A stockpile of nuclei during decay
Teaching Guidance for 14-16
Piles of nuclei: changing but not being removed
It's quite helpful to get students to visualise radioactive decay in terms of a
stockpile of atoms containing nuclei. The word
stockpile fosters an image of a pile of atoms. This remains constant, but the nuclei inside the atoms are transmuted, and so the atoms are transmuted. During radioactive decay, the number of atoms stays constant but the number of nuclei of any sort can vary with time. If you're focusing on a single-step decay then the stockpile of nuclei you start off with will gradually get depleted (stock levels of that kind of nucleus go down: stock levels of all kinds of nuclei are constant).
Some things (the numbers of particular kinds of nuclei) change; some stay the same (the total number of nuclei). Apples do change to oranges, but here the number of pieces of fruit stay the same.
This is, of course, very different from fission and fusion, where nuclei split or join. The number of atoms will therefore change.