Question

Which of the following does not represent the correct order of the properties indicated:
A. ${{\text{O}}^{2 - }}\, > {\text{Ne}}\,\,{\text{ > }}\,\,{\text{M}}{{\text{g}}^{2 + }}\, > \,{\text{A}}{{\text{l}}^{3 + }}$ (Size)
B. ${{\text{N}}^{3 - }}\, > {{\text{O}}^{2 - }}\, < \,{\text{F}}\,\,{\text{ < }}\,\,{\text{N}}{{\text{a}}^ + }$ (nuclear charge)
C. ${\text{Li}}\,{\text{ > Be}}\,{\text{ < }}\,{\text{B}}\,{\text{ < }}\,\,{\text{C}}$ (electron gain enthalpy)
D. ${\text{Li}}\,{\text{ < Na}}\,\,{\text{ > }}\,{\text{K}}\,\,{\text{ > }}\,\,{\text{Cs}}$ (IE1)

Answer 1

Hint: On increasing the electron number the size increases whereas on decreasing electron number size decreases. On increasing proton number, the nuclear charge increases. Left to right in the periodic table electron gain enthalpy increases and down in a group ionization energy decreases.

Complete step by step answer:
Oxygen anion, neon, magnesium ion, and aluminium ion all are of period third elements and all have the same electrons so, the size of the ions depends upon the effective nuclear charge and electron-electron repulsion only.
As the electron-electron repulsion increases the size increases so, the anion has a maximum size than cation and neutral atom and definitely, the neutral atom will have a larger size than cation.
As the positive charge increases, the effective nuclear charge increases the size decreases.
So, the decreasing order of size is, ${{\text{O}}^{2 - }}\, > {\text{Ne}}\,\,{\text{ > }}\,\,{\text{M}}{{\text{g}}^{2 + }}\, > \,{\text{A}}{{\text{l}}^{3 + }}$. So, option (A) is incorrect.
The anion of nitrogen, oxygen, fluorine, and sodium cation all are the same group elements. Positive charge means an atom is losing electrons so, after the removal of an electron, proton number increases, so the nuclear charge increases. So, cation has the highest nuclear charge than a neutral atom. In anion after adding the electron, the proton number decreases, so the nuclear charge decreases. So, the anion has less nuclear charge than a neutral atom.
So, the increasing order of nuclear charge is ${{\text{N}}^{3 - }}\, > {{\text{O}}^{2 - }}\, < \,{\text{F}}\,\,{\text{ < }}\,\,{\text{N}}{{\text{a}}^ + }$. So, option (B) is incorrect.
Lithium, beryllium, boron and carbon all are the same group elements. In a period, left to right the electron gain enthalpy increases. Beryllium has a fully-filled electronic configuration, so its electron gain enthalpy is almost zero.
So, the increasing order of electron gain enthalpy is, ${\text{Li}}\,{\text{ > Be}}\,{\text{ < }}\,{\text{B}}\,{\text{ < }}\,\,{\text{C}}$ (electron gain enthalpy). So, option (C) is incorrect.
Lithium, sodium, potassium and cesium all are the same group elements. On going down in a group, the ionization potential decreases.
The reason for decreases in ionization potential is as follows:
- Decrease in effective nuclear charge.
- Increases in size.
So, the decreasing order of electron gain enthalpy is, ${\text{Li}}\,{\text{ > Na}}\,\,{\text{ > }}\,{\text{K}}\,\,{\text{ > }}\,\,{\text{Cs}}$ so, the given order is incorrect.

So, option (D) is correct.

Note: The species having the same number of electrons are known as isoelectronic species. For the same period order of size is anion > neutral atom > cation. The nuclear charge is directly proportional to the number of protons and indirectly proportional to the number of electrons.