Most of the volume of an atom is taken up by the , and most of the mass of an atom is found in the .

a. nucleus; electron clouds
b. nucleus; nucleus
c. electron clouds; nucleus

Electron cloud, nucleus.  Atoms are actually mostly empty space due to the fact that the electron cloud takes up a lot of space compared to the nucleus which is very small and dense.  The nucleus does however contain the protons and neutrons which give atoms their mass.  Electrons have such a small mass that we usually don't even take their mass into consideration saying that both protons and neutrons are 1amu in mass while electrons are 0amu in mass.

I hope this helps.  Let me know if anything is unclear.

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