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?

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⁺

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

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

The first ionization enthalpy of sodium (Na) is 5.1eV. Using this information, determine the value of the electron gain enthalpy of \(\mathrm{Na^+}\):

1. +10.2 eV 

2. –5.1 eV

3. –10.2 eV 

4. +2.55 eV

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

1. $C{a}^{2+}>K^{+}>S^{2-}>C{l}^{-}$

2. $C{l}^{-}>S^{2-}>C{a}^{2+}>K^{+}$

3. $S^{2-}>C{l}^{-}>K^{+}>C{a}^{2+}$

4. $K^{+}>C{a}^{2+}>C{l}^{-}>S^{2-}$

The correct order of increasing electron affinity for the elements, O, S, F and Cl is:

Among the elements Ca, Mg, P and Cl, the correct order of increasing atomic radii is:
1. Cl < P < Mg < Ca
2. P < Cl < Ca < Mg
3. Ca < Mg < P < Cl
4. Mg < Ca < Cl < P

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. $\left[Ne\right]$ $3s^2$ $3p^1$