Question

The metal which shows most variable valencies is:
A.Iron
B.Aluminum
C.Nickel
D.Cobalt

Answer 1

We know that valence electrons are those that are present in the outermost shell of an atom. In simple words, we can say that electrons that could be gained or lost during a chemical reaction are known as valence electrons.

Complete step by step answer: We can define valency as the number of electrons that are gained or lost by an atom to maintain the stability for attaining the electronic configuration of the noble gas which is nearest to the element.
Now let us define variable valency.
We can define variable valency as a phenomenon that is generally found in transition elements. It is present in the second last shell of these elements and is not completely filled with electrons.
We have to know that some elements exhibit variable valency due to differences in electronic configurations. An atom of elements could sometimes lose more electrons than those present in its outermost shell.
Let us consider the example of iron, Iron reacts with oxygen to form iron (II) oxide and iron (III) oxide. In iron (II) oxide, the oxidation state/valency of iron is +2 and in iron (III) oxide, the oxidation state/valency of iron is +3. We can term as variable valency.
Aluminum does not belong to the transition element. Hence option (B) is incorrect.
The given table contains transition elements along with their variable valencies.

Elements	Outer electronic configuration	Oxidation states
Iron	$3{d^6}4{s^2}$	$+ 2$,$+ 3$,$+ 4$,$+ 5$,$+ 6$
Cobalt	$3{d^7}4{s^2}$	$+ 2$,$+ 3$,$+ 4$
Nickel	$3{d^8}4{s^2}$	$+ 2$,$+ 3$,$+ 4$

Though the common oxidation state of all three elements is $+ 2$. The compounds of nickel and cobalt predominantly exhibit +2 in all their compounds, where the most common oxidation states of iron is +2 and +3. Option (C) and (D) are incorrect.
We can see that the element that has the highest valencies is iron.
And hence Option (A) is correct.

Note:
The elements of the d-block show variable oxidation because their outermost electrons are in two different sets of orbitals. The two sets of orbitals are $\left( {n - 1} \right)d$ and $ns$. The difference in energy is less, hence both the energy levels can be used in the formation of bonds. This is one reason why transition elements show variable oxidation state. The ability of d-block elements to show variable valency is attributed to the availability of additional electrons in $\left( {n - 1} \right)d$ orbital that are closer to the valence $ns$ orbital in energy levels. Some of the elements that show variable oxidation states are $Cu$, $Ti$, $Cr$, $Mn$ etc. The most common oxidation state of 3d series is $+ 2$. The stability of higher oxidation state increases down a group.