Answer:
meter the mean free path of hydrogen atoms in interstellar space.Explanation:
The mean free path equation is given as:

Where"
d = diameter of hydrogen atom in meters
n = number of molecules per unit volume
We are given: d = 100 pm = 



meter the mean free path of hydrogen atoms in interstellar space.
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Gold has a very high density of about 19.32g/cm^3 while Aluminum has a low density of 2.7 gm/cm^3 which means gold can pack more amount of matter in a comparatively small space as compared to Aluminum.
Rubidium is a alkali metal.