Question: when two smaller atoms combine into a larger atom, _______ has occurred

when two atoms combine into a larger atom,  nuclear  fusion has occurred,  fusion is the coming together of two or more atoms  to form a single atom, in a fusion a new atom entirely different from the participating atoms or reactant are form, in other word we can say a new atom is crated in a nuclear fusion reaction.

nuclear fission is different from fusion, because in it, a single large atom is split into two smaller but unequal atom,  with both chemical and nuclear properties completely different from that of the larger atom.