A s-block element (M) reacts with oxygen to form an oxide of the formula \(\mathrm{MO}_{2}\). The oxide is pale yellow in colour and paramagnetic. The element ( M ) is:

JEE Main · 2021 · 20 Jul 2021 · Shift 1 · s-Block Elements

A s-block element (M) reacts with oxygen to form an oxide of the formula

MO_{2}

. The oxide is pale yellow in colour and paramagnetic. The element ( M ) is:

(A)Na
(B)K✓ Correct
(C)Mg
(D)Ca

Answer: (B)

Solution

An oxide of formula

M O_{2}

from a monovalent s-block metal is a superoxide, built from the

O_{2}^{-}

ion, which holds one unpaired electron in a

π^{*}

orbital and is therefore paramagnetic and coloured. Only the heavier alkali metals form superoxides on burning in air, because their large cations of low charge density stabilise the bulky

O_{2}^{-}

ion; potassium gives the pale yellow

K O_{2}

. Sodium burns to the diamagnetic peroxide

N a_{2} O_{2}

, while magnesium and calcium, with small divalent cations, give the normal oxides MgO and CaO. The element M is potassium.

JEE Main · 2021 · 20 Jul 2021 · Shift 1 · Single correct option

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JEE Main · 2021 · 20 Jul 2021 · Shift 1 · s-Block Elements

Solution

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