The sBlock Elements - Result Question 4
####4. Which one of the alkali metals, forms only, the normal oxide, $M_2 O$ on heating in air?
(a) $Rb$
(b) $K$
(c) $Li$
(d) $Na$
[2012]
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Solution:
- (c) All the alkali metals when heated with oxygen form different types of oxides for example lithium forms lithium oxide $(Li_2 O)$, sodium forms sodium peroxide $(Na_2 O_2)$, while $K, Rb$ and $Cs$ form their respective superoxides.
$Li^{+}$being smallest, combines with small anion $O^{2-}$ to form stable $Li_2 O$.
The $Na^{+}$being relatively larger than $Li^{+}$and weaker positive field than $Li^{+}$, combines with $O_2^{2-}$ (peroxide ion) to form $Na_2, O_2 \cdot K^{+}, Rb^{+}$and $Cs^{+}$ being relative larger than $Na^{+}$and with weaker positive field than $Na^{+}$combines with $O_2^{-}$ (superoxide ion) to for $KO_2, RbO_2$ and $CsO_2$ respectively.
