Which of the following undergoes change during a chemical reaction?

a. atomic mass of elements in reactants

b. element identities of atoms in reactants

c. total number of electrons in reactants

d. chemical bonds between atoms in reactants

D. chemical bonds between reactants.

In a chemical reaction, the reactants break and establish new bonds between atoms by exchanging electrons. The overall atomic mass doesn't change. Element identities only change with a loss or gain of protons, which happen in nuclear reactions. The total number of electrons does not change, just become exchanged or shared between atoms.