Answer: Electrons
Explanation: In chemical reactions, elements and compounds change in order to form different substances. This changes can be:
1. oxidation state: which is define as the charge of the atom
2. bonding: atoms in a compound are bonded, this bonds can break so atoms rearrange to form different substances
In this changes electrons are involve. The charge depends on the amount of negative charge of electron respect to positive charge of proton in nucleus which remain the same, otherwise we would be talking about nuclear reactions.
Also, bonding depends on the electrons that atoms can give, receive or share to form ionic or covalent bonds.