So,
An atom is composed of a positively-charged nucleus and negatively-charged electrons. The protons and neutrons are packed very closely in the center, called the nucleus. The electrons are usually much further away from the center.
If we imagine a basic reaction, there are two atoms coming together. Since the protons and neutrons buried deep inside the atom, they cannot "react" with other atoms. Rather, the electrons, who are the most exposed, are the ones that change in a reaction.
