GCSE Physics Modelling nuclear equations
Instructions and answers for teachers These instructions should accompany the OCR resource ‘Modelling nuclear equations’ activity which supports OCR GCSE Physics.
The Activity:
This resource comprises of 2 tasks. Understanding what happens when a radioactive nucleus undergoes decay is the key to grasping nuclear equations. This activity is designed to build on and develop understanding about the atom and radioactive decay, as well as explaining the concept of nuclear equations. This activity involves using role play to model radioactive decay and also includes card matching activities with extension tasks.
This activity offers an This activity offers an opportunity for English opportunity for maths
skills development. skills development. Associated materials:
‘Modelling nuclear equations’ Lesson Element learner activity sheet and cards.
Testing prior knowledge: what is in a nucleus?
Introduce the activity by determining prior knowledge of atomic structure through questioning – an example could be to ask learners to recall what an atom of helium contains and how it is commonly written by scientists. It is important to ensure that learners understand:
• what is not found in the nucleus,
• the charges of protons, electrons and neutrons,
• what is meant by the terms atomic number and mass number,
• how we can describe the nucleus using chemical shorthand.
Activity
To determine prior knowledge and correct any misconceptions, the learners should be asked to model what they think an atom of helium looks like. Use 8 learners for this activity as it is important that the ‘single’ neutron ‘cards’ are held jointly by two learners. Explain that the reason for this will become clear when beta decay is considered.
The polonium-215 nucleus
Explain that the class will be examining the radioactive decay of polonium-215.
Write down the chemical notation, and discuss how to model this nucleus which contains 84 protons and 131 neutrons. Again, it is important that the ‘single’ neutron ‘cards’ are held jointly by two learners.
What happens when a radioactive nucleus emits an alpha particle?
Using questioning revise alpha emission (uses and dangers), what alpha is (helium nucleus) and how it differs from the helium atom they modelled earlier. Guide the class to the question of how to model an alpha particle. Next, ask how the learners in the nucleus could demonstrate alpha emission.
Discuss how the nucleus has changed and whether it is still a polonium-215 nucleus. Once the conclusion is made that it is now a nucleus of lead–211, write this down along with the representation of the alpha particle.
Steer the class to the conclusion that one nucleus of polonium-215 has decayed to produce one nucleus of lead-211 and an alpha particle and complete the nuclear equation.
Ask the learners to ascertain if any particle has been destroyed during this process. Using the learner model prove that the new total number of protons and neutrons is identical to the initial number. Explain that this is important and that mass numbers and atomic numbers on both sides of a nuclear equation must always be equal.
At this point, understanding can be consolidated using written exercises. Learners can either make up their own or use the alpha decay equation cards.
What happens when a radioactive nucleus emits a beta particle?
Most learners are familiar with the fact that beta decay involves the emission of an electron, but become confused when they realise that this emission is from the nucleus. Using a learner model can help overcome some of this confusion.
From prior knowledge learners should recap their knowledge of the properties, uses and danger of beta decay. It is worth ensuring that learners understand the shorthand notation that represents a beta particle. Particularly, important is the representation of mass and charge.
New ‘particle’ volunteers, with directions from other learners in the class are to now try and ‘construct’ another polonium-215 nucleus. However, this time each of the pairs of learners holding the single neutron cards are to be given an envelope. Unbeknown to the learners, two of the envelopes contain nothing; the third contains two cards - a single proton and a single electron.
Once the envelopes are opened, discuss with learners what this might mean. Guide the discussion to the conclusion that the neutron which has no charge, has split into a positively charged proton and a negatively charged electron. With the learners who have the two new cards relieved of their single neutron card and left standing independently holding respective electron and proton cards, ask the class if there is anything wrong with this scenario. It may be necessary to remind learners that they are looking at the contents of the nucleus of an atom. Once this has been realised, the learner representing the electron is ‘ejected’ from the nucleus.
As with the alpha emission, discuss how the nucleus has changed and whether it is still a polonium-215 nucleus. Once a class have arrived at the conclusion that it is now a nucleus of astatine-215, write this down along with the representation of the beta particle and complete the nuclear equation. If any learners question the imbalance of charge in the equation, you may want to mention that a particle called an antineutrino which is also charged is also released – this is KS5 material.
