Alpha Decay

Alpha decay or α-decay is a type of radioactive decay in which the atomic nucleus emits an alpha particle thereby transforming or decaying into a new atomic nucleus. Here the atomic mass number of the newly formed atom will be reduced by four and the atomic number will be reduced by two. The emitted alpha particle is also known as a helium nucleus. The mass of the alpha particles is relatively large and has a positive charge.

Ernest Rutherford distinguished alpha decay from other forms of radiation by studying the deflection of the radiation through a magnetic field. The deflection of alpha decay would be a positive charge as the particles have a +2e charge.

Equation for Alpha Decay

Following is the general alpha decay equation:

\(_{Z}^{A}\textrm{X}\rightarrow _{Z-4}^{A-4}\textrm{Y}+_{2}^{4}\textrm{He}\)


  • \(_{Z}^{A}\textrm{X}\) is the parent nucleus
  • A is the total no.of nucleons
  • Z is the total no.of protons
  • \(_{Z-4}^{A-4}\textrm{Y}\) is the daughter nucleus
  • \(_{2}^{4}\textrm{He}\) is the released alpha particle

Alpha Decay DefinitionWhat Happens In Alpha DecayAlpha Decay FormulaAlpha Decay EquationAlpha Decay ExampleGamow Theory of Alpha DecayRadioactive DecayFAQs For Alpha Decay

What Is Alpha Decay?

Alpha decay is a type of radioactive decay in which a particle with two neutrons and two protons is ejected from the nucleus of a radioactive atom.

Alpha Decay Symbol

Alpha decay is denoted by α or He2+ or α2+

What Happens In Alpha Decay?

Alpha decay occurs in very heavy elements like uranium, thorium, and radium. They are called parent nucleus and they are basically unstable. Because the nuclei of these atoms have a lot more neutrons in their nuclei than protons, that is, they have too large a proton to neutron ratio, which makes these elements neutron-rich. This richness makes alpha decay possible. Thus, emitting its two protons and two neutrons in the form of an alpha particle and a forming of new daughter nucleus and attains a very stable configuration. Alpha decay can be described like this:

  • The nucleus of these nuclei (parent nucleus) rich atoms splits into two parts.
  • The alpha particle goes zooming off into space.
  • The nucleus left behind (daughter nucleus) has its atomic number reduced by 2 and its mass number reduced by 4.

Alpha Decay Formula

In general, the following is the way to write the formula for alpha decay:

\(_{Z}^{A}\textrm{X}\rightarrow _{Z-2}^{A-4}\textrm{X’}+_{2}^{4}\textrm{α}\)


  • \(_{Z}^{A}\textrm{X}\) is the parent nucleus.
  • \(_{Z-2}^{A-4}\textrm{X’}\) is the daughter nucleus.
  • \(_{2}^{4}\textrm{α}\) is the emitted alpha particle.

Alpha Decay Equation

In nuclear physics, Alpha decay Equation for thorium-232 can be written as –

\(_{90}^{232}\textrm{Th}\rightarrow _{88}^{228}\textrm{Ra}+_{2}^{4}\textrm{He}\)


  • \(_{90}^{232}\textrm{Th}\) is the unstable Thorium-232 parent nucleus.
  • \(_{88}^{228}\textrm{Ra}\) is the stable Radium-228 daughter nucleus.
  • \(_{2}^{4}\textrm{He}\) is the emitted alpha particle.

Alpha Decay Example

Ample of Alpha decay example can be given. Some of them are-

\(_{93}^{237}\textrm{Np}\rightarrow _{91}^{233}\textrm{Pa}+_{2}^{4}\textrm{He}\)
\(_{78}^{175}\textrm{Pt}\rightarrow _{76}^{171}\textrm{Os}+_{2}^{4}\textrm{He}\)
\(_{64}^{149}\textrm{Gd}\rightarrow _{62}^{145}\textrm{Sm}+_{2}^{4}\textrm{He}\)

Alpha Decay Of Uranium 238

Alpha decay of Uranium 238 heavy nucleus into Thorium 234 with the emission of an alpha particle is given as:

\(_{92}^{238}\textrm{Ur}\rightarrow _{90}^{234}\textrm{Th}+_{2}^{4}\textrm{He}\)

Alpha Decay

Gamow Theory of Alpha Decay

The Geiger–Nuttall law or Geiger–Nuttall rule relates to the decay constant of a radioactive isotope with the energy of the alpha particles emitted. This relation also states that half-lives are exponentially dependent on decay energy, so that very large changes in half-life make comparatively small differences in decay energy, and thus alpha particle energy.

