The missing number is 1/10,000 (i.e. the nucleus radius is about ten thousand times smaller than the atom’s radius).

Key idea

  • A typical atomic radius is about 10−1010^{-10}10−10 m, while a typical nuclear radius is about 10−1410^{-14}10−14–10−1510^{-15}10−15 m.
  • Dividing 10−1010^{-10}10−10 m by 10−1410^{-14}10−14–10−1510^{-15}10−15 m gives a factor of about 10410^{4}104–10510^{5}105, meaning the atom is roughly ten thousand to one hundred thousand times larger in radius than its nucleus.
  • In many textbook questions, this is rounded so that:
    • “The radius of a nucleus is about 1/10,000 the radius of the atom.”

So the completed sentence is:

“The radius of a nucleus is about 1/10,000 the radius of the atom.”