Which one of the following pairs of nuclei are isotones ?

The total energy of an electron in the first excited state of hydrogen atom is about $$-$$ 3.4 eV. Its kinetic energy in this state is

In the reaction $${}_1^2$$H + $${}_1^3$$H $$ \to $$ $${}_2^4$$He + $${}_0^1$$n, if the binding
energies of $${}_1^2$$ H, $${}_1^3$$H and $${}_2^4$$He are respectively a, b and c (in MeV),
then the energy (in MeV) released in this reaction is

The half life of radian is about 1600 years. Of 100 g of radium existing now, 25 g will remain unchanged after