AQA GCSE Physics

Revision Notes

4.2.5 Beta Decay

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, downloadable IGCSE & GCSE Physics revision notes

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 new nuclei will increase its atomic number by 1 in order to maintain the overall atomic number before and after the decay
  • The following equation shows carbon-14 undergoing beta decay
    • It forms nitrogen-14 and a beta particle
    • Beta particles are written as an electron in this equation

Beta decay equation, downloadable IGCSE & GCSE Physics revision notes

The carbon nucleus emits a beta particle, causing its charge to increase. This means it changes into a new element

Worked Example

A nucleus with 11 protons and 13 neutrons undergoes beta decay. It forms magnesium, which has the element symbol Mg.

Worked example beta decay, downloadable IGCSE & GCSE Physics revision notes

Which is the correct isotope of magnesium formed during the decay?

ANSWER:  D

Step 1: Calculate the mass number of the original nucleus

    • The mass number is equal to the number of protons plus the number of neutrons
    • The original nucleus has 11 protons and 13 neutrons

11 + 13 = 24

    • The mass number of the original nucleus is 24

Step 2: Calculate the new atomic number

    • During beta decay a neutron changes into a proton and an electron
    • The electron is emitted as a beta particle
    • The neutron has an atomic number of 0 and the proton has an atomic number of 1
    • So the atomic number increases by 1

11 + 1 = 12

    • The new nucleus has an atomic number of 12

Step 3: Calculate the new mass number

    • Protons and neutrons both have a mass number of 1
    • Changing a neutron to a proton will not affect the mass number
    • The new nucleus has a mass number of 24 (the same as before)

Exam Tip

There is a second form of beta decay during which a proton changes into a neutron. This is called beta-plus decay and it is not required to know about it for your exam. Only use the information here for your GCSE.

Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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