Radioactive Decay - Meaning Types , FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 04:46 PM IST

In this article, we define radioactive decay is, radioactive decay law, concept of decay constant, disintegration constant, types of radioactive decay, radioactive equilibrium, and the difference between radioactive equilibrium and chemical equilibrium.

What is radioactive decay?

Radioactive decay is a process by which the nucleus of an unstable atom loses by emitting radiation. It is also called nuclear decay, radioactivity, or nuclear disintegration.

Explain the decay process.

The decay process or radioactive process is a property of the nuclei of an atom. A material is called a radioactive or radioactive element if it contains unstable nuclei. By this process, the nucleus becomes more stable by emitting its energy. The mass and charge are conserved in the decay process.

Theory of radioactivity:

Radioactivity or the radioactive decay process involves the continuous emission of radiation by unstable nuclei in the form of radiation. The nuclei of the atom emit ionizing particles by losing their energy spontaneously. Due to the loss of energy, two types of atoms called parent nuclide and daughter nuclide are formed. The parent nuclide is the decaying nucleus and the daughter nuclide is obtained from the parent nuclide. Radioactive decay is called a nuclear transformation that yields a daughter nuclide that has protons or neutrons. This condition is not applicable in gamma decay, a type of radioactive decay. A new element is obtained as the number of protons in the atom changes.

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Radioactive Decay - Meaning Types , FAQs

Types of radioactive decay:

Radioactive decay is classified into three types:

Alpha decay: The nuclear decay process in which alpha particle is emitted is called alpha decay.

It is represented as, E=(m_i-m_f-m_p)c²

Here, E is the energy, c is the speed of light, mi is the mass of nucleus before decay, m_f is mass of nucleus after decay, and m_p is the mass of the particle emitted in the decay process. Consider an example for the alpha decay of Th-230.

90230Th→24He+88226Ra

In this equation, thorium-230 is converted to radium-226 with the emission of an alpha particle.

Beta-decay: In the decay process, if the beta particle is emitted then it is called beta decay. The nucleus emits a high-energy electron in the form of a beta particle. The beta particle is considered an electron as it is an electron ejected from the splitting of the neutron. An example for the beta decay of Th-234 is represented as:

90234Th→-10e+91234Pa

In the above equation, thorium-234 is converted to Pa-234 with the emission of a beta particle.

Gamma decay: A nuclear disintegration in which gamma rays are emitted is called gamma decay. Consider the example of U-238.

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92238U→24He+90234Th+200

In this equation, uranium-238 is converted to thorium-234 with the emission of gamma rays along with the alpha particle.

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State radioactive decay law.

Radioactive decay law states that “the rate of radioactive disintegration is directly proportional to the number of nuclei of the elements present at that time”.

If N is the number of nuclei and dN is the number of radioactive decay per unit time dt then, the law is represented as,

dN/dt=N

dN/dt=-λN

In the above equation, λ is the radioactive constant.

Derive the law of radioactive decay.

From the radioactive decay equation,

dN/dt=-λN ……………………………. (1)

Consider at time t=0, the number of nuclei is N0 and at time t, the number of nuclei is N.

On integrating equation (1),

$\int_{N_0}^{N}\frac{dN}{N}=-\int_{0}^{t}\lambda dt$

log_eN-log_eN₀=-λt

log_e(N/N0)=-λt

N/N₀=e^-λt

N=N₀e^-λt

The above equation is the formula for the radioactive decay law.

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Rate of disintegration or rate of radioactive decay:

The rate of disintegration or rate of radioactive decay is the number of radioactive nuclei reducing per unit time. The rate of disintegration is denoted by(-dN_t/dt).

What is the decay constant?

Decay constant lambda (λ) is the time taken by the nucleus of an atom to reduce half of its initial value. It is also defined as the reciprocal of the time in which the number of atoms in a radioactive element reduces to 36.8% of the initial value.

$\lambda=\frac{2.303}{t}log_{10}(N_0/N)$

SI unit of the decay constant is s^-1.

Radioactive disintegration series:

A radioactive nucleus undergoes a series of decay. In a radioactive disintegration series, the decay process is continued until the unstable nuclei become stable nuclei.

The half-life of a radioactive element:

The time taken by the radioactive element to reduce half of its initial value is called the half-life of a radioactive element. It is represented by T_1/2.

T_1/2=ln2/λ=0.693/λ

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Frequently Asked Questions (FAQs)

1. Define decay meaning in physics.

Decay is the process in which an object gets decomposed.

2. What is the radioactive element definition?

The meaning of radioactive element is the element having unstable nuclei in its atom.

3. Define radioactive nuclei.

In the decay process, unstable nuclei that emit radiation are called radioactive nuclei.

