Which alkali metals are useful as electrodes in photoelectric cells?

Which metal has a greater tendency to form metal oxide?

Fill in the blanks:
$(1)$ $Li-Mg$ alloy is used to prepare ......
$(2)$ Colors of superoxides of alkali metals are ...... or ......
$(3)$ On combustion of lithium in the presence of air,it produces ...... and ......
$(4)$ On heating,lithium nitrate converts into ......

$X$ and $Y$ are two metals. When burnt in air,$X$ forms only oxide while $Y$ forms oxide and nitride. The metals $X$ and $Y$ may be

Beryllium is placed above magnesium in the second group. Beryllium dust,therefore,when added to $MgCl_2$ solution will:

$A$ solution of sodium in liquid ammonia is blue in colour due to