The energy of an electron in the first orbit of $He^+$ is $-871.6 \times 10^{-20} \ J$. The energy of an electron in the first orbit of hydrogen is ...... .

The figure shows six spectral lines in an emission spectrum arising from energy level transitions. Which of the following spectral lines does not appear in the absorption spectrum?

The statements
$i$. In filling a group of orbitals of equal energy,it is energetically preferable to assign electrons to empty orbitals rather than pair them into a particular orbital.
$ii$. When two electrons are placed in two different orbitals,energy is lower if the spins are parallel.
Are valid for

The surface of sodium metal is irradiated with radiation of wavelength $x$ nm. The kinetic energy of ejected electrons is $2.8 \times 10^{-20}$ $J$. The work function of sodium is $2.3$ eV. The value of $x$ is . . . . . . $\times 10^2$ nm.

For a hydrogen atom,the energy of an electron in the first excited state is $-3.4 \ eV$. The kinetic energy $(KE)$ of the same electron in the hydrogen atom is $x \ eV$. The value of $x$ is . . . . . . $\times 10^{-1} \ eV$. (Nearest integer)

Circumference of the first orbit of hydrogen atom is given by the formula :