Which were the major problems before the scientists after the discovery of sub-atomic particles?

(N/A) The major problems faced by scientists after the discovery of sub-atomic particles were:
$1$. To account for the stability of the atom.
$2$. To compare the behavior of one element with another in terms of both physical and chemical properties.
$3$. To explain the formation of different kinds of molecules by the combination of different atoms.
$4$. To understand the origin and nature of the characteristics of electromagnetic radiation absorbed or emitted by atoms.