Radioactivity teaching order
Don't worry about atoms to begin with
A common approach to teaching radioactivity is to start with the idea of the nucleus, then talk about alpha, beta and gamma, then move on to half-life and finish with a few words about uses and dangers.
We take a different approach based on Radiation and Radioactivity, a scheme developed by Professor Robin Millar at the University of York.
Professor Millar argues that many students have problems coping with the details of atomic theory but that many ideas about radioactivity can be described, even if not fully explained, without having to worry about exactly where the radiation came from.
Uses and dangers come first
For example we can simply describe the different effects of alpha, beta and gamma radiation. We can also state that radioactivity decreases with time without explaining why. We use the idea of half-life as simply a short-hand way of comparing how long different sources are radioactive for.
Once we have these two ideas then the uses and dangers of radioactivity can be introduced fairly painlessly early on. For example tracers are always gamma sources (because gamma radiation can pass through things easily) with a short half-life (so they stop being radioactive fairly quickly after we've used them).
The early history of radioactivity uses mostly pre-university concepts
We introduce the atom and the nucleus using an historical approach. Many of the experiments carried out by Rutherford in particular (who did so much more than just his famous alpha scattering experiment) are similar to those carried out in classrooms around the world. Except he was doing them for the first time and had to think very hard about what they meant.
The history of atomic theory also gives a very good feel for how scientific theories are developed: the inspired guesses, the good luck, the dead ends, the meticulous experiments, the order-of-magnitude feel, the rivalries and the friendships.
Atomic concepts to explain stability and changes with time
Once we're happy with the structure of the atom then we can explain radioactivity as an atomic phenomenon that makes the nucleus of atoms more stable.
Radioactivity decreases with time because the more nuclei there are that have decayed, the fewer there are left to decay.