Answer: The neutron
Explanation:
An atom consists of 3 parts, proton, electron and neutron. The proton and neutron occupy the nucleus of the atom while the electron evolve around it.
The number of proton determines the atomic number of the element while the number of proton and neutron determines the mass number of the element.
The electron is what takes part in chemical reaction and not the proton and neutron.
Therefore, the number of proton and neutron always remain the same before and after chemical reactions.
Isotopes involves same element with same proton number but different mass numbers. The difference in the number of neutron is what is responsible for the differences in the mass number.
