Chemical reactions only trade or share the electrons on the outside of atoms. Nuclear reactions alter the deep center of the atom instead. This deep change can transform one element into a completely different element.
The strong nuclear force mostly controls these heavy changes. The weak nuclear force steps in for specific processes like beta decay. These reactions release or absorb energy according to the famous E=mc2 equation.
This rule means a tiny amount of mass becomes a massive amount of energy. Nuclear reactions happen naturally all the time through simple radioactive decay. Scientists can also trigger these reactions on purpose inside controlled laboratories.
The mass number counts the total protons and neutrons in a nucleus. The atomic number counts just the protons in that same nucleus. Both of these numbers remain perfectly conserved from start to finish.
Fission and fusion are the most energetic types of nuclear reactions. Fission happens when a heavy nucleus splits into smaller pieces. Fusion occurs when two light nuclei combine into a much heavier one.
Fission provides the massive energy used in modern nuclear power plants. Fusion acts as the intense energy source for the sun and stars. A common student mistake is thinking these reactions break basic conservation laws. The mass changes slightly to create energy, but the total particles balance out perfectly.