The energy difference between the first two levels of hydrogen atom is $$10.2\,eV$$   for another element of atomic number 10 and mass number 20, this will be

Solution :
For $$H$$ atom
$${E_2} - {E_1} = 10.2\,eV$$
For $$Z= 10,$$
$$\eqalign{ & {E_1} = \frac{{ - 13.6 \times {{10}^2}}}{{{1^2}}} = - 1360\,eV \cr & {E_2} = \frac{{ - 13.6 \times {{10}^2}}}{{{2^2}}} = - 3.4 \times 100 = - 340\,eV \cr & \therefore {E_2} - {E_1} = - 340 - \left( { - 1360} \right) = 1020\,eV \cr} $$