$A$ metal when left exposed to the atmosphere for some time becomes coated with green basic carbonate. The metal in question is

Select the correct statement.

When compared with alkaline earth metals,the alkali metals have

Fill in the blanks:
$(1)$ Among all alkali metals,.......... is a radioactive element.
$(2)$ The half-life of $^{223}Fr$ is ..........
$(3)$ Among all alkali metal ions,............ has the highest ionization enthalpy.
$(4)$ Alkali metals are identified by the ........... test.

Which of the following is Indian saltpetre?

The compound which does not exist is