Nuclear fission differs from nuclear fission because nuclear fusion reactions

Nuclear reactions have two main types: fission reactions and fusion reactions.  When two light nuclei are joined to create a heavier and more stable nucleus, this process is called the fusion reaction. In the fission reaction, the heavy nucleus split into two nuclei with a smaller mass number.  In other words, fission is splitting of atoms although not all atoms undergo fission, only few went through normal circumstances (that is a fact).  

C) electrons encircling a nucleus with neutrons and protons