The emission spectrum of $CO_2$ gas is:

If elements with principal quantum number $n > 4$ were not allowed in nature,then the number of possible elements would be

In a $He-Ne$ laser,the metastable state exists in:

Consider the following two statements $A$ and $B$ and identify the correct choice in the given answers:
$A :$ Line spectra are due to atoms in the gaseous state.
$B :$ Band spectra are due to molecules.

In the absorption spectrum of $Na$,the missing wavelength$(s)$ are:

The band spectra (characteristic of molecular species) is due to emission of radiation from which state of matter?