What kinds of changes occur during chemical reactions, and how do these changes affect the atoms and masses of the reactants and products?

changes that happen during a chemical reaction:

Change of odor. Change of color (for example, silver to reddish-brown when iron rusts). Change in temperature or energy, such as the production (exothermic) or loss (endothermic) of heat.

how they affect:

In a chemical reaction, reactants contact each other, bonds between atoms in the reactants are broken, and atoms rearrange and form new bonds to make the products.