4. What is radioactivity? Define decay constant class 12.

The process in which radiation is emitted in the form of particles or photons of high energy is called radioactivity.

The decay constant is the reciprocal of the time by which the left number of nuclei in the element decreases to 1/e times the number of nuclei before the decay.

5. What is the time required to decay from N to N0?

t=2.303log10(N0N)

Here λ is the decay constant.

6. What is the activity of radioactive samples?

The number of disintegration per second is the activity of the radioactive sample.

7. What is the decay of radioactive isotopes?

Decay of radioactive isotopes is the process of emitting radiation where the time period of the isotope is measured in half-life.

8. What is the use of the law of radioactive disintegration?

The law of radioactive disintegration is used in the calculation of decay rate.

9. What are the three main types of radioactive decay?

The three main types of radioactive decay are:

10. How does alpha decay change the atomic number and mass number of an element?

In alpha decay, the atomic number decreases by 2 and the mass number decreases by 4. This is because an alpha particle (consisting of 2 protons and 2 neutrons) is emitted from the nucleus, changing the element into a different one with fewer protons and neutrons.

11. What is the difference between beta-minus and beta-plus decay?

Beta-minus decay involves the emission of an electron, converting a neutron into a proton and increasing the atomic number by 1. Beta-plus decay involves the emission of a positron, converting a proton into a neutron and decreasing the atomic number by 1. Both processes keep the mass number constant.

12. Why doesn't gamma decay change the atomic number or mass number of an element?

Gamma decay only involves the emission of high-energy electromagnetic radiation (gamma rays) from an excited nucleus. It doesn't involve the release of any particles, so the number of protons and neutrons in the nucleus remains unchanged, preserving both the atomic number and mass number.

13. How is carbon-14 dating used to determine the age of organic materials?

Carbon-14 dating uses the known half-life of carbon-14 (about 5,730 years) to estimate the age of organic materials. Living organisms continuously exchange carbon with their environment, maintaining a constant ratio of carbon-14 to carbon-12. After death, this exchange stops, and the carbon-14 decays at a predictable rate, allowing scientists to calculate the time since the organism died based on the remaining carbon-14.

14. What is radioactive decay?

Radioactive decay is the process by which an unstable atomic nucleus spontaneously releases energy in the form of radiation, transforming into a more stable nucleus. This process occurs naturally and cannot be influenced by external factors like temperature or pressure.

15. Why do some atoms undergo radioactive decay while others don't?

Atoms undergo radioactive decay when their nuclei are unstable, typically due to an imbalance between protons and neutrons. Stable atoms have a balanced ratio of protons to neutrons, while unstable atoms have an excess of either, leading to radioactive decay as they seek a more stable configuration.

16. What is the half-life of a radioactive isotope?

The half-life of a radioactive isotope is the time it takes for half of the original amount of the isotope to decay. It's a measure of the rate of decay and is unique to each isotope, ranging from fractions of a second to billions of years.

17. What is the relationship between radioactive decay and nuclear fission?

Radioactive decay is a spontaneous process where unstable nuclei emit radiation to become more stable. Nuclear fission, on the other hand, is the splitting of heavy atomic nuclei into lighter nuclei, often induced by bombarding them with neutrons. While both processes involve changes in nuclear structure, fission is typically controlled and used in nuclear reactors, whereas decay occurs naturally.

18. How does radioactive decay contribute to the Earth's internal heat?

Radioactive decay of elements like uranium, thorium, and potassium in the Earth's crust and mantle generates heat. This process, known as radiogenic heating, is a significant source of the Earth's internal heat, contributing to phenomena such as plate tectonics, volcanic activity, and the Earth's magnetic field.

19. How do scientists detect and measure radioactive decay?

Scientists use various instruments to detect and measure radioactive decay, including:

20. How does the concept of binding energy per nucleon explain the stability of iron?

The binding energy per nucleon is a measure of nuclear stability. It increases with atomic number up to iron-56, which has the highest binding energy per nucleon. This explains why iron is the most stable element and why fusion reactions in stars typically stop at iron. Elements heavier than iron are formed through other processes, such as supernovae.

21. How does the shell model of the nucleus help explain magic numbers and nuclear stability?

The shell model of the nucleus, analogous to electron shells in atoms, proposes that protons and neutrons occupy energy levels or "shells" within the nucleus. Nuclei with completely filled shells (corresponding to magic numbers) are more stable. This model helps explain why certain numbers of protons or neutrons result in particularly stable nuclei, influencing radioactive decay rates.

