Question

Identify the most stable ion from the following:
A.${\text{F}}{{\text{e}}^ + }$
B.${\text{F}}{{\text{e}}^{2 + }}$
C.${\text{F}}{{\text{e}}^{3 + }}$
D.${\text{F}}{{\text{e}}^{4 + }}$

Answer 1

We must have the knowledge of writing electronic configuration for the iron, following the required law for filling of electrons in an orbital. Most stable are those electronic configurations which are either fully filled or half filled.

Complete step by step answer:
The order of filling of electrons following Aufbau principle as follow:
$1{\text{s }}2{\text{s }}2{\text{p }}3{\text{s }}3{\text{p 4s 3d 4p}}...$
Pauli Exclusion Principle says that a maximum of two electrons can be filled in a particular orbital that too with opposite spins.
s subshell has one orbital so it can have a maximum of 2 electrons. p subshell has 3 orbital and can have a maximum of 6 electrons and d subshell, hence 5 electrons and can accommodate a maximum of 10 electrons.
Let us write electronic configuration for each of the ion:
Electronic configuration of iron ${\text{Fe}}$ is:
$1{{\text{s}}^2}2{{\text{s}}^2}2{{\text{p}}^6}3{{\text{s}}^2}3{{\text{p}}^6}3{{\text{d}}^6}4{{\text{s}}^2}$
Each positive charge is formed by removing 1 electron and hence the electronic configuration of subsequent cations is.
Electronic configuration of iron ${\text{F}}{{\text{e}}^ + }$ is:
$1{{\text{s}}^2}2{{\text{s}}^2}2{{\text{p}}^6}3{{\text{s}}^2}3{{\text{p}}^6}3{{\text{d}}^6}4{{\text{s}}^1}$
Electronic configuration of iron ${\text{F}}{{\text{e}}^{2 + }}$ is:
$1{{\text{s}}^2}2{{\text{s}}^2}2{{\text{p}}^6}3{{\text{s}}^2}3{{\text{p}}^6}3{{\text{d}}^6}4{{\text{s}}^0}$
Electronic configuration of iron ${\text{F}}{{\text{e}}^{3 + }}$ is:
$1{{\text{s}}^2}2{{\text{s}}^2}2{{\text{p}}^6}3{{\text{s}}^2}3{{\text{p}}^6}3{{\text{d}}^5}4{{\text{s}}^0}$
Electronic configuration of iron ${\text{F}}{{\text{e}}^{4 + }}$ is:
$1{{\text{s}}^2}2{{\text{s}}^2}2{{\text{p}}^6}3{{\text{s}}^2}3{{\text{p}}^6}3{{\text{d}}^4}4{{\text{s}}^0}$
d orbital can accommodate a maximum of 10 electrons in it. So if either 10 electrons are filled or 5 electrons are filled then the electronic configuration will be most stable. As we see that in the electronic configuration of ${\text{F}}{{\text{e}}^{3 + }}$d orbitals have 5 electrons. Hence most stable electronic configuration is of ${\text{F}}{{\text{e}}^{3 + }}$.

Hence, the correct option is C.

Note:
Iron is a d block element. Its atomic number is 26 with group 8 of periodic table and is metallic in nature. It is one of the most common elements on earth crust and forms many complexes due to its variable oxidation state.