Nuclear Chemistry I: _{Radiation, half-life, and nuclear reactions}

The discovery of radiation and radium

Radium is one of the earlier radioactive elements to be discovered, and it is still used today in the treatment of cancer. Radium was discovered by Marie Curie, arguably one of the most famous scientists in the world. Marie Curie was born Maria Sklodowska in Poland in 1867. Sklodowska grew up in poverty in Warsaw, Poland. At the time, scientists were exclusively men, and even admission into a university was reserved for men. So it was that although Sklodowska graduated first in her class from high school, she was banned from attending university in Poland. Not accepting that rejection as the final say in her education, Sklodowska attended a “Flying University” in Poland, so called because the university held classes secretly in changing locations, including in private homes, to avoid the restrictions that were in place at the time, such as those against women attending. Marie was totally committed to her education, in fact, a few years before she had made a pact with her sister Bronya - they would move to Paris and Marie would work to help pay for Bronya’s education, and then Bronya would do the same for Marie. And so, in 1891, Marie moved to Paris to join her sister and attend the Sorbonne. Sklodowska continued to struggle in Paris, lacking in money, but working hard to prove herself as a scientist. By a chance encounter, she came to work in the lab of a young scientist named Pierre Curie. Pierre and Marie would eventually marry and change the course of science with their groundbreaking research into the newly discovered phenomenon of radioactivity.

The Curies started a family together as both continued with their research. In the late 1890s Marie faced a milestone, she needed to choose a topic for her doctoral research, and no woman had yet been awarded a doctorate in science. Curie’s decision was influenced by two important discoveries. In 1895, the German physicist Wilhelm Roentgen discovered a strange type of radiation which were called X-rays, that could travel through solid objects and even yield images of peoples’ bones. And in 1896, Frenchman Henri Becquerel made a unique, and accidental discovery. Becquerel placed uranium salts and photographic plates in a laboratory drawer overnight to use in an experiment. The next day, he was surprised to see that the plates, quite expensive and difficult to obtain at the time, looked as if they had been exposed to light. The uranium salts had released energy to change the photographic plates.

Curie chose to study Becquerel’s “uranium rays” for her thesis and her work yielded important discoveries. Working in a small, damp storage room as a lab, Marie’s experiments led her to propose that the rays were a property of the very atoms of uranium contained in the salts and she began testing other elements, eventually discovering that thorium also emitted these strange rays. Curie named the phenomenon “radioactivity,” based on the Latin name for rays.

Pierre was so intrigued by Marie’s discovery that he soon joined her in her work, and the Curies sought to learn more about radioactivity. They separated pitchblende, a black uranium-containing ore, to remove uranium. To their surprise, the remaining ore was still radioactive even with the uranium removed! They worked laboriously to separate a new element, polonium (named for Marie’s home country of Poland), from the remaining ore. But the remaining ore was still radioactive! They removed a second new element that was one million times more radioactive than uranium. They named this element radium, again using the Latin term for “ray.”

Separating elements from pitchblende was very hard work. Ten tons of pitchblende yielded just one milligram of radium. Unknowingly, in the process, the Curies were exposed to high doses of radiation and suffered from radiation sickness as they worked with the most radioactive natural element ever discovered. They were chronically weak, coughed, and had regular burns on their skin. Their sacrifices in the lab to discover elements and learn more about radioactivity eventually led to them, and Henri Becquerel, being awarded a Nobel Prize in physics in 1903 - the first ever Nobel Prize awarded to a woman - for their discovery of spontaneous radiation. In 1911, Marie Curie was actually awarded a second Nobel Prize in chemistry for her work with radioactivity, the first person, and still one of only two people, who have ever won two Nobel prizes in science.

Radioactivity

So what exactly is “radioactivity”? Radioactivity is the spontaneous release of energy from certain atoms. It is what Becquerel observed when his photographic plates became exposed and darkened. And it is the same energy that the Curies could not see, but that made them ill and burned their hands. So how do you study something that you can’t see? That was a challenge that faced scientists searching to learn more about the energy released from these processes.

