The word atom, in Greek, means indivisible.
This meaning was generally accepted by the scientific community, that atoms were the basic particles of all matter and could not be divided.
It was also generally accepted that matter and energy were independent from each other and could not be converted between one another.
With the study of nuclear chemistry, it was found that that the protons, and neutrons that make up the nucleus could be split apart from each other and put back together.
In unstable nuclei, this happens naturally, and the nuclei are said to be radioactive.
When this happens, a lot of energy is given off.
Some particles can also be converted into energy.