Question

Which one of the following is the correct order of the size of the ions?
A. \[{\text{N}}{{\text{a}}^ + } > {\text{M}}{{\text{g}}^{2 + }} > {{\text{F}}^ - } > {{\text{O}}^{2 - }}\]
B. \[{{\text{O}}^{2 - }} > {{\text{F}}^ - } > {\text{N}}{{\text{a}}^ + } > {\text{M}}{{\text{g}}^{2 + }}\]
C. \[{\text{M}}{{\text{g}}^{2 + }} > {\text{N}}{{\text{a}}^ + } > {{\text{F}}^ - } > {{\text{O}}^{2 - }}\]
D. \[{{\text{O}}^{2 - }} > {{\text{F}}^ - } > {\text{M}}{{\text{g}}^{2 + }} > {\text{N}}{{\text{a}}^ + }\]

Answer 1

Hint: First of all, after reading the question carefully, compare the number of shells in each species. If they all are equal in number, compare the positive charge and negative charge. You can get the correct option.

 Complete step by step answer: The correct option is “B” and remaining options all are wrong.
Initially comparing the number of shells, all species of ions are having the same number of shells. Each species is having two shells. As all given ions are having the same number of shells, now compare according to the charge.
If positive charge increases, the size of the ion decreases. Because if positive charge is more, the effective nuclear charge is more which leads to the smaller size of the ion. If negative charge increases, the size of the ion increases. Because if negative charge is more, the effective nuclear charge is less which leads to the larger size of the ion. Now compare in the options.
Oxygen ion comes first as it is having highest negative charge when compared to the remaining ions and it is followed by fluorine ion as it is having negative charge whereas sodium and magnesium ions contain positive charge. Comparing sodium ion and magnesium ion, sodium ion is larger in size than magnesium ion. Because sodium ions contain less positive charge than magnesium ions.

Note: Do not get confused between the terms. The reason for larger size of ions which contain more negative charge is an increase in positive charge also increases effective nuclear charge. Less effective nuclear charge is favourable for more size.