One of the first pieces of evidence that helped scientists uncover the secrets of radiation came from Ernest Rutherford, a Cambridge University physicist born in New Zealand and who later became famous for his work uncovering the structure of the atom (see our Atomic Theory I module for more information on these later experiments of Rutherford’s). For his graduate work in 1898, Rutherford designed a set of simple experiments to study the mysterious rays emanating from uranium. He placed an increasing number of aluminum foil sheets between a uranium source and a detector. Rutherford observed that some of the radioactivity disappeared if he placed just one thin sheet of foil in front of the detector. However, some of the radiation traveled through and could still be seen by the detector. Rutherford correctly theorized that there were at least two types of radioactive particles coming from the uranium source. The first type, which was blocked by one thin sheet of foil, he called alpha radiation. And the second type, he called beta radiation. Rutherford repeated the experiment many times with different radioactive sources and different metallic foils and he added in electromagnets to the set up to see if either of the particles he had discovered were charged. He observed that alpha rays were positively charged particles and beta rays were negatively charged particles.

Paul Villard, a French chemist, took Rutherford’s work further in 1900. He used a lead screen to eliminate the alpha rays, and a magnetic field to eliminate the beta rays. And yet Villard still detected radiation. This radiation was powerful enough to travel through lead, and had no charge as it was not attracted or repelled by the magnets. Rutherford confirmed this discovery and eventually named this third type of radiation “gamma rays.”

These experiments began to uncover the properties of radiation, but they gave little information about what caused radioactivity. At the time of its discovery, no one knew. Many scientists believed that radiation was energy that atoms had previously absorbed and which was being reemitted. This hypothesis was later proved false, and Rutherford and English radiochemist Frederick Soddy would go on to find the correct answer in 1901. Rutherford and Soddy’s relationship began not as a result of collaboration, but as a result of conflict. Rutherford had proposed that radiation was not previously absorbed energy, but was caused by the break up of atoms. Soddy did not believe this to be true, and actually debated Rutherford on the idea, motivating them to work together. Rutherford and Soddy began working with the radioactive element thorium. They observed that when thorium emitted radiation, a gas was released. They collected and studied this gas and realized that it consisted of something entirely different from thorium, not a new chemical compound, but an entirely new element - the element radon, which had fewer subatomic particles than thorium. Rutherford and Soddy correctly theorized that radioactive particles were actually part of the parent element being released, and when these particles left the original element, it transformed into another element, releasing energy in the process. This “theory of atomic disintegration” was controversial at the time. But Rutherford and Soddy’s careful experiments and abundant data eventually convinced others of the process.

Radioactive elements are unstable and transform into other elements. While scientists started to piece this together at the turn of the 20th century, the discovery of the atomic nucleus in 1911 and the observation of protons in 1919, both by Rutherford, helped scientists better understand radioactivity and its different forms.

Radioactive isotopes

Over the first few decades of the 20th century, a view of atomic structure had appeared from the many experiments that were conducted that helped scientists understand the atom and radiation. Rutherford’s experiments had led to a realization that atoms had small dense nuclei that contained the atom’s protons, and that electrons resided outside of the nucleus of the atom. Scientists also understood that radiation was caused by part of the nucleus being ejected from the atom. But some things still did not add up. Experiments with alpha particles, which were now known to contain protons, suggested that they were too heavy to only contain protons. And there was something wrong with the masses of atoms. By this time scientists were able to measure both the charge and mass of atoms, and charge increased in distinct whole number ratios - but mass did not. If one atom had twice as many protons and electrons as another, why was its mass not also double?

A discovery in 1932 by a British scientist, James Chadwick, helped answer these questions and shed light on the phenomenon of radioactivity and atomic structure. Fittingly, Chadwick made this discovery by using radiation itself. Chadwick bombarded beryllium atoms with alpha particles, and observed a strange radiation being emitted. This radiation consisted of a small particle, but had no charge. Chadwick had discovered the existence of the neutron in the nucleus. The neutron is a neutral particle with a mass about equal to that of a proton. The discovery helped explain a lot. The “heavy” mass of the alpha particle could be explained by the existence of the neutron, in fact two of them, in each alpha particle. And the ratio of mass to charge in atoms could now be explained. The neutron also explained another observation by Frederick Soddy. Soddy had continued studying the breakdown, or decay, of radioactive elements, and had found that some elements had more than one atomic mass. Chadwick’s discovery explained why - the nuclei of different forms of the element had different numbers of neutrons but the same number of protons as other forms. Because the different forms had the same number of protons, their chemical properties and location on the periodic table was the same. And so a family friend of Soddy, Dr. Margaret Todd, suggested he call the forms of elements with different atomic masses “isotopes” meaning “same place” in Greek.

