which part of the atom allows one atom to react with another one?

The part of the atom that allows one atom to react with another is the valence electrons , which are the electrons in the outermost energy level (or shell) of the atom. These valence electrons are involved in forming chemical bonds by being shared, gained, or lost between atoms. The attraction between the nuclei of atoms and the valence electrons of other atoms causes them to bond together, creating molecules. This bonding can happen through sharing electrons (covalent bonds) or transferring electrons (ionic bonds), making atoms more stable by filling their outer electron shells.