22. How does radioactive decay contribute to the formation of elements in the universe?

Radioactive decay plays a crucial role in element formation through several processes:

23. What is the significance of the "island of stability" in superheavy elements?

The "island of stability" is a hypothetical region of superheavy elements with relatively long half-lives, predicted by nuclear physics models. It's significant because it suggests that some very heavy elements might be more stable than their neighbors, potentially leading to new elements with unique properties and applications in science and technology.

24. How does radioactive decay affect the composition of Earth's atmosphere over time?

Radioactive decay influences Earth's atmosphere in several ways:

25. What is the significance of secular equilibrium in radioactive decay chains?

Secular equilibrium occurs in a decay chain when the half-life of the parent isotope is much longer than that of its daughters. In this state, the activity (decay rate) of each daughter isotope becomes equal to that of the parent. This concept is important in understanding the behavior of natural decay series and in applications like radioactive dating and environmental monitoring.

26. How does radioactive decay contribute to the Earth's magnetic field?

Radioactive decay in the Earth's core and mantle contributes to convection currents in the liquid outer core. These currents of electrically conducting material, combined with the Earth's rotation, generate the geodynamo that produces the Earth's magnetic field. Thus, radioactive decay indirectly supports the maintenance of the Earth's magnetic field, which is crucial for protecting life from harmful solar radiation.

27. How does the concept of nuclear shell structure explain "magic" proton and neutron numbers?

The nuclear shell model, analogous to electron shells in atoms, proposes that nucleons (protons and neutrons) occupy distinct energy levels within the nucleus. "Magic" numbers (2, 8, 20, 28, 50, 82, and 126) correspond to completely filled shells, resulting in exceptionally stable nuclei. This model explains why nuclei with these numbers of protons or neutrons are more resistant to radioactive decay and more abundant in nature.

28. What is the difference between ionizing and non-ionizing radiation?

Ionizing radiation, such as that emitted during radioactive decay, has enough energy to remove electrons from atoms, creating ions. This can be harmful to living tissues. Non-ionizing radiation, like radio waves or visible light, doesn't have enough energy to ionize atoms and is generally less harmful.

29. What is the concept of radioactive equilibrium?

Radioactive equilibrium occurs when the rate of production of a radioactive isotope equals its rate of decay. This can happen in a decay chain where a parent isotope decays into a daughter isotope, which then decays further. When equilibrium is reached, the ratio of parent to daughter isotopes remains constant over time.

30. How does radioactive decay relate to the concept of binding energy?

Binding energy is the energy required to break apart a nucleus into its constituent protons and neutrons. Radioactive decay occurs when a nucleus can achieve a more stable configuration (higher binding energy per nucleon) by emitting particles or energy. The difference in binding energy before and after decay is released as radiation.

31. What is the significance of the "magic numbers" in nuclear stability?

Magic numbers (2, 8, 20, 28, 50, 82, and 126) represent the number of protons or neutrons that form complete shells in the nucleus. Nuclei with these numbers of protons or neutrons are particularly stable and less likely to undergo radioactive decay, explaining why some isotopes are more abundant in nature than others.

32. How does the strong nuclear force influence radioactive decay?

The strong nuclear force holds protons and neutrons together in the nucleus, counteracting the electromagnetic repulsion between protons. Radioactive decay occurs when the balance between these forces is unstable. For example, in heavy nuclei, the strong force may not be sufficient to overcome the electromagnetic repulsion, leading to alpha decay.

33. What is the difference between artificial and natural radioactivity?

Natural radioactivity occurs in elements found in nature, such as uranium and thorium. Artificial radioactivity is induced in stable elements through processes like neutron bombardment or nuclear reactions. While the decay process is the same for both, artificial radioactive isotopes are created in laboratories or nuclear reactors.

34. How does the neutron-to-proton ratio affect nuclear stability?

The neutron-to-proton ratio is crucial for nuclear stability. Light stable nuclei have roughly equal numbers of protons and neutrons. As atomic number increases, stable nuclei require more neutrons than protons to counteract the increased electromagnetic repulsion. Nuclei with ratios far from the stable range are more likely to undergo radioactive decay.

35. What is the role of the weak nuclear force in beta decay?

The weak nuclear force is responsible for beta decay. It allows for the conversion of neutrons to protons (or vice versa) within the nucleus. This force facilitates the emission of electrons (beta-minus decay) or positrons (beta-plus decay), changing the atomic number of the element while maintaining the same mass number.

36. How does radioactive decay relate to nuclear transmutation?

Radioactive decay is a natural form of nuclear transmutation, where one element changes into another. For example, when uranium-238 undergoes alpha decay, it transmutes into thorium-234. Nuclear transmutation can also be artificially induced through nuclear reactions, but radioactive decay is a spontaneous process that occurs without external intervention.