Isotopes are atoms of the same element with different numbers of neutrons. For example, 99% of all carbon exists as carbon with an atomic mass of 12. Atoms of this isotope of carbon contain 6 protons and 6 neutrons, which combine to give it a mass of 12. However, a very small percentage of carbon has an atomic mass of 14. Since this is still carbon, it has to have 6 protons, so what differs is that this isotope contains 8 neutrons. Isotopes are indicated by writing the element’s symbol followed by a dash and the mass. In this case, C-12 is the common isotope and C-14 is the less common isotope. C-14 is also unstable, which means it undergoes radioactive decay, which we will learn more about in just a bit.

Another way to indicate isotopes is by writing them in isotopic notation. Figure 3 is a model of the notation. The symbol of the element is written with a superscript number and a subscript number to the left of the symbol. The superscript number gives the atomic mass; the subscript number gives the atomic number. Figure 3 shows what this model looks like.

So, for carbon, its two forms would be written as:

Some isotopes of elements are unstable and decay to become smaller, more stable atoms, releasing radiation in the process. These are called “radioisotopes.” For example, carbon-12 is the stable isotope of the element, and C-14 is a radioisotope of carbon.

Types of radioactive decay

So let’s take a closer look at the types of radioactive decay that can occur in radioisotopes. Remember that these were identified by Rutherford and Villard, as discussed earlier in this module. The types of radioactive decay still bear the names given to them by these scientists.

“Alpha decay” is the loss of an alpha (α) particle. An alpha particle is the same as a helium nucleus in that it contains two protons and two neutrons. It is represented as \( \ce{^4_2He} \). Remember that the superscript “4” represents the atomic mass of the particle, and the subscript “2” represents the atomic number. Since this particle has two protons and no electrons, it has a charge of +2. The loss of an alpha particle can be represented or modeled with a nuclear equation. For example, Equation 1 gives the nuclear equation model for the alpha decay of radium-226 to become radon-222, a more stable radioisotope. Figure 4 provides a diagram of the decay process.

Notice a few things about Equation 1. First, as originally observed by Soddy, when radium undergoes radioactive decay, it turns into a smaller element. Second, both atomic mass and atomic number are conserved on the reactant side and the product side. Meaning, the atomic mass on the reactant side, 226, equals the sum of masses on the reactant side (222 + 4 = 226). And this is true for the atomic number as well. The mass on the left, 88, equals the sum on the right (86 + 2). Particles are not lost (or gained) during this decay. The number of particles leaving radium as an alpha particle equals the number of fewer particles that the resulting radon atom contains.

“Beta decay” is the loss of a beta (β) particle. A beta particle turns out to be an electron or \(\ce{^0_-1e}\) with a charge of -1. The loss of a beta particle can also be represented with a nuclear equation. Equation 2 gives the nuclear equation for the beta decay of lead-214 to become bismuth-214, a more stable radioisotope.

Take a close look at equation 2. What do you notice? The atomic mass of the starting lead atom and the resulting bismuth atom are the same at 214. But what about the atomic number? It actually increases—how can this be? Well, the electron lost from lead is not released from the electron shells—it is ejected from the nucleus. In beta decay, a neutron itself is unstable and decays by releasing both an electron and a proton. The proton is retained in the nucleus, and thus the atomic number of the resulting element increases. The electron is ejected as the radioactive particle.

“Gamma decay” is the loss of energy only and does not itself lead to a change in the nucleus of an atom. Gamma (γ) rays are often emitted along with alpha and beta particles. Gamma rays are represented as \(\ce{^0_0\gamma}\) in a nuclear equation. Of the three types of radiation we have discussed, gamma rays have the greatest penetrating power because they are pure energy, as shown in Figure 5. Even a thin piece of paper can stop alpha particles; beta particles travel through paper but can be stopped by something denser like aluminum, but thick lead shielding is needed to reduce gamma rays.

