$***$ Challenge Problem. Compounds A and $B$ are inorganic salts with the same elemental composition; they contain metal $x,$ nonmetal $Y,$ and oxygen. An equimolar mixture of A and $B$ has a mass of $5\mathrm{.}09g.$ The following information is available for these compounds:

A and $B$ contain the same oxoanion.

In the given mixture, A contains $2\mathrm{.}41{10}^{22}$ oxygen atoms.

Molar ratio of metal $x$ in $A$ to $x$ in $B$ is $1$:$3.$

There is twice as many atoms $Y$ in $B$ than in $A.$

In B$,$ mass ratio of $x$:$Y=2\mathrm{.}71.$

B contains $8$ oxygen atoms per formula unit.

Identify compounds A and $B.$

What is the unique physical property of metal $x?$ Name all other metals that have the same property.

Nonmetal $Y$ forms several allotropes, one of which is stored under water. Why?