Lesson Explainer: Radioactivity Chemistry

In this explainer, we will learn how to explain the concept of radioactivity.

In 1896, French scientist Henri Becquerel was researching uranium and its possible connection to the newly discovered X-ray. Becquerel theorized that uranium might absorb sunlight and release it as X-rays. To test this theory, he wrapped a photographic plate in black paper, placed uranium salts on top, and set it in the sun. When the photographic plate was developed, Becquerel could clearly see an outline of the uranium crystals. He tried placing objects in between the plate and the crystals, and when the plate was developed, an outline of the object was visible.

The experiments seemed to support Becquerelโ€™s hypothesis, but he continued to conduct more experiments. At the end of February of that year, the sky above Paris was cloudy for several days. Becquerel placed the photographic plates and uranium in a drawer and waited for a sunny day. After several days, he decided to go ahead and develop the plates expecting to see a very weak image. To his surprise, the image was just as clear as when the uranium was placed in the sun. This led Becquerel to propose that uranium produced rays without sunlight.

His continued research showed that these rays did not exhibit the same behavior as X-rays. He further showed that the rays could cause the discharge of electrified particles, now known as ionization.

In 1898, Marie Curie, a doctoral student of Becquerelโ€™s, and her husband Pierre continued research on uranium and discovered that the elements polonium and radium also produced rays. Marie Curie coined the term radioactivity to describe the spontaneous emission of particles and/or radiation.

Definition: Radioactivity

Radioactivity is a property of matter that exhibits spontaneous emission of particles and/or radiation.

Significant work by Ernest Rutherford and Paul Villard on the penetrating ability (the ability to travel through a medium) and behavior of radioactive emission in electric and magnetic fields led to the discovery of alpha, beta, and gamma radiation.

Alpha radiation, also called alpha particles or alpha rays, consists of fast-moving particles that each contain two protons and two neutrons. An alpha particle has a mass of 4 unified atomic mass units and a net charge of 2+. Alpha particles can be represented by the Greek letter ๐›ผ or the nuclide notation 42He, as an alpha particle has the same composition as a helium nucleus.

As alpha particles are positively charged, when alpha radiation is passed through an electric or magnetic field, the radiation will be repelled by the positively charged side of the field and attracted by the negatively charged side of the field.

Due to the size of the particle, alpha particles have low penetrating ability and can be stopped by skin, a piece of paper, or approximately ten centimeters of air.

Beta radiation, also called beta particles or beta rays, consists of high-energy electrons or positrons. These high-energy particles travel much faster than alpha particles and have an approximate mass of 11800 unified atomic mass units. Electrons emitted as beta radiation have a net negative charge and can be represented by the Greek letter ๐›ฝ๏Šฑ or the nuclide notation 0โ€“1e, the notation of an electron.

As ๐›ฝ๏Šฑ particles can be negatively charged, when this kind of beta radiation is passed through an electric or magnetic field, the radiation will be repelled by the negatively charged side of the field and attracted by the positively charged side of the field. Beta particles are more strongly deflected by a field than alpha particles.

Beta particles are significantly smaller than alpha particles and therefore have greater penetrating ability. Beta radiation can pass through skin and paper but can be stopped by a thin sheet of aluminum.

Gamma radiation, or gamma rays, is a high-frequency electromagnetic wave that travels at the speed of light. Gamma radiation does not have mass or charge as it does not consist of particles. Gamma radiation can be represented by the Greek letter ๐›พ or the nuclide notation 00๐›พ.

As gamma radiation does not have a charge, it is unaffected by electric and magnetic fields.

As gamma radiation has no mass and travels as a wave of electromagnetic energy, it has the greatest penetrating ability. It can pass through aluminum and most materials. Sheets of lead a few centimeters thick or a meter or more of concrete are often used to shield against gamma radiation. However, even these materials may not be able to absorb all of the gamma radiation that passes through.

Example 1: Recognizing the Penetrating Ability of Ionizing Radiation

The following questions relate to the ability of different types of ionizing radiation to penetrate various substances.

  1. Which type of ionizing radiation is able to pass through aluminum but is stopped by large quantities of concrete or several centimeters of lead?
    1. ๐›ฝ particles
    2. ๐›ผ particles
    3. ๐›พ rays
  2. Which type of ionizing radiation is able to pass through paper but is stopped by aluminum sheeting?
    1. ๐›ฝ particles
    2. ๐›ผ particles
    3. ๐›พ rays
  3. Which type of ionizing radiation could be stopped by a human hand?
    1. ๐›ฝ particles
    2. ๐›ผ particles
    3. ๐›พ rays

Answer

Part 1

The three primary types of ionizing radiation are alpha particles (๐›ผ), beta particles (๐›ฝ), and gamma rays (๐›พ). Alpha particles have a greater mass than beta particles. Gamma rays are electromagnetic waves and thus have no mass. As alpha particles have the greatest mass, they exhibit the least penetrating ability. As gamma rays have no mass, they exhibit the greatest penetrating ability. Beta particles have a mass in between alpha particles and gamma rays. Thus, the penetrating ability of a beta particle is greater than that of an alpha particle but less than that of a gamma ray.

