In terms of chemical bonds and electrons, what kinds of changes occur between atoms when substances undergo chemical reactions?
The modern chemical bond is conceived to be a region of high electron density between 2 positively charged nuclei, such that internuclear repulsion is minimized, and a net attractive force results.
A chemical bond is typically described as the superposition of 2 electrons, which form a region of HIGH electron density between neighbouring nuclei. Because atomic nuclei are strongly positively charged, each nucleus interacts attractively with the electron cloud, and can approach to close distances, before internuclear repulsion occurs. The mean internuclear distance is quoted as the average bond length.
Of course, between different atomic nuclei, different, stable, internuclear attractions may occur, and this gives rise to the formation of new chemical bonds (and gives rise to Chemistry in fact).