Now that we understand these three types of radioactive decay, we can get back to our discussion of radiation therapy. Just how does this work? Two types of radiation therapy are used to treat cancer: external beam radiation therapy and internal radiation therapy. Doctors use medical imaging, like magnetic resonance imaging (MRI), to determine the exact location of cancer cells. They also determine the type of therapy and radiation dosage needed to destroy the cancer cells.

During external beam radiation therapy, a machine, such as a Gamma Knife, is positioned over the patient to precisely release gamma rays at a location over the cancer cells. Gamma rays damage the DNA inside of cells so that, over time, the cancer cells stop dividing and die. External beam radiation therapy is used to treat many types of cancer.

Other types of cancer are treated with internal radiation therapy. A source of radiation is placed by the cancer cells. As the radioactive isotope decays and releases radiation, the cancer cells are killed over time. Brachytherapy uses a solid radiation source; by comparison, systemic therapy uses a liquid source of radiation that travels through the blood.

Cancer treatments may include radiation therapy only or radiation therapy combined with surgery, chemotherapy, or immunotherapy. Doctors recommend treatment on a case-by-case basis.

Let’s review the three types of radiation:

While these are the more common types of nuclear decay processes, it’s important to note that there are others. Neutron emission (observed by Chadwick), electron capture, cluster decay, and other types of pathways exist by which nuclear decay can occur.

Half-life and its applications

As you have seen, unstable nuclei release particles and energy as radiation when they undergo nuclear decay. Depending on the element being observed, this process can take fractions of seconds or billions of years. The decay’s speed depends upon how unstable an atom is: Very unstable atoms decay quicker than others. But the nuclei do not all decay at once. Instead, they decay independently and randomly. One cannot predict exactly when a particular nucleus will decay, but the probability of a nucleus undergoing decay can be calculated. When this probability is placed in the context of a unit of time, it is called a “decay constant.” This leads us to a concept for measuring decay over time called “half-life.”

The half-life of a radioisotope is the time required for half of the atoms of the radioactive substance to break down or undergo decay. Mathematically, the amount of a radioactive substance that remains after a certain amount of time can be calculated with the equation shown below (Equation 3).

N = amount of radioactive substance remaining

N₀ = initial amount of radioactive substance

e = a constant, approximately 2.71828

k = decay rate/half-life

t = time

Equation 3 tells us that the amount of radioactive material that remains after a given time is a product of the initial amount multiplied by the constant e raised to a negative exponent that is the product of the decay rate (half-life and time elapsed). Let’s see how this works. The Curie’s first isolated radium in their lab, and radium-226 has a half-life of 1,600 years and a decay rate of 0.693. Using the equation, we can calculate how much of a 1.0 g sample of radium-226 remains after 100 years.

$$N = N_0e^{-kt}$$

N = ?

N₀ = 1.0 g

e = a constant, approximately 2.71828

k = 0.693/1600 years

t = 100 years

N = (1.0 g) \(e^{-(\frac{0.693}{1600})(100)}\)

N = 0.958g radium-226 remaining

So, it should be no surprise that the Curies’ laboratory notebooks are still radioactive. Even after 100 years, most of the trace amount of radium that contaminated them is still present. The amount remaining can also be determined graphically. Figure 6 is a graph showing the amount of radium-226 remaining over time for a 1g sample. Note that at 1,600 years, or one half-life, half of one gram remains; at 3,200 years, or two half-lives, one-fourth of one gram remains; and at 4,800 years, or three half-lives, only one-eighth of one gram remains. How long will it take for the sample to decay so that only one-sixteenth of one gram remains?

Decay chains

Radioisotopes rarely turn into a stable element after just one decay process. Instead, they go through a “decay chain,” or sequence of nuclear reactions, to reach stability. For example, when radium-226 decays, it releases an alpha particle to become radon-222. But radon itself is radioactive and undergoes decay to Polonium. You can see in Figure 7 that the radium decay chain actually contains seven distinct steps. And while the first few steps release alpha particles, beta particles will also be released as the decay chain continues (Figure 7).

Radioisotopes rarely turn into a stable element after just one decay process. Instead, they go through a “decay chain,” or sequence of nuclear reactions, to reach stability. For example, when radium-226 decays, it releases an alpha particle to become radon-222. But radon itself is radioactive and undergoes decay to Polonium. You can see in Figure 7 that the radium decay chain actually contains seven distinct steps. And while the first few steps release alpha particles, beta particles will also be released as the decay chain continues (Figure 7).