37. What is the significance of decay chains in radioactive elements?

Decay chains are series of radioactive decays that occur as unstable isotopes transform into more stable ones. They are significant because they explain how different radioactive elements are related and how they contribute to the overall radioactivity in nature. For example, the uranium-238 decay chain ends with the stable lead-206 isotope after several intermediate decays.

38. What is the relationship between radioactive decay and nuclear fusion in stars?

While radioactive decay and nuclear fusion are different processes, they are both important in stellar evolution. Fusion in stars creates heavier elements up to iron, while the energy released from radioactive decay of elements like nickel-56 contributes significantly to the brightness of supernovae. After a star's death, radioactive decay continues in the remnants, influencing the composition of the interstellar medium.

39. What is the difference between decay constant and half-life?

The decay constant (λ) is the probability of a single atom decaying per unit time, while the half-life (t₁/₂) is the time it takes for half of a sample to decay. They are related by the equation: t₁/₂ = ln(2) / λ. The decay constant is useful in calculations, while half-life is more intuitive for understanding the timescale of decay.

40. What is the role of neutrinos in beta decay?

Neutrinos play a crucial role in beta decay. In beta-minus decay, an antineutrino is emitted along with the electron, while in beta-plus decay, a neutrino is emitted with the positron. The emission of these nearly massless particles ensures conservation of energy, momentum, and angular momentum in the decay process. The discovery of neutrinos was prompted by the continuous energy spectrum observed in beta decay.

41. How does the concept of quantum tunneling apply to alpha decay?

Quantum tunneling explains how alpha particles can escape the nucleus despite not having enough energy to overcome the potential barrier classically. The alpha particle has a small probability of "tunneling" through the barrier, which is why alpha decay rates can be predicted and why some heavy nuclei are more prone to alpha decay than others.

42. What is the difference between spontaneous and stimulated nuclear decay?

Spontaneous nuclear decay occurs naturally without external influence, governed by the inherent instability of the nucleus. Stimulated decay, on the other hand, is induced by external factors such as bombardment with particles or high-energy radiation. While most radioactive decay is spontaneous, stimulated decay is important in nuclear reactors and some medical applications.

43. How does the mass defect relate to nuclear binding energy and radioactive decay?

The mass defect is the difference between the mass of a nucleus and the sum of its constituent nucleons' masses. This mass difference, when converted to energy (E=mc²), represents the binding energy of the nucleus. Nuclei with lower binding energy per nucleon are more likely to undergo radioactive decay to achieve a more stable configuration with higher binding energy per nucleon.

44. What is the significance of the N=Z line in the chart of nuclides?

The N=Z line on the chart of nuclides represents nuclei with an equal number of protons and neutrons. Light stable nuclei tend to lie along or near this line. As atomic number increases, stable nuclei require more neutrons than protons, deviating from the N=Z line. This trend helps explain why heavier elements tend to be more neutron-rich and prone to radioactive decay.

45. How does radioactive decay contribute to geothermal energy?

Radioactive decay of elements like uranium, thorium, and potassium in the Earth's crust and mantle generates heat. This radiogenic heat contributes significantly to the Earth's internal heat budget, which drives geothermal processes. Geothermal energy harnesses this heat for power generation and heating applications, making radioactive decay an indirect source of renewable energy.

46. What is the relationship between radioactive decay and nuclear isomers?

Nuclear isomers are excited states of atomic nuclei that have measurable half-lives. While not all excited states are isomers, those that are can undergo gamma decay to reach their ground state. This process is a form of radioactive decay, but it doesn't change the element or isotope. The study of isomers provides insights into nuclear structure and energy levels.

47. How does the concept of branching ratio apply to radioactive decay?

The branching ratio describes the probability of a radioactive nucleus decaying through different modes. For example, a nucleus might have a 70% chance of undergoing beta decay and a 30% chance of alpha decay. Branching ratios are important for understanding the overall decay process of an isotope and its daughters, especially in complex decay chains.

48. What is the role of radioactive decay in the formation and evolution of planetary atmospheres?

Radioactive decay influences planetary atmospheres in several ways:

49. What is the significance of the "valley of stability" in nuclear physics?

The "valley of stability" is a region on the chart of nuclides where the most stable isotopes for each element are found. It represents the optimal neutron-to-proton ratio for nuclear stability. Nuclei far from this valley are more prone to radioactive decay as they attempt to reach a more stable configuration. Understanding this concept helps predict the behavior of unstable isotopes and guides research in nuclear physics.

50. How does radioactive decay relate to the concept of nuclear transmutation?

Radioactive decay is a natural form of nuclear transmutation, where one element changes into another. For instance, when uranium-238 undergoes alpha decay, it transmutes into thorium-234. This process occurs spontaneously in nature and is distinct from artificial transmutation, which involves inducing nuclear reactions to create new elements or isotopes.