Decay Equations (Edexcel IGCSE Physics (Modular))

Alpha, beta, gamma & neutron emission

Alpha decay

• During alpha decay, an alpha particle is emitted from an unstable nucleus

• A completely new element is formed in the process

Alpha decay usually happens in large unstable nuclei, causing the overall mass and charge of the nucleus to decrease

• An alpha particle is a helium nucleus

  • It is made of 2 protons and 2 neutrons

• When the alpha particle is emitted from the unstable nucleus, the mass number and atomic number of the nucleus changes

  • The mass number decreases by 4

  • The atomic number decreases by 2

• Alpha decay can be represented by the following nuclear equation:

• Where:

  • is the initial element X with mass number A and atomic number Z

  • is the new element Y

  • is an alpha particle

Beta decay

• During beta decay, a neutron changes into a proton and an electron

  • The electron is emitted and the proton remains in the nuclei

• A completely new element is formed because the atomic number changes

Beta decay often happens in unstable nuclei that have too many neutrons. The mass number stays the same, but the atomic number increases by one

• A beta particle is a high-speed electron

• It has a mass number of 0

  • This is because the electron has a negligible mass, compared to neutrons and protons

• Therefore, the mass number of the decaying nuclei remains the same

• Electrons have an atomic number of -1

  • This means that the atomic number of the new nucleus will increase by 1 to balance the overall atomic number before and after the decay

• Beta decay can be represented by the following nuclear equation:

• Where:

  • is the initial element X with mass number A and atomic number Z

  • is the new element Y 

  • is a beta particle

Gamma decay

• During gamma decay, a gamma ray is emitted from an unstable nucleus

• The process that makes the nucleus less energetic but does not change its structure

Gamma decay does not affect the mass number or the atomic number of the radioactive nucleus, but it does reduce the energy of the nucleus

• The gamma ray that is emitted has a lot of energy, but no mass or charge

• Gamma decay can be represented by the following nuclear equation:

• Where:

  • is the element X with mass number A and atomic number Z

  • is a gamma ray

• Notice that the mass number and atomic number of the unstable nucleus remains the same during the decay

Neutron emission

• A small number of isotopes can decay by emitting neutrons

• When a nucleus emits a neutron:

  • The atomic number (number of protons) does not change

  • The mass number (total number of nucleons) decreases by 1

• Neutron emission can be represented by the following nuclear equation:

• Where:

  • is the element X with mass number A and atomic number Z

  • is a neutron

• Notice that the atomic number remains the same during the decay but the mass number has changed

  • This means an isotope of the original element has formed

Decay equations

• Radioactive decay events can be shown using nuclear decay equations

• A decay equation is similar to a chemical reaction equation as

  • the particles present before the decay are shown before the arrow

  • the particles produced in the decay are shown after the arrow

• In a decay equation:

  • the sum of the mass numbers before and after the reaction must be the same

  • the sum of the atomic numbers before and after the reaction must be the same

• The following decay equation shows polonium-212 undergoing alpha decay

• When a nucleus of polonium-212 decays, a nucleus of lead-208 forms and an alpha particle is emitted

• To check if the equation is balanced:

  • mass number:  212 = 208 + 4 

  • atomic number:  84 = 82 + 2

• The sum of the numbers are the same on each side, so the equation is balanced