How I sequence: radioactivity
This blog is part of the Chat Sequencing project. You can find out more about the project here.
Atomic structure is the fourth topic in the AQA GCSE for Science/Physics. It feels like quite a standalone bit of the GCSE with students arriving with a lot of misconceptions from TV and popular culture. As a chemist, this topic has always felt quite disjointed and messy so I decided I needed to upskill with CPD and identifying how others set out this scheme.
I used to teach in the order set out in the specification but have since made some adjustments to help students in terms of cognitive load and a more natural progression of ideas. I have put the specification order on the left and the order I teach on the right.
The following is my curriculum narrative for the changes I have made:
Structure of the atom
This is common content with Chemistry so should already be familiar to students. However, recapping this content in detail allows me to double-check this knowledge is secure. In addition to making pupils aware that electron shells can also be referred to as energy levels as this will be useful when we look at gamma radiation later on.
Development of the atomic model
Again, this is common content with chemistry so allows for students to feel a sense of achievement early doors in the topic. But I do ensure I go deep on the Rutherford scattering experiment as the alpha particles are going to pop up again. In my more discovery learning days, I would have had this as the first lesson with students doing a carousel activity but this is no longer my style.
Mass number and atomic number
Fluency with the periodic table will prove useful throughout this topic in addition to a deep understanding of what the mass number and atomic number are actually telling us about the structure of an atom.
Changing the number of protons/electrons/neutrons
Now that students have a clear understanding of atomic structure, mass number and atomic number, I spend a lot of time building to three core facts: changing the number of protons = the atom becomes a different element, changing the number of electrons = the atom becomes an ion,
changing the number of neutrons = the atom becomes an isotope.
This is a threshold concept, if students are not understanding this then the rest of the topic feels pretty inaccessible and much more challenging. By the end of this section, I would expect students to explain/draw what happens when an atom loses or gains any sub-atomic particle. This always takes multiple lessons but pays dividends for this topic and also their base chemistry knowledge.
Stable and Unstable Isotopes
Now that there is a solid foundation of what an isotope is and is not, we can add the next layer. As explored by many others including Ruth Ashbee and Joe Rowing, the term “radioactive” on its own is not especially useful in this topic so I have opted to not use it as it is too nebulous and instead stick to explaining that some isotopes are stable and so will not change whereas some are unstable. These unstable isotopes undergo radioactive decay to become stable, and during this process, radiation is emitted. The key concept I want students to understand is the difference between the process of radioactive decay vs radiation being the waves or particles that are emitted.
At this stage, it is not important what the radiation is (although some students do have educated guesses) but why it is being emitted.
Detecting radiation
The random nature of radioactive decay can now be introduced as there is an understanding of what radioactive decay is. Students are also aware of the radius of atoms and so can understand that we cannot see when atoms decay and thus we measure the byproduct which is radiation being emitted. Again it doesn't yet matter what type of radiation is being emitted but that we can measure these emissions in the form of a count-rate.
Contamination and Irradiation
At this stage, the specification introduces the different types of radiation. However, I feel this then overcomplicates the remainder of the topic and isn’t necessary to understand contamination, irradiation, background radiation and half-life. As they can all be explained using “radiation” as a general term of emissions during radioactive decay. Therefore I next explain contamination and irradiation using terms already introduced: unstable isotopes and radiation.
Background Radiation (Triple)
Triple students then cover background radiation and the different sources this comes from. This slots in nicely after contamination and irradiation due to the many hinterland examples that can be included such as Chernobyl, space travel and the banana equivalent dose. As well as some retrieval on measuring radiation levels.
Half-Life (including Triple content)
For combined students, I have previously taught this at the end of the topic but students seem to find it cognitively overloading and conflate the properties of different types of radiation to half-life. Therefore I now teach it earlier as students have all the underpinning knowledge: that unstable isotopes decay, that radioactive decay is random and that we can measure the radiation emitted as a proxy for decay.
The section of the specification shown below is the triple only content. I will cover the first part in this section for all students as it is not a complex idea that different unstable isotopes will have different half-lives. The part on hazards I cover towards the end of the topic.
I introduce the idea of half-life pictorially, then numerically, graphically and finally as a ratio of stable:unstable atoms. Students find this idea quite abstract and therefore tricky; confidence can take several lessons to develop.
Types of nuclear radiation
I have previously taught this section much earlier in the course and as a very knowledge recall heavy section. I have found that teaching this later in the topic allows students to have a greater understanding, instead of rote learning the four types of radiation and very little else about the topic. Another tweak is linking the properties to the structure of the radiation. E.g. Beta particles can travel further in air than alpha particles because they are much smaller. And because they are much smaller they are less likely to collide with other atoms so will be less ionising.
Nuclear equations
The strong foundation from the beginning of the topic will payout here as students will understand what the mass number and atomic number represent. As well as being clear on why some types of radiation cause an atom to become a different element but some do not. This also builds on the model of unstable isotopes becoming stable through emitting radiation as shown below.
Hazards of radiation
I have often taught the hazards of radiation when discussing the types of radiation or when explaining how we measure the radiation being emitted. But I now teach this at the end of the topic bringing the following strands together:
Type of radiation emitted- range in air, penetration power, ionising power.
Contamination or irradiation
The half-life of the isotope- as a shorter half-life = a likelihood of a higher dose.
In addition to proximity, safety measures and time exposed.
This is the final lesson for combined students so is a nice way to recap a lot of the content without being a classic “revision lesson”.
Nuclear fission and fusion (Triple)
I introduce nuclear fission as another way an unstable atom can become stable, especially for larger atoms. Before going into the detail (see the diagram at the top) and applications. Followed up with the same for nuclear fusion. This final bit doesn't feel like it fits in anywhere in this topic so I currently just have it as an add-on, on the end. Which doesn’t feel very satisfying!
I’d love to hear what you think of this sequence and the narrative behind it.
The booklet I use that goes with this topic can be found here.