At this point remind learners that just as with alpha decay equations, mass numbers and atomic numbers on both sides of a nuclear equation must always be equal.
Consolidate understanding in the same way as for alpha decay, either by writing equations or use of card matching activity.
What happens when a radioactive nucleus emits gamma radiation?
Begin with a revision of the nature of gamma radiation. Followed by asking learners how they would represent gamma radiation in shorthand form. Discussion should lead to the conclusion that gamma radiation has no mass and no charge and has the symbol .
Learners can then be challenged as to how the emission of gamma radiation would affect the nucleus of an atom. Consolidate understanding by asking how gamma decay could be illustrated using the learner model.
Note that at this level it is sufficient for learners to understand that gamma radiation does not affect the atomic or mass number of a nucleus. Therefore, it is not usually included in nuclear equations.
Alpha and beta decay learner sheets
These are essentially card matching activities designed to assess learners’ understanding of how to construct nuclear equations. They can be completed individually, in pairs or larger groups; as a race or completed within a set time. Assessment can be formal via the board or peer assessed by encouraging learners to evaluate each other’s work.
Extension activity sheet
This contains two extension activities:
1. This activity involves working out the decay chain for thorium-232. Then laying out the decay equation cards in the correct order. It works well as a challenging activity for groups, although it could be carried out on an individual basis. 2. The second extension activity builds on the first, looking at the decay chain for uranium-235 and challenging learners to work out how it may decay to form a stable element..
Equipment/preparation required:
Designate an area of the classroom to be the nucleus
A4 size coloured sheets/cards labelled as follows:
4 green cards labelled ‘neutron’ or ‘n’
1 green card labelled ‘128 neutrons’ or ‘128n’
3 red cards labelled ‘proton’ or ‘p’
2 red card labelled ’82 proton’ or ‘82p’
2 blue cards labelled ‘electron’ or ‘e’
3 large envelopes
Access to periodic tables alpha/beta decay nuclear equation sheets and thorium-232 cards as required
Task 1 Alpha decay equations
The following five isotopes will undergo radioactive decay by emitting an alpha particle. Cut out and use the cards below to complete the nuclear equations for all five isotopes.
232 210 Th → 224Ra → 221Fr → 209Bi → Po → 1. 90 2. 88 3. 87 4. 83 5. 84
Task 2 Beta decay equations
The following five isotopes will undergo radioactive decay by emitting a beta particle. Cut out and use the cards below to complete the nuclear equations for all five isotopes.
1. 2. 3. 4. 5.
Extension activities
Task 1 Thorium-232 decay chain
Most radioactive nuclei undergo a series of radioactive decay transformations before finally becoming a stable nucleus. Such a series of transformations is called a decay chain. Thorium-232 is just radioactive element that experiences several before becoming a stable nucleus.
Below is the incomplete decay chain for thorium-232. Using your knowledge of alpha and beta decay and the set of cards you have been given, complete this decay chain identifying the final and stable isotope.
Task 2 Uranium – 235 decay chain
The isotope 235U decays into another element, emitting an alpha particle. What is the element? This element decays, and the next, and so on until a stable element is reached. The complete list of particles emitted in this chain is:
What is the stable element X? You could write down each element in the series, but there is a quicker way, can you work it out?
Answers to Learner activities
Alpha decay equations
232 228 4 1. 90Th→+88 Ra 2He
224 220 4 2. 88Ra→+86 Rn 2 He
221 217 4 3. 87Fr→+85 At 2 He
209 205 4 4. 83Bi→+81 Tl 2 He
210 206 4 5. 84Po→+82 Pb 2 He
Beta emission equations
1.
206 206 0 2. 80Hg→ 81 Tl+− 1 e
210 210 0 3. 83Bi→ 84 Po+− 1 e
209 209 0 4. 82Pb→ 83 Bi+− 1 e
14 14 0 5. 6C → 7N+ -1 e
Answers to Extension activities:
Thorium -232
Uranium-235
4 α 0 The complete decay chain involves the loss of seven alpha particles ( 2 ) and four beta particles ( −1e ). This represents a loss of 7 × 4, ie 28, in mass number and (7 × 2 – 4), ie 10, in atomic number. X is therefore an isotope with mass number (235 – 28), ie 207, and atomic number (92 – 10), ie 82. This is
208 lead, 82 Pb .
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