The average life of an excited state of hydrogen atom is of the order 10−8s .The number of revolutions made by an electron when it is in state n=2 and before it suffers a transition to state n=9  are

Velocity of electronvn=2.19×106Znms−1The distance travelled by electron in 10−8  s Second Bohr’s Orbit         =2.19×106×1×10−82m         =1.095×10−2 mThe circumference of second orbit =2π r2                  =2π×0.529×10−10×22                  =13.3×10−10m∴  Number of revolutions=1.095×10−213.3×10−10=8.23×106