Which of the following alkaline earth metal nitrides is volatile?

Which of the following elements can be represented by the electronic configuration $[Kr]\,5s^2$?

In the given reaction cycle,$X$,$Y$ and $Z$ respectively are:
$CaCl_2 + Na_2CO_3 \rightarrow X + Y$
$X \xrightarrow{Z} CaCl_2$

As the alkaline earth metals (except $Be$) tend to lose their valence electrons readily,they act as:

Plaster of Paris is hardened by

Observe the following reactions:
$Dimer \longleftarrow \text{Vapour phase } MCl_2 \xrightarrow{1200 \ K} \text{linear monomer}$.
What is $M$?