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Radioactivity – Definition, Laws, Occurrence, Applications

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  • Last Updated : 28 Apr, 2022

A nucleus is the positively charged center of an atom made up of protons and neutrons in chemistry. The “atomic nucleus” is another name for it. The word “nucleus” is derived from the Latin nucleus, which is a derivative of the word nux, which signifies nut or kernel.

What is Radioactivity?

The ability of certain forms of matter to emit energy and subatomic particles spontaneously is known as radioactivity. It’s essentially a property of individual atomic nuclei. 

An unstable nucleus will spontaneously dissolve, or decay, into a more stable structure, but only in a few precise ways, for as by generating specified particles or electromagnetic energy. Several naturally occurring elements, as well as artificially manufactured isotopes of the elements, have radioactive decay as a feature. The half-life of a radioactive element is the amount of time it takes for one-half of any given quantity of the isotope to decay. 

Some nuclei have half-lives of more than 1024 years, while others have half-lives of less than 1023 seconds. The daughter of the parent isotope, the outcome of radioactive decay, may be unstable, in which case it will decay as well. The process continues until a stable nuclide has been formed.

Laws of Radioactivity

  • The decay of the nucleus produces radioactivity.
  • Temperature and pressure have no effect on the rate of nucleus disintegration.
  • The rule of conservation of charge governs radioactivity.
  • The offspring nucleus has distinct physical and chemical properties than the mother nucleus.
  • Alpha, beta, and gamma particles are always present when energy is emitted by radioactivity.
  • The amount of atoms present at the time determines the rate of decay of radioactive compounds.

Alpha Decay

Alpha decay is a type of radioactive decay in which unstable atomic nuclei emit a helium nucleus (alpha particle) and transition into a more stable element in the process. The alpha particle, which was thrown out, is made up of four nucleons: two neutrons and two protons. 

The ratio of protons to neutrons in the parent nucleus is reduced by alpha radiation, resulting in a more stable structure. The nucleus of a helium atom is identical to an alpha particle. Ernest Rutherford, who employed alpha particles in his gold foil scattering experiment, conducted the first observations and investigations of alpha decay. 

The atomic number of the radioactive sample changes as the alpha particle, which is made up of two protons and a neutron, exits the nucleus. After alpha decay, the element left behind is two atomic numbers and four mass numbers smaller. For example, Uranium 23592 decays to form Thorium 23190.


Occurrence of alpha decay

Only the most massive components undergo alpha decay. The element’s nucleus should be big enough or unstable enough to undergo spontaneous fission-like modifications. In such elements, it’s the most common type of degradation. The alpha particles that are emitted from the nucleus have an energy level of roughly 5 MeV and a speed of about 5% of light. Because alpha particles lack electrons, they have a charge of +2. An alpha particle reacts violently with its surroundings due to its charge and heavy mass, quickly losing all of its energy. A few inches of air can halt their forward movement.

This type of radioactive decay reacts most aggressively with the human body due to its heaviness and charge. They have a strong ionising power, which allows them to cause havoc on tissue. The victim’s body develops blisters and burns as a result of an alpha radiation overdose.

Applications of radioactivity

  • The alpha emitter americium-241 is utilized in household smoke detectors in the United States.
  • The alpha particles emitted by the Americium sample ionize the air in the smoke detector chamber, causing a small current to flow.
  • When smoke enters the chamber, it produces a current drop, which sets off the alarm. Although Alpha particles have a very low range, they are extremely lethal when they come into contact with each other.
  • When eaten, alpha emitters come into intimate contact with tissue and are lethal in such situations, and are thus employed in assassination attempts through radiation poisoning.

Advantages and Disadvantages of Radioactivity

Advantages of radioactivity are:

  • Gamma rays are employed in radiotherapy to eliminate malignant cells.
  • Cobalt-60 is used to kill cancer-causing cells.
  • The interior regions of the body are scanned with gamma rays.
  • Gamma rays kill bacteria in food and extend its shelf life by preventing deterioration.
  • The age of the rocks can be determined using radioactive radiations and the amount of argon present in the rock.

Disadvantages of radioactivity are:

  • The effects of a high dose of radioactive radiation on the body could be fatal.
  • The cost of radioactive isotopes is high.

Sample Problems 

Problem 1: Write the alpha-decay reaction for Cf−246?


The helium atom, with atomic number 2 and atomic mass number 4, is lost by the radioactive atom. The radioactive atom is transferred to an atom with a two-digit atomic number and a four-digit atomic mass.

The unbalanced equation,

24698Cf → 42He + X

The superscript of X is :
246−4 = 242

The subscript of X is:
98−2 = 96

Thus, the X has the atomic number 96.

The element with the atomic number 96 is curium (Cm).

So, the alpha-decay equation for Cf -246 is:

24698Cf → 24296Cm + 42He

Problem 2: What are the uses of radioactivity?


  • Smoke detectors in the home are made of this material.
  • Sterilization of medical instruments
  • It’s a tool that’s used to diagnose and cure ailments.
  • It is used to generate electricity

Problem 3: What are the units of radioactivity? 


Curie and Rutherford are the units of radioactivity.

1C = 3.7 × 104 Rd is the relationship between Curie and Rutherford.

Problem 4:  Which elements can undergo alpha decay?


Alpha decay can occur in any element heavier than lead. Lighter elements, on the other hand, show no signs of radioactive decay. Alpha decay is a typical phenomenon in heavier elements than bismuth, which has an atomic number of 83. Alpha decay can also be seen in rare earth elements such as neodymium, which has an atomic number of 60, and lutetium, which has an atomic number of 71.

Problem 5: A stationary uranium nucleus scatters an α-particle with an energy of 5 MeV through 180°.What is the order of the distance of the closest approach ? 


Distance of closet approx:−

r0 = ze 2e/4π∈0E


1/2mv2 = 1/4π∈0  q1q2/r​

r = 9×109×2×92×(1.6×10−19)2/5×106×1.6×10−19

= 5.3×10−14m


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