radioactive materials in factories that irradiate foods, and of radioactive pollution that may escape from the facilities (Priesnitz).

Some applications of nuclear technology do not involve the production, transportation, or potential leakage of highly radioactive materials; radioactive dating is one of these, used to find the ages of rocks and organic matter.  There are several types of radioactive dating, two of which are carbon-14 dating and potassium-argon dating.  Scientists can use carbon-14 dating to find the ages of organic matter by measuring its ratio of carbon-14 to carbon-12.  (Plants and the animals that eat them take in carbon-14 when they are alive, but it decays with a certain half-life after they die, so that its ratio to the normal carbon-12 decreases as with age (“Radioactive Dating”).)   Though it is often a useful tool, carbon-14 dating is not a perfect technique, because it only works for matter about 70,000 years old or younger—at that point, too much of the carbon is decayed to make the method effective (Nelson).  Because carbon-14 dating only works for organic material, other types of dating must be used to discover the ages of rocks, etc.  One of these is potassium argon dating, by which one can date inorganic matter by looking at the ratio of 40K (potassium) to 40Ar (argon, a product of 40K decay that can be trapped in rock when released from the rock with the potassium) (“Radioactive Dating”).  This method, too, has its limitations, because argon can escape from rock, making dates obtained from the dating process inaccurate (Nelson).

Yet another use for nuclear science, as mentioned in the “Fission and Fusion” section, is power generation, via nuclear fission.  Nuclear reactors, powered by uranium pellets, use fission chain reactions to produce a steady heat.  This heat turns the cooling agent of the reactor (often water) into steam, which powers turbines that generate electricity.  The chain reactions are controlled by rods of “nuclear poison,” or materials like xenon that absorb some of the neutrons produced by fission, preventing the rate of fission from rising.  Nuclear is a “clean” source of energy in that it gives off no greenhouse gases or other air pollutants, but radioactive fission products are produced which must be transported and stored, as well as radioactive waste, consisting of tools and clothing used in environments with “radioactive dust.”  There is also the possibility of a nuclear meltdown, such as the Chernobyl incident, which causes widespread pollution via toxic, potentially cancer-causing radioactive material (“Nuclear energy”).

Finally, nuclear science has been applied to weapons, to build atomic bombs (like those dropped on Japan) and hydrogens bombs, which are even more powerful.  The nuclear reactions which these weapons use (as noted before) are fission and fusion (see “Fission and Fusion” section for fusion of hydrogen), with elements such as uranium, plutonium, lithium, and hydrogen[1] (Nave; “Fat Man: Implosion-Type Bomb”).  The advantages of nuclear weapons are questionable (some people suggest that because we have these weapons of mass destruction, we are unlikely to use them or even to engage in war (Spulak[2])), while the advantages of nuclear warfare seem fairly nonexistent, unless one desires to destroy one’s enemy entirely, and perhaps injure one’s own forces.  A speaker at the Monterey Institute of International Studies cited a 1970s study which reported that “even [in] an attack where you try to use nuclear weapons surgically, the radiation spreads all across the country,” which would pose potential danger to any of the attacker’s army which might be within that large area (“Are Nuclear Weapons Useful?”).  So far, the only problem which nuclear warfare has solved has been the Pacific Theater of WWII—and it solved that problem rather one-sidedly, destroying two Japanese cities.



[1] I could not find specific information on nuclear reactions used in nuclear weapons—no sources that I found told specifically what fission products elements broke down into.
[2] This source is nearly 20 years old, and so may well be outdated, but it was all that I could find for possible advantages of nuclear weapons.  I could find no advantages for actual nuclear warfare.

Should we use nuclear chemistry?  It is a difficult question to answer—how can we ever weigh the benefits and drawbacks on a fair scale?  Both sides have their ethical questions—should we let more people die of cancer, and should we burn more fossil fuels, to avoid possible radioactive pollution?  But we know that accidents happen, and radioactive materials harm people and environments—should we really use this technology, if it can be so dangerous?

I believe that we should continue to use nuclear technology, taking as much care as we possibly can to prevent nuclear accidents.  We benefit from nuclear technology in so many ways—our crops are healthier, fewer greenhouse gases are released into the air, and we can diagnose diseases much sooner and treat them more effectively.  Perhaps, too, power from nuclear sources does not need to be as hazardous as it is, if we can find a way to make fusion a reasonable means of generating electricity.  My view may be somewhat limited, and as I have never personally experienced a nuclear accident, I may be rather naïve, but—in my current situation—it seems that the benefits of using nuclear chemistry outweigh the drawbacks.

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