Alpha Decay

Alpha Decay Definition

Alpha decay or α-decay refers to any decay where the atomic nucleus of a particular element releases 42He and transforms into an atom of a completely different element. This decay leads to a decrease in the mass number and atomic number, due to the release of a helium atom.

To understand this entirely, consider this alpha decay example. Suppose element Z has mass number ‘a’ and atomic number ‘b’. During α decay, this element changes to X. Take a look at the equation below. 

abZ → a-4b-2X + 42He

Thus, you can see that the mass number and the atomic number balances out on both sides of this equation.

Alpha Decay Example Problems

Now, using the same concept, solve the following problem. A Uranium nucleus, 23892U undergoes alpha decay and turns into a Thorium (Th) nucleus. What would be the mass and atomic number for this resulting nucleus after the decay?

Solution – 

23892U → 238-492-2Th + 42He → 23490Th + 42He

Therefore, the resulting Thorium nucleus should have 234 mass number and 90 atomic number.

Alpha Decay Equation

Alpha decay formula can be written in the following way – 

AZX → A-4Z-2Y + 42α

In this equation, AZX represents the decaying nucleus, while A-4Z-2Y is the transformed nucleus and 42α is the alpha particle emitted.

Understanding Q Value of Alpha Decay

In Physics and Chemistry, Q-value is defined as the difference between the sum of rest masses of original reactants and sum of final product masses. In simpler terms, you can say that the Q-value is the difference between the final and initial mass-energy of the decayed products.

For alpha decay equations, this Q-value is,

Q = (mX – mY – mHe) c2 

The energy Q derived from this decay is divided equally into the transformed nucleus and the Helium nucleus.   

Gamow Theory of Alpha Decay

Gamow Theory or Geiger-Nutall law defines the relationship between the energy of an alpha particle emitted with the decay constant for a radioactive isotope. It was derived by John Mitchell Nutall and Hans Geiger in 1911, hence the name for this law. 

With this rule, it becomes abundantly clear that shorter-lived isotopes emit greater energy when compared to isotopes with longer lives. However, α decay is just one type of radioactive decay. A nucleus can undergo beta and gamma decay as well.

What is Beta Decay?

In beta decay, the radioactive isotope emits an electron or positron. This decay occurs by following the radioactive laws, just as an alpha decay does. An example of beta decay is – 

3215P → 3216S + e- + v- 

What is Gamma Decay?

The last form of radioactive decay is gamma decay. Here, a high-energy radioactive nucleus can lower its energy state by emitting electromagnetic radiation. Gamma decay is common for the daughter nucleus formed after α decays and ß decays. 

This happens because daughter nuclei in both these forms of decay are in a heightened state of energy. To return to a stable state, these nuclei emit electromagnetic radiation in the forms of one or multiple gamma rays. 

Exercise

14964Gd undergoes α decay to form one nucleus of Sm. Calculate the atomic and mass number of the daughter nucleus.

Solution – 

14964Gd → 149-464-2Sm + 42He → 14562Sm + 42He

As per the alpha decay equation, the resulting Samarium nucleus will have a mass number of 145 and an atomic number of 62.         

The isotope element that emits radiation is known as the Radioactive Element. This element is also the object that undergoes radioactivity. 

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FAQ (Frequently Asked Questions)

1. What is alpha decay?

Alpha decay is the process of transformation of a radioactive nucleus by emitting helium. During this transformation, the initial element changes to another completely different element, undergoing a change in mass and atomic number as well.

2. Which elements can undergo alpha decay?

All elements heavier than lead can undergo alpha decay. However, lighter elements do not exhibit radioactive decay of any kind.

3. What is the use of the Geiger-Nutall Law?

Geiger-Nutall law establishes a relation between the decay constant of a radioactive isotope and the energy of the emitted alpha particle.