Class 12 > Unit # 27: Nuclear Physics > Radioactive Decay Process

Talha's Physics Academy

Alpha ($\alpha$), Beta ($\beta$), and Gamma ($\gamma$) Decay

1. Alpha ($\alpha$) Decay

Definition: Alpha decay occurs when an unstable heavy nucleus emits an alpha particle, which is identical to a helium nucleus containing 2 protons and 2 neutrons (${}_{2}^{4}\text{He}$).

General Equation

${}_{Z}^{A}\text{X} \rightarrow {}_{Z-2}^{A-4}\text{Y} + {}_{2}^{4}\text{He}$

• Mass number ($A$) decreases by 4.
• Atomic number ($Z$) decreases by 2.

Example

Uranium-238 emits an alpha particle and transforms into Thorium-234:

${}_{92}^{238}\text{U} \rightarrow {}_{90}^{234}\text{Th} + {}_{2}^{4}\text{He}$

2. Beta ($\beta$) Decay

(i) Negative Beta ($\beta^-$) Decay

Definition: Negative beta decay occurs when a neutron converts into a proton inside the nucleus, emitting an electron ($\beta^-$ particle) and an antineutrino.

General Equation

${}_{Z}^{A}\text{X} \rightarrow {}_{Z+1}^{A}\text{Y} + {}_{-1}^{0}\text{e} + \bar{\nu}$

• Mass number remains unchanged.
• Atomic number increases by 1.

Example

Carbon-14 emits a beta particle and changes into Nitrogen-14:

${}_{6}^{14}\text{C} \rightarrow {}_{7}^{14}\text{N} + {}_{-1}^{0}\text{e} + \bar{\nu}$

(ii) Positive Beta ($\beta^+$) Decay

Definition: Positive beta decay occurs when a proton converts into a neutron inside the nucleus, emitting a positron ($\beta^+$ particle) and a neutrino.

General Equation

${}_{Z}^{A}\text{X} \rightarrow {}_{Z-1}^{A}\text{Y} + {}_{+1}^{0}\text{e} + \nu$

• Mass number remains unchanged.
• Atomic number decreases by 1.

Example

Carbon-11 emits a positive beta particle and changes into Boron-11:

${}_{6}^{11}\text{C} \rightarrow {}_{5}^{11}\text{B} + {}_{+1}^{0}\text{e} + \nu$

3. Gamma ($\gamma$) Decay

Definition: Gamma decay occurs when an excited nucleus releases excess energy in the form of a gamma ray, which is high-energy electromagnetic radiation.

General Equation

${}_{Z}^{A}\text{X}^* \rightarrow {}_{Z}^{A}\text{X} + \gamma$

• Mass number remains unchanged ($A$).
• Atomic number remains unchanged ($Z$).

Example

An excited Cobalt nucleus emits a gamma ray to transition to its stable ground state:

${}_{27}^{60}\text{Co}^* \rightarrow {}_{27}^{60}\text{Co} + \gamma$

© 2026 Talha's Physics Academy. All rights reserved.