Which of the following triads do not follow Dobereiner's law of triads? 

1.	Li, Na, K	2.	Ca, Sr, Ba
3.	Be, Mg, Ca	4.	Cu, Ag, Au

Element of the third period that is expected to exhibit positive electron gain enthalpy is: 

For the second-period elements, the correct increasing order of first ionisation enthalpy is:

Amongst the elements with the following electronic configurations, which one of them may have the highest ionisation energy?

1. $\left[Ne\right]$ $3s^2$ $3p^3$

2. $\left[Ne\right]$ $3s^2$ $3p^2$

3. $\left[Ar\right]$ $3d^{10}$ $4s^2$ $4p^3$

4. $\left[Ne\right]$ $3s^2$ $3p^1$

Among ${\mathrm{P}}_2{\mathrm{O}}_5, {\mathrm{As}}_2{\mathrm{O}}_3, {\mathrm{Sb}}_2{\mathrm{O}}_3 \mathrm{and} {\mathrm{Bi}}_2{\mathrm{O}}_3$, the most acidic oxide is:

1.  $P_2{O}_5$

2.  $A{s}_2{O}_3$

3.  $S{b}_2{O}_3$

4.  $B{i}_2{O}_3$

Which ion is isoelectronic with Be²⁺?
1. $H^{+}$
2. $L{i}^{+}$
3. $N{a}^{+}$
4. $M{g}^{2+}$

The correct order of ionic radii is:

1. $H^{-}>H^{+}>H$ $$
2. $N{a}^{+}>F^{-}>O^{2-}$
3. $F^{-}>O^{2-}>N{a}^{+}$
4. $N^{3-}>M{g}^{2+}>A{l}^{3+}$

The species Ar, K⁺ and Ca²⁺ contain the same number of electrons. In which order do their radii increase?
1.  Ar < K⁺ < Ca²⁺
2. Ca²⁺ < Ar < K⁺
3. Ca²⁺ < K⁺ < Ar
4. K⁺ < Ar < Ca²⁺

The formation of the oxide ion O^2– (g), from the oxygen atom requires first an exothermic and then an endothermic step as shown below, 
$\mathrm{O}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}\to {\mathrm{O}}^{-};$ ${∆}_{\mathrm{f}}{\mathrm{H}}^0=–141$ $\mathrm{kJ}$ ${\mathrm{mol}}^{-1}$
${\mathrm{O}}^{-}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}\to {\mathrm{O}}^{2-}\left(\mathrm{g}\right);$ ${∆}_{\mathrm{f}}{\mathrm{H}}^0=+780$ $\mathrm{kJ}$ ${\mathrm{mol}}^{-1}$

Thus, the process of formation of O^2– in the gas phase is unfavorable even though O^2– is isoelectronic with neon. It is due to the fact that: