The spin quantum numbers $+1/2$ and $-1/2$ indicate that...

$A$ hydrogen atom emits a photon corresponding to an electron transition from $n = 5$ to $n = 1$. The recoil speed of the hydrogen atom is almost......$m/s$ (mass of proton $\approx 1.6 \times 10^{-27} \ kg$).

The crystal structure of $NaCl$ is:

$A$ beam of ultraviolet light of all wavelengths passes through hydrogen gas at room temperature,in the $x-$ direction. Assume that all photons emitted due to electron transition inside the gas emerge in the $y-$ direction. Let $A$ and $B$ denote the lights emerging from the gas in the $x$ and $y$ directions respectively.

The emission spectrum of $CO_2$ gas is:

If in nature there may not be an element for which the principal quantum number $n > 4,$ then the total possible number of elements will be