Question

A s-block element (M) reacts with oxygen to form an oxide of the formula $M{O}_2$. The oxide is pale yellow in colour and paramagnetic. The element (M) is

• A. K
• B. Na
• C. Mg
• D. Ca

Answer 1

The correct option is A K

Reactivity towards air:
Alkali metals tarnish in dry air due to the formation of their oxides which in turn react with moisture
to form hydroxides.
They burn vigorously in oxygen forming oxides:
Lithium monoxide
$4Li+O_2⟶2L{i}_{2}O$
Sodium peroxide
$2Na+O_2⟶N{a}_2{O}_2$
Other superoxide:
$M+O_2⟶M{O}_2$
Where M = K, Rb, Cs
The superoxide $O_2^{-}$ ion is stable only in the presence of larger cations such as K, Rb, Cs.

Be and Mg are inert to oxygen due to the formation of an oxide film on their surface.
Calcium, strontium and barium are readily attacked by air to form oxides and nitrides.