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. $\left[Ne\right]$ $3s^2$ $3p^1$

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

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?

The correct trend of atomic size among the following is:

 1. F > N > O > C

 2. Rb > Na > K > Li

 3. Be > B > C > N

 4. Ne >He > Ar > Kr

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