Helium is the lightest noble gas and the second most abundant element (after hydrogen) in the universe. (a) The radius of a helium atom is 3.1x10-11 m; the radius of its nucleus is 2.5x10-15 m. What fraction of the spherical atomic volume is occupied by the nucleus (V of a sphere 5 4/ 3πr3)? (b) The mass of a helium-4 atom is 6.64648x10-24 g, and each of its two electrons has a mass of 9.10939x10-28 g. What fraction of this atom’s mass is contributed by its nucleus?

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