Question

Which of the following is the most corrosion-resistant metal known?
A.Iron
B.Bismuth
C.Rhenium
D.Iridium

Answer 1

Hint: The d-block or transition elements of the periodic table exhibit a variety of properties due to the effect of incompletely filled d-orbitals. When all the orbitals are singly occupied by an electron, the orbital is relatively more stable than other configurations although it is obvious that that orbital will be even more stable in case of completely filled orbital.

Complete step by step answer:
Iridium is considered as the ultimate corrosion resistant metal that exists. It is a d-block element of the periodic table, considered as an element of \[Pt\] platinum group element. Corrosion is the phenomenon of oxidation of the outer surface of a metal in presence of air and humidity.
Iridium, \[Ir\] does not react even with aqua-regia or any acid or base.
The electronic configuration of Ir (Atomic Number 77) is \[[Xe].4{{f}^{14}}5{{d}^{7}}.6{{s}^{2}}\].
Its d-shell 5 orbitals have electrons arranged in the following manner:
In this arrangement, the d-shell has 3 out of 5 orbitals which are singly occupied which is a stable configuration as all orbitals are filled. Thus this element is quite stable in terms of getting oxidized.
\[Ir\] has most common oxidation states +3 and +4. But the values of 3rd and 4th ionization enthalpy is much more than other metals ($5100 kJmol^-1$). This makes it less susceptible to corrosion and oxidation.
Hence option (D) is the correct answer.

Additional information: \[Pt-Ir\] alloy (9:1 ratio by mass respectively) cylinder used to be placed at the International Bureau of Weights and Measures as the unit to measure 1 kg mass due to its inertness property.

Note:\[Ir\] is a brittle and very dense metal. It is also used to make devices needed for high temperatures and in electrical contacts.
\[Ir\] is the second most dense metal next to Osmium and is mixed with it to make pen tips.