That interesting topic is...
...Magic numbers.
The numbers that determine fissionable material have some interesting qualities and a whole thread could be devoted to the significance. In the old days 3 and 7 were considered magic but the modern magic numbers in atoms relate to the spacing of particles in the nucleus.
An interesting thing I was shown, way back when, is to take a penny and place as many pennys as you can around it so that they touch the center penny.
When you add them up you'll see why seven was considered lucky and significant..and this can serve as an example to see also how certain arrangements and numbers in atoms are also significant to their properties.
SDAI-Tech1