Fission? Fusion?

Today, we learned a new type of energy: nuclear energy. To start this, we have been introduced two types of nuclear reaction: fission and fusion.

In short, nuclear fission is a type of reaction when a radioactive atom splits into small particles due to external source (ex. extra neutron). Nuclear fission usually releases large amount of energy, and is usually used in power generator and nuclear weapon.

Nuclear fusion is a type of reaction when multiple small particles come together and form a heavier nucleus. Depends on the mass of particles involved, nuclear fusion can release or absorb energy. It is the type of reaction that occurs in the sun.

Most of fusion reactions are completed by adding protons into the atom. For instance, two hydrogen could fuse a helium; 2 helium could fuse a Boron; 2 Boron could fuse a carbon...

Something intrigued me from today's learning is how a small interaction could cause massive amount of energy to release. I also realized how atoms that have lots of electrons tend to split into small parts, which made me to think that one should not be greedy, or you are more likely going to lose everything.

Something confused me is the relationship between mass and energy. According to Law of Conservation of Mass-Energy, mass can transform into energy, and energy into mass, such that the total mass-energy in an isolated system remains constant. How can this statement explain the vast amount of energy released through a nuclear reaction process? Also, isotopes tend to have less neutrons than an atom, does that mean isotopes are more active than atoma? If so, would the energy required for an atom to undergo nuclear reaction be the same for its isotopes?