Question: why is there radioactive substances ?

James Lees answered on 15 Mar 2020:

Radioactive substances exist because some things are unstable.

Imagine that you have a Jenga tower that’s been played with a lot. This is unstable and because of breezes or table vibrations sometimes even without touching it it can fall down.

In some atomic nuclei the energy levels are unstable and in physics things want to be stable. So randomly they can ‘collapse’ in a form which is losing some of that energy as radiation.