The crystal structure of $NaCl$ is:

Continuous emission spectrum is produced by

The energy required to remove one electron from a helium atom is $24.6 \, eV$. What is the energy required to remove both electrons from the helium atom (in $, eV$)?

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

The colour of the positive column in a gas discharge tube depends on

$A$ neon sign does not produce