When $Mg$ burns in air,it produces

Why are alkali metals not found in nature?

$A$ metal $M$ reacts with nitrogen gas to afford $M_3N$. $M_3N$ on heating at high temperature gives back $M$ and on reaction with water produces a gas $B$. Gas $B$ reacts with aqueous solution of $CuSO_4$ to form a deep blue compound. $M$ and $B$ respectively are

The metal that is extracted from sea water is

The dark red colour of bombs in fireworks is due to the presence of:

How many of the following metals give oxides and nitrides when burnt in air?
$Be, Na, Mg, Ba, Sr, Li, K$