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Wave Optics

Question
CBSEENPH12038611

Which state of the triply ionized Be+++ has the same orbital radius as that of the ground state of hydrogen? Compare the energies of two states.

Solution
Radius of nth orbit is given by, r = n2h24π2mKZe2 

  i.e.,   r  n2Z 

For hydrogen,  Z = 1, n = 1 in ground state. 

         n2Z = 121 = 1 

For Berylium, Z = 4, as orbital radius is same,n2Z=1 

             n2 = 1 × Z = 1× 4 = 4n = 4 = 2 

Hence n = 2.

Thus, second level of triply ionised Be+++ has same radius as the ground state of hydrogen. 

Now energy of electron in nth orbit is given by, E=-2π2mK2Z2e4n2h2 

               E Z2n2

          E(Be)EH = Z2/n2BeZ2/n2 = 16/41/1 = 4 

Therefore, energy of ionized Be is greater than the energy of hydrogen in the ground state.