The radius of hydrogen atom in its ground state is $5.3\times {10}^{-11} \mathrm{m}$. After collision with an electron it is found to have a radius of $21.2 \times {10}^{-11} \mathrm{m}$. What is the principal quantum number $n$of the final state of the atom?

• A

  $n=4$
• Correct answer
  B

  $n=2$
• C

  $n=16$
• D

  $n=3$

Hence, the correct answer is option: 'B'.