The radius of hydrogen atom in its ground state is 5.3  ×  10−11  m. After collision with an electron it is found to have a radius of 21.2  ×  10−11  m. The principal quantum number of the final state of the atom is

Radius  of H - atom  in  its  ground  state,r1=5.3  ×  10−11  metresRadius  of Bohr  atom  in  its  nth  orbit,rn=5.3  ×  10−11 n2z metresFor  H−atom,    z=1  and  rn=21.2  ×  10−11  metresn=21.2  ×  10−115.3  ×  10−111/2 = 2