The element having electronic configuration $[Kr]4d^{10} 4f^{14} 5s^2 5p^6 6s^2$ belongs to

Identify the $CORRECT$ decreasing order of melting point of cluster of sodium atoms depending on size.

When substance $A$ reacts with water,it produces a combustible gas $B$ and a solution of substance $C$ in water. When another substance $D$ reacts with solution $C$,it produces the same gas $B$,even when reacting with dilute sulfuric acid at room temperature. $A$ gives a golden yellow flame in a Bunsen burner flame. What are $A, B, C,$ and $D$ respectively?

Write balanced chemical equations for the reactions between:
$(a)$ $Na_{2}O_{2}$ and water
$(b)$ $KO_{2}$ and water
$(c)$ $Na_{2}O$ and $CO_{2}$

What is caused by an excess of $Na^+$ in the human body?

Alkali metals .........