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

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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