What takes up the most space in an atom, the nucleus it the electron cloud?

Nucleus. Electrons have practically no mass compared to protons and neutrons.

The basic structure of any atom includes a nucleus made of protons and neutrons and a series of tiny electrons which orbit around it in a space called the electron cloud. ... The mass of an atom's nucleus is split nearly evenly between the protons and neutrons, though neutrons are slightly larger.

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