amount of neutrons number of electrons size of protons and neutrons size of electrons and electron cloud
I gave my students this multiple choice question: Two forms of boron are naturally occurring, boron-10 and boron-11. What difference exists between a boron atom with a 10 atomic mass units (amu) and a boron atom with 11 atomic mass units (amu)?

Only one answer is correct among these 4 choices:
- amount of neutrons
- number of electrons
- size of protons and neutrons
- size of electrons and electron cloud

Could you tell me which one is correct?

amount of neutrons