Which of the following alkali metals has a density greater than water?

The compound which does not exist is

Select the correct statement from the following.

Choose the correct order of density of the alkali metals.

$P \xrightarrow{\Delta } Q$ (metallic solid) $+ R \uparrow + S \uparrow$
$X \xrightarrow{\Delta } Y$ (amphoteric oxide) $+ R \uparrow + S \uparrow$
$P$ and $X$ are respectively:

Ions of an element of group $1$ participate in the transmission of nerve signals and transport of sugars and amino acids into cells. This element imparts yellow colour to the flame in flame test and forms an oxide and a peroxide with oxygen. Identify the element and write chemical reaction to show the formation of its peroxide. Why does the element impart colour to the flame?