What else do you notice in this decay chain (Figure 7)? Each arrow represents a nuclear equation that can be modeled and has its own half-life. Some, like the decay of polonium-218, have a half-life measured in minutes. Others, like the decay of lead-210, have half-lives measured in years. The rate of decay of each different product is independent of others.

Radium-226 is not the only radioactive isotope that goes through a decay chain. Uranium is found naturally in our environment, in rocks, soil, and even water, and there are three naturally occurring radioisotopes of uranium: U-238, U-235, and U-234. More than 99% of all uranium found in the environment is U-238. U-234 makes up less than 1% of the uranium found naturally but gives off more radiation than U-238. And U-235 is the radioisotope used in nuclear reactors and weapons.

Though rare, uranium is an important element. The small amount of radiation emitted by naturally occurring uranium as it decays makes up a significant portion of the background radiation around us all the time. As it turns out, U-238 has a half-life of 4.5 billion years! This means that half of a sample of U-238 changes into a more stable form in 4.5 billion years as the remaining sample continues to break down. Uranium is important to the planet as it releases heat inside the Earth as it decays. Uranium and other natural radioactive elements inside the Earth, like thorium-232 and potassium-40, account for about half of the heat given off deep inside the Earth.

Understanding radioactive decay is especially important to modern science. For example, one way that scientists date very old objects is by radioactive dating. Carbon dating is one example of this. Recall that carbon-14 is a radioactive element, and since all living organisms on earth contain carbon, they also contain traces of C-14. By understanding the decay of this element over time, scientists can date the remains of animals or plants that lived tens of thousands of years ago. To better understand how carbon-14 dating works, you can read our Uncertainty, Error, and Confidence module.

Radioactivity and medical therapies

The half-life of radioactive isotopes is also important in radiation therapy. Recall that the Curies observed the effects of radiation on their own health. Radium caused skin burns, leading physicians to believe that tumors could be treated with radium. In 1903, American surgeon Dr. Robert Abbe was one of the first physicians to use radium for experiments in cancer treatment. Abbe found that placing a tumor in contact with a source of radium caused the tumor to stop growing. Physicians continued to experiment to determine the dosage of radium needed and ways to deliver it safely to the patient.

As research about the use of radium and other radioisotopes for treating cancer grew in the early part of the 20th century, findings were published in the prestigious journals of the day, such as the Journal of the American Medical Association. However, this did not mean that all physicians began to learn about this life-saving technique. For example, at the time, African American physicians were barred from joining the American Medical Association due to racist policies, let alone being given a sample of radium for research. So, they formed the National Medical Association in 1895 to share information and released their first journal in 1909. While the African American physicians did not have access to the Journal of the American Medical Association, they relied on pioneers like L. Greeley Brown. Brown was an African American radiologist who had read about the treatment of uterine tumors with radium. In 1918, he published an article in the Journal of the National Medical Association about research on treating uterine tumors with radium to inform his colleagues of the groundbreaking use of radium in treating cancer.

However, not all uses of radium turned out so well. As it became more available in the 1920s, radium was considered a “wonder drug.” Radium salts were placed in all sorts of household products, such as toothpaste, cosmetics, and even water. Figure 8 is an image of a tin that contained cigarettes branded with radium. The thinking went: If radium could shrink cancerous tumors, it must be healthy. Sadly, thousands of people were sickened, and many died due to the careless use of radium in household products. Radium quickly disappeared from household products; however, radium-223 is still used today as a life-saving treatment for bone cancer.

The discovery of radiation, radium, radioactivity, types of radioactive decay, and half-life have all contributed to medical success stories in treating cancer with radiation therapy. This has happened in just over a century of experimentation, research, and innovation. As Madame Curie said in a speech at Vassar College in 1921:

We must not forget that when radium was discovered, no one knew that it would prove useful in hospitals. The work was one of pure science. And this is a proof that scientific work must not be considered from the point of view of the direct usefulness of it. It must be done for itself, for the beauty of science, and then there is the chance that a scientific discovery may become like the radium, a benefit for humanity.