Question

In iron atoms$\left( {Z = 26} \right)$, the differentiating electron enters sublevel.
A) $4d$
B) $3d$
C) $4p$
D) $5p$

Answer 1

We know that the Iron is a transition metal and it exhibits more than one oxidation state. The most common oxidation states of iron are$+ 2$,$+ 3$ In order to identify the type of iron cation, the charge of the compound is written in parenthesis using Roman numeral.

Complete step by step answer:
We must remember that the iron is represented by a symbol of Fe which means ferrum derived from latin word. As we know that iron belongs to the transition metal of the table. In transition metal, the differentiating electrons go through the penultimate shell. As we know the atomic number of iron is 26 and we can write the electronic configuration of iron as,
$1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}3{d^6}$ or $\left[ {Ar} \right]4{s^2}3{d^6}$
Penultimate shell is that the shell immediately presents previous to the valence or the last shell. i.e., the second last shell of the atom. Thus the differentiating electron enters into $3d$ shell.
Therefore, the option B is correct.
The electronic configuration clearly shows thus in iron, there are no electrons in 4d, 4p and 5p orbitals. Therefore, the options A, C and D are wrong.

Therefore, the correct option is B. .

Note: Now, let us discuss the variable valency.
The capability of the transition metal to show variable valency is usually attributed to the availability of more electrons in the $\left( {n - 1} \right)d$ orbital which is closer the outermost $ns$ orbital in energy levels. Variable valency is generally found in transition elements of the periodic table and it happens as the second last shell of transition metal and it is not filled with electrons.
Let us understand why elements exhibit variable valency.
Some of the transition elements show variable valency because of the different electronic configuration. Sometimes an atom of an element loses its electron from the penultimate shell and thus has more than $1$ valency.