As per this rule, short-lived isotopes emit more energetic alpha particles than long-lived ones. This law was stated by Hans Geiger and John Mitchell Nuttall in the year 1911, hence the name was dedicated to these physicists.

Radioactive Decay

The spontaneous decay or breakdown of an atomic nucleus is known as Radioactive Decay. This decay in a nucleus causes the release of energy and matter from the nucleus.

The most common forms of Radioactive decay are:

  1. Alpha Decay (Helium nucleus is emitted)
  2. Beta Decay (Electrons are emitted)
  3. Gamma Decay (High energy photons are emitted)

This is also termed as Nuclear Decay or Radioactivity. The element or isotope which emits radiation and undergoes the process of radioactivity is called Radioactive Element.

General Questions on Alpha Decay

Q1: What is the atomic mass number of the emitted Alpha particle?

A: The atomic mass number of the emitted Alpha particle is four.

Q2: Write the general form of Alpha decay.

A: The general form of alpha decay is

\(_{Z}^{A}\textrm{X}\rightarrow _{Z-2}^{A-4}\textrm{X’}+_{2}^{4}\textrm{α}\)


  • \(_{Z}^{A}\textrm{X}\) is the parent nucleus.
  • \(_{Z-2}^{A-4}\textrm{X’}\) is the daughter nucleus.
  • \(_{2}^{4}\textrm{α}\) is the emited alpha particle.

Q3: Which type of radioactive decay is shown below?

\(_{93}^{237}\textrm{Np}\rightarrow _{91}^{233}\textrm{Pa}+_{2}^{4}\textrm{He}\)

A: The radioactive decay is shown above is Alpha decay.

Q4: What is the number of protons present in the alpha particle?

A: The number of protons present in the Alpha particle is two.

Q5: What is the number of neutron present in the alpha particle?

A: The number of neutrons present in the Alpha particle is two.

Q6: How is Alpha decay denoted?

A: Alpha decay is denoted by α or He2+ or α2+

Q7: What product do we get when Uranium 238 undergoes alpha decay?

A: When Uranium 238 undergoes Alpha decay we get Thorium 234.

\(_{92}^{238}\textrm{Ur}\rightarrow _{90}^{234}\textrm{Th}+_{2}^{4}\textrm{He}\)

Q8: If \(_{90}^{232}\textrm{Th}\) undergoes Alpha decay, name the daughter nucleus formed.

A: If \(_{90}^{232}\textrm{Th}\) undergoes Alpha decay,the daughter nucleus formed will be \( _{88}^{228}\textrm{Ra}\) \(_{90}^{232}\textrm{Th}\rightarrow _{88}^{228}\textrm{Ra}+_{2}^{4}\textrm{He}\)

Q9: Name the alternate name of the Alpha particle.

A: Alpha particles are alternatively named as doubly ionized helium nuclei(He2+)

Q10: What is the mass of the emitted Alpha particle?

A: The mass of the Alpha particle is 6.644657230(82) × 10−27 Kg.

Q11: What is the penetration depth of an Alpha particle?

A: The penetration power of Alpha rays is low. Generally few centimeters of air or by the skin.

Q12: What is the polarity of charge of the Alpha Particle?

A: The emitted Alpha particle is positively charged.

Q13: What is the average Kinetic Energy of the emitted Alpha Particle?

A: The average Kinetic energy of the emitted Alpha particle is approximately 5MeV.

Q14: What is the velocity of the Alpha particle?

A: Average velocity of the emitted Alpha particle is in the vicinity of 5% of that of c.

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Test your knowledge on Alpha Decay


  1. If one alpha and two beta particles emitted from the radioactive element then what will be the relationship
    between the parent and daughter element?

    1. The relation between any parent and daughter element is that the rate of decay of a radioactive isotope is dependent on the amount of parent isotope that is remaining.
      Note that, here the term isotope refers to the combination of elements that are obtained with different number of neutrons.

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