Decay Equations (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note
Exam code: 8465
Written by: Ashika
Updated on
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?
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)
Examiner Tips and Tricks
There is a second form of beta decay during which a proton changes into a neutron. This is called beta-plus decay - you might come across it while revising, but you don't need to know about it for your exam. Only use the information here for your GCSE.
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
Here is an example of Uranium-238 undergoing gamma decay
Notice that the mass number and atomic number of the unstable nuclei remains the same during the decay
Although the Uranium nucleus is unchanged in structure, its energy reduces during gamma decay
Neutron Emission
Some unstable nuclei may also emit a neutron during decay
Neutron emission is a separate process from gamma decay
Emitting a neutron reduces the mass number by 1 but does not change the atomic number
Examiner Tips and Tricks
Gamma decay equations are the easiest to construct because there is no change to the element or the mass number. The only change is that energy is released and we don't show that in the equation. However, gamma decay usually happens at the same time as alpha and beta decay, so watch out for that in exam questions.
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