Consider a hydrogen atom with its electron in the $n$th orbit. An electromagnetic radiation of wavelength 90 nm is used to ionize the atom. If the kinetic energy of the ejected electron is 10.4 eV , then the value of $n$ is ( $h c=1242 \mathrm{eV} \mathrm{nm}$ )

The mass of proton is 1.0073 u and that of neutron is $1.0087 \mathrm{u}(\mathrm{u}=$ atomic mass unit). The binding energy of ${ }_2 \mathrm{He}^4$ is

The total energy of an electron in the second excited state of hydrogen atom is about $$-1.51 \mathrm{~eV}$$. Its kinetic energy in this state is