For the alkali metals,which of the following increases with increasing atomic number?

Alkali metals dissolve in liquid ammonia to give a deep blue solution. This blue color is due to which of the following?

Which one of the following is an $s$-block element?

The oxide of $Cs$ is:

$KO_2 + CO_2 + H_2O \xrightarrow{\text{more } CO_2} [X] + [Y]$
Products $[X]$ and $[Y]$ are respectively

Fill in the blanks:
$(1)$ Lithium on reaction with nitrogen present in air is denoted by .....
$(2)$ Potassium gives ...... colour in oxidizing flame.
$(3)$ ....... and ........ metals are used in photoelectric cells.
$(4)$ When alkali metal dissolves in ammonia,it produces a blue-coloured solution. This colour is due to .......