OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Balance equations representing the emission of alpha, beta or gamma…
Full specification wording
Radioactive emissions: balance equations representing the emission of alpha, beta or gamma radiation in terms of the masses, and charges of the atoms involved
An emission equation is balanced when the mass numbers and the atomic (proton) numbers on both sides agree.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 7 of 12
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An emission equation is balanced when the mass numbers and the atomic (proton) numbers on both sides agree. Write the parent nuclide on the left and the daughter nuclide plus the emitted particle on the right. An alpha particle is a helium nucleus, so it carries mass number 4 and charge 2⁺; emitting it lowers the mass number by 4 and the atomic number by 2. A beta particle is an electron emitted when a neutron changes into a proton, so it carries mass number 0 and charge 1⁻; emitting it leaves the mass number unchanged and raises the atomic number by 1. A gamma photon carries mass number 0 and charge 0, so emitting it changes neither number. For example, uranium-238 emitting an alpha particle gives thorium-234: 238 = 234 + 4 and 92 = 90 + 2. Check both totals before choosing an answer.
What earns the marks
- Balances the mass numbers by making the total on the left equal the total on the right.
- Balances the atomic numbers by making the total charge on the left equal the total on the right.
- Assigns an alpha particle a mass number of 4 and a charge of 2⁺.
- Assigns a beta particle a mass number of 0 and a charge of 1⁻.
- Assigns a gamma photon a mass number of 0 and a charge of 0.
- Deduces the daughter nuclide by subtracting the emitted particle's mass number and charge from the parent's values.
Common mistakes and how to put them right
Treats a beta particle as having a mass number of 1.
Put it right by giving it mass number 0 and charge 1⁻.
Raises the atomic number when an alpha particle is emitted.
Put it right by lowering it by 2.
Changes the mass number when a gamma photon is emitted.
Put it right by leaving both numbers unchanged.
Writes ionic charges in a postfix form.
Put it right by using superscript signs such as 2⁺ and 1⁻.
Exam tips
- Write the mass-number sum and the charge sum separately beneath each side before selecting an option.
- Recall the fixed values for each particle: alpha 4 and 2⁺, beta 0 and 1⁻, gamma 0 and 0.
- Test each option by checking both numbers, since an option may balance mass but not charge.
Try the lesson
The start of the Radioactive emissions: balance equations representing the emission of alpha, beta or gamma radiation in terms of the masses, and charges of the atoms involved lesson, made from checked teaching notes for this specification point.
Have a go first
A radium-226 nucleus emits an alpha particle and becomes radon-222. What is the mass number of the alpha particle emitted?
Listen
An emission equation is balanced when the mass numbers and the atomic (proton) numbers on both sides agree. Write the parent nuclide on the left and the daughter nuclide plus the emitted particle on the right. An alpha particle is a helium nucleus, so it carries mass number 4 and charge 2⁺; emitting it lowers the mass number by 4 and the atomic number by 2. A beta particle is an electron emitted when a neutron changes into a proton, so it carries mass number 0 and charge 1⁻; emitting it leaves the mass number unchanged and raises the atomic number by 1. A gamma photon carries mass number 0 and charge 0, so emitting it changes neither number. For example, uranium-238 emitting an alpha particle gives thorium-234: 238 = 234 + 4 and 92 = 90 + 2. Check both totals before choosing an answer. A strong answer earns marks for these points. Balances the mass numbers by making the total on the left equal the total on the right. Balances the atomic numbers by making the total charge on the left equal the total on the right. Assigns an alpha particle a mass number of 4 and a charge of 2⁺. Assigns a beta particle a mass number of 0 and a charge of 1⁻. A common slip: Treats a beta particle as having a mass number of 1. Instead, giving it mass number 0 and charge 1⁻. Exam tip: Write the mass-number sum and the charge sum separately beneath each side before selecting an option.
Key words
Say what each word means before you turn the card.
Conservation of mass number
Try answering aloud or in your head before turning the card.
Conservation of charge
Try answering aloud or in your head before turning the card.
Nuclear decay equations
Try answering aloud or in your head before turning the card.
See every definition
- Conservation of mass number
- An emission equation is balanced when the mass numbers on both sides agree, making the total on the left equal the total on the right.
- Conservation of charge
- An emission equation is balanced when the atomic (proton) numbers on both sides agree, making the total charge on the left equal the total on the right.
- Nuclear decay equations
- The parent nuclide is written on the left and the daughter nuclide plus the emitted particle on the right, with both totals checked before choosing an answer.
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