In terms of shielding, alpha particles can be stopped by a piece of paper or skin, beta particles can be stopped by a thin sheet of aluminum, and gamma rays can be largely stopped by thick lead or concrete.

The type of ionizing radiation that can pass through aluminum but is stopped by large quantities of concrete or several centimeters of lead is ๐›พ rays. The correct answer is C.

Part 2

The type of ionizing radiation that can pass through paper but is stopped by aluminum sheeting is ๐›ฝ particles. The correct answer is A.

Part 3

The type of ionizing radiation that could be stopped by a human hand is ๐›ผ particles. The correct answer is B.

Example 2: Representing Ionizing Radiation with Nuclide Notation

Nuclide notation is used to represent different types of ionizing radiation. Which type of ionizing radiation is represented by the nuclide notation 42He?

  1. Gamma rays
  2. Beta particles
  3. X-rays
  4. Alpha particles

Answer

In nuclide notation, the top-left value is the mass number and the bottom-left value is the atomic number. The atomic number also represents the number of protons. Therefore, this type of ionizing radiation must have two protons. The mass number is the sum of the number of protons and neutrons: massnumbernumberofprotonsnumberofneutrons=+4=2+๐‘›2=๐‘›.

Therefore, this type of ionizing radiation must have two neutrons. The type of radiation that has two protons and two neutrons is an alpha particle. The correct answer is answer choice D.

Radiation may be ionizing or nonionizing. Ionizing radiation is radiation that carries enough energy to remove electrons from atoms.

Definition: Ionizing Radiation

Ionizing radiation is radiation that carries enough energy to remove electrons from atoms.

Definition: Nonionizing Radiation

Nonionizing radiation is radiation that does not carry enough energy to remove electrons from atoms.

The figure below shows the electromagnetic spectrum separated into ionizing and nonionizing radiation. Alpha particles, beta particles, neutrons, and cosmic rays, a collection of high-energy particles, are also forms of ionizing radiation.

Example 3: Identifying an Example of Ionizing Radiation

Which of the following is an example of ionizing radiation?

  1. IR rays
  2. Cosmic rays
  3. Light rays
  4. Microwaves
  5. Radio waves

Answer

Ionizing radiation is radiation that carries enough energy to remove electrons from atoms. Looking at the electromagnetic spectrum, we can separate radiation into ionizing and nonionizing radiation in the ultraviolet range.

In addition, some types of particle radiation are ionizing. These include alpha particles, beta particles, neutrons, and cosmic rays. We can see in the electromagnetic spectrum diagram that IR rays (infrared radiation), visible light, microwaves, and radio waves have less energy than UV radiation and are nonionizing. Cosmic rays contain high-energy protons and atomic nuclei that can remove electrons from atoms. Therefore, answer choice B is an example of ionizing radiation.

Ionizing radiation is harmful to biological systems. The high-energy radiation can break bonds in molecules important for cellular function or ionize water molecules that can proceed to react with DNA, proteins, or enzymes. Nonionizing radiation does not have enough energy to cause the same type of cellular damage; however, it can cause burns. Depending on the energy of the ionizing radiation and the length of exposure, cells may be able to repair themselves, immediately die, be unable to replicate, or mutate into cancerous cells.

Example 4: Recognizing Why Nonionizing Radiation Does Not Cause Permanent Cellular Damage

In general terms, why does nonionizing radiation not cause permanent damage to living cells?

  1. Nonionizing radiation passes straight through living organisms.
  2. Nonionizing radiation only travels short distances.
  3. Nonionizing radiation does not have enough energy to damage cells.
  4. Nonionizing radiation cannot penetrate living cells.
  5. Cell exposure to nonionizing radiation is minimal.

Answer

Looking at the electromagnetic spectrum, we can separate radiation into ionizing and nonionizing radiation in the ultraviolet range.

Ionizing radiation also includes alpha, beta, and neutron particle radiation. Ionizing radiation has short wavelengths, high frequency, and high energy while nonionizing radiation has long wavelengths, low frequency, and low energy. The high energy of ionizing radiation can break bonds in molecules in a cell or ionize molecules in a cell. Ionizing radiation cell damage is often permanent. Nonionizing radiation, however, does not have enough energy to break bonds in molecules or ionize molecules in a cell. Exposure to nonionizing radiation can lead to localized heating of tissue and possible burns but will not result in permanent cell damage. In general, nonionizing radiation does not cause permanent damage to living cells because it does not have enough energy to damage cells. The correct answer is C.

