The formation of the oxide ion O^2-(g), from oxygen atom requires first an exothermic and

then an endothermic step as shown below,

$\mathrm{O}\left(\mathrm{g}\right)<mspace\; width="1em"></mspace>+<mspace\; width="1em"></mspace>\mathrm{e}-<mspace\; width="1em"></mspace>\stackrel{<mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace>}{\to }<mspace\; width="1em"></mspace>{\mathrm{O}}^{-}\left(\mathrm{g}\right);<mspace\; width="1em"></mspace>{∆}_{\mathrm{f}}\mathrm{H}°<mspace\; width="1em"></mspace>=<mspace\; width="1em"></mspace>-141{\mathrm{kJmol}}^{-1}$
${\mathrm{O}}^{-}\left(\mathrm{g}\right)<mspace\; width="1em"></mspace>+\hspace{0.17em}{\mathrm{e}}^{-}<mspace\; width="1em"></mspace>\stackrel{<mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace><mspace\; width="1em"></mspace>}{\to }{\mathrm{O}}^{2-}\left(\mathrm{g}\right);<mspace\; width="1em"></mspace>{∆}_{\mathrm{f}}\mathrm{H}°<mspace\; width="1em"></mspace>=<mspace\; width="1em"></mspace>+780<mspace\; width="1em"></mspace>\mathrm{kJ}<mspace\; width="1em"></mspace>{\mathrm{mol}}^{-1}$

Thus, process of formation of O^2- in gas phase is unfavourable even though O^2- is

isoelectronic with neon. It is due to the fact that

1. electron repulsion outweighs the stability gained by achieving noble gas configuration

2. O^- ion has comparatively smaller size than oxygen atom

3. Oxygen is more electronegative

4. addition of electron in oxygen result in large size of the ion

Which of the following ions is isoelectronic with Be²⁺?

1. H⁺

2. Li⁺

3. Na⁺

4. Mg²⁺

The correct order of the decreasing ionic radii among the following isoelectronic species is : 

1. Ca²⁺ > K⁺ > S^2- > Cl^-

2. Cl^- > S^2- > Ca²⁺ > K⁺

3. S^2- > Cl^- > K⁺ > Ca²⁺

4. K⁺ > Ca²⁺ > Cl^- > S^2-

The correct order of increasing electron gain enthalpy with a negative sign for the elements O, S, F, and Cl is : 

1. Cl < F < S < O                   

2. O < S < F < Cl

3. F < S < O < Cl                   

4. S < O < Cl < F

Which one of the following compounds is a peroxide ?

1. KO₂

2. BaO₂

3. MnO₂

4. NO₂

The sequence of ionic mobility in aqueous solution is

1. K⁺>Na⁺>Rb⁺>Cs⁺

2. Cs⁺>Rb⁺>K⁺>Na⁺

3. Rb⁺>K⁺>Cs⁺>Na⁺

4. Na⁺>K⁺>Rb⁺>Cs⁺

Which one of the following arrangements does not give the correct picture of the trends indicated against it?

(1) F₂ > Cl₂ > Br₂ > I₂ :Oxidising power

(2) F₂ < Cl₂ > Br₂ > I₂ : Electron gain enthalpy

(3) F₂ > Cl₂ > Br₂ > I₂ : Bond dissociation energy

(4) F₂ > Cl₂ > Br₂ > I₂: Electronegativity

Identify the correct order of the size of the following:

1.  Ca²⁺<K⁺<Ar<S^2-<Cl^-

2.  Ca²⁺<K⁺<Ar<Cl^-<S^2-

3.  Ar<Ca²⁺<K⁺<Cl^-<S^2-

4.  Ca²⁺<Ar<K⁺<Cl^-<S^2-

An element whose IUPAC name is Ununtrium (Uut) belongs to:

1. s-Block elements                       

2. p-Block elements

3. d-Block elements                              

4. Inner transition elements .