Two radioactive substances A and B have decay constants 5$$\lambda $$ and $$\lambda $$ respectively. At t = 0 they have the same number of nuclei. The ratio of number of nuclei of A to those of B will be (1/e)² after a time interval

The total energy of electron in the ground state of hydrogen atom is $$-$$ 13.6 eV. The kinetic energy of an electron in the first excited state is

The radius of germanium (Ge) nuclide is measured to be twice the radius of $${}_4^9$$Be. The number of nucleons in Ge are

In a radioactive material the activity at time t₁ is R₁ and at a later time t₂, it is R₂. If the decay constant of the material is $$\lambda $$, then