Alpha radiation is the most ionizing of the three main types of radioactivity. Alpha radiation consists of large, relatively slow-moving, positively charged particles that quickly remove electrons from surrounding atoms. The ionization caused by alpha particles will occur over a very short distance. This means that alpha radiation in a biological system will do a great deal of damage in a concentrated area. Luckily, as alpha radiation can be stopped by the skin, it is generally only a health concern if inhaled or ingested.

Beta particles generally have more energy than alpha particles but are smaller and carry half the charge magnitude. Beta particles still ionize atoms but will ionize fewer atoms over a larger distance when compared to alpha particles.

Gamma radiation is a highly energetic electromagnetic wave. Gamma radiation is not charged, but its energy can be transferred to an electron to cause the electron to be emitted from an atom. As gamma radiation travels faster than alpha and beta radiation, it spends the least amount of time in contact with biological tissue and is the least ionizing of the three main types of radioactivity. However, gamma exposure poses the greatest external health concern as it can pass through the human body.

The table below summarizes the information we have discussed so far.

Radiation NameAlphaBetaGamma
Type of RadiationParticleParticleElectromagnetic wave
Symbol๐›ผ๐›ฝ๏Šฑ๐›พ
Nuclide Notation42He0โ€“1e00๐›พ
Deflection in a FieldDeflected toward the negatively charged side of the field-Deflected toward the positively charged side of the field
-Deflected to a greater degree than alpha radiation
No deflection
Penetrating AbilityStopped by skin or a piece of paperStopped by a thin sheet of aluminumLargely stopped by several centimeters of lead or thick concrete
Relative SpeedSlowFaster than alpha, slower than gammaFast
Relative Ionizing AbilityHighLess ionizing than alpha, more ionizing than gammaLow

We are exposed to ionizing radiation daily from naturally occurring and man-made sources including radium in the air and soil, cosmic rays, and potassium-40 in bananas. Our daily ionizing radiation exposure poses little risk. However, increased exposure can result in ionizing radiation injuries that can manifest quickly or over the course of several years. These injuries, often referred to as radiation sickness, may include burns, nausea, hair loss, and ulcers. Severe radiation exposure can lead to increased bacterial infections, cancer, and early death.

Scientists have found a number of ways to use ionizing radiation to our benefit. Americium-241, an alpha-particle source, is found in smoke detectors. The alpha particles ionize the surrounding air. A change in the amount of ionization due to the presence of smoke will cause the alarm to sound. Gamma radiation and X-rays can be used to irradiate food to prolong shelf life and eliminate organisms such as Salmonella and E. coli that cause foodborne illnesses.

In hospitals, gamma radiation and X-rays can be used to sterilize equipment and are used in X-ray, CT, and PET machines to produce detailed images of the human body. Small amounts of radioactive isotopes, called radiotracers, can be given to a patient. The specific radiotracer will accumulate in a particular area of the body and emit radiation that can be detected to produce more detailed images. Radioactive isotopes can also be given as a treatment. Iodine-131 is often given as a thyroid cancer treatment. The iodine is absorbed by the thyroid and releases beta and gamma radiation that can destroy nearby thyroid cells.

Example 5: Recognizing the Practical Applications of Ionizing Radiation

Ionizing radiation has many practical applications. In which of the following devices or processes is ionizing radiation not used?

  1. Smoke detectors
  2. Medical imaging
  3. Radars
  4. Disinfecting medical instruments
  5. Food irradiation

Answer

Americium-241, an alpha-particle source, is commonly found in smoke detectors. Medical imaging often uses X-rays, gamma rays, and radioactive tracers. A radar is a detection system that uses radio waves. Medical instruments can be disinfected, and food can be irradiated with gamma rays or X-rays.

Looking at the electromagnetic spectrum, we can separate radiation into ionizing and nonionizing radiation in the ultraviolet range.

We can see that gamma rays and X-rays are forms of ionizing radiation, while radio waves are a form of nonionizing radiation. Ionizing radiation also includes alpha-, beta-, and neutron-particle radiation. Therefore, the only application that does not use ionizing radiation is a radar. The correct answer is C.

Key Points

  • Henri Becquerel discovered that atoms could spontaneously emit rays.
  • Marie Curie coined the term radioactivity to describe the ability of an atom to emit radiation.
  • The three main types of radiation are alpha, beta, and gamma.
  • Radiation may be ionizing or nonionizing.
  • Ionizing radiation can cause permanent cell damage or cell death.
  • Ionizing radiation can be safely used in smoke detectors, for food irradiation, or in various medical procedures and treatments.

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