Which of the following third-period elements has a positive electron gain enthalpy?

1.	Na	2.	Al
3.	Cl	4.	Ar

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

Among the following oxides: \(\mathrm{P_2O_5}​, \mathrm{As_2O_3}​, \mathrm{Sb_2O_3}, and~ \mathrm{Bi_2O_3}\)​, which one exhibits the highest acidic character?

1.  $P_2{O}_5$

2.  $A{s}_2{O}_3$

3.  $S{b}_2{O}_3$

4.  $B{i}_2{O}_3$

The most non-metallic element among the given elements 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. $\left[Ne\right]$ $3s^2$ $3p^2$

2. $\left[Ne\right]$ $3s^2$ $3p^3$

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

4. $\left[Ne\right]$ $3s^2$ $3p^1$

Which of the following ionic species is isoelectronic with \(\mathrm{Be^{2+}}\)?

Which of the following represents the correct order of ionic radii?

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, O²⁻(g), from an oxygen atom occurs in two successive steps:

O(g) + e⁻ → O⁻(g)  ΔH = −141 kJ mol⁻¹

O⁻(g) + e⁻ → O²⁻(g)  ΔH = +780 kJ mol⁻¹

Although O²⁻ is isoelectronic with neon, its formation in the gaseous state is energetically unfavourable. What is the main reason for this?