OK, so far I understand. As I keep delving, and thank you for your simple, non-insulting answers, I looked up what it means to split an atom. And amazingly (yes, it amazes me) -- here's what the definition is, in part:
"Nuclear fission
An extremely complex nuclear reaction representing a cataclysmic division of an atomic nucleus into two nuclei of comparable mass. This rearrangement or division of a heavy nucleus may take place naturally (spontaneous fission) or under bombardment with neutrons, charged particles, gamma rays, or other carriers of energy (induced fission)" Splitting the atom | Article about Splitting the atom by The Free Dictionary
(My reaction -- wow.)
My conclusion so far is concordant with "an extremely complex nuclear reaction..." And no wonder. Yes, no wonder it is termed as "extremely complex."
There are two primary types of nuclear reactions: fission and fusion.
I'm not sure that the reactions themselves are that complex. The *conditions* for the reaction can be problematic to produce, though. The main reason is that the nuclei of the atoms are shielded by the electrons surrounding them.
Fission is where a large nucleus is split into two (sometimes more) smaller nuclei. Fusion is where two small nuclei merge together to form a larger one. To make a large nucleus (like that of uranium) fission, there has to be a source of neutrons directed to that nucleus to make it (more) unstable. It will then split to form a variety of possible smaller nuclei. This is the basis of the atomic bomb.
Fusion has to be at very high temperatures and pressures to get through the shielding effects of the electrons. Stars operate on fusion (not fission): the hydrogen nuclei merge to form larger nuclei (like helium, lithium, boron, carbon, oxygen, etc). This is possible because it is typically millions of degrees in the core of a star and the pressure is also very high. Even at those temperatures and pressures, though, the reactions tend to be very slow.
One of the challenges is to produce fusion reactions here on Earth. Because we want them to happen fairly quickly and because we cannot get the high pressures like in the cores of stars, this requires *very* high temperatures (hundreds of millions of degrees to billions of degrees). To produce such temperatures on Earth is complicated by the fact that we don't want to vaporize the labs surrounding the reactors. Fusion is also used in a hydrogen bomb, but needs a fission bomb as a 'trigger'. So that is a fairly complicated type of thing as well.
The nuclear reactions here are *very* different than chemical reactions. Chemical reactions do not involve the nucleus of the atoms at all and the atoms do not change the type of element they are. In nuclear reactions, the type of atom can change.