If isotopes are chemically alike but physically different propose which subatomic particles are

All isotopes of the same element have the same number of protons. Number of neutrons in different atoms of the same element is free to vary. Since, different elements react differently, different elements have different numbers of protons. Atoms with the same number of protons and vary neutrons behave chemically similar. The number of electrons in a neutral atom depends on the number of protons. All chemical reactions involve transfer or sharing of electrons it stands to reason, neutrons are NOT responsible for chemical reactivity. It's arguable whether protons or electrons dictate chemical reactivity certainly the SUBATOMIC nuclear particle that dictates reactivity is the PROTON.

i believe the answer is 5,000