Question

Which element is present in ruby as impurity?
A. $Cu$
B. $Cr$
C. $Au$
D. $Zn$

Answer 1

The element which is present in ruby as impurity is a hard silvery metal with a blue tinge. It is used in the preparation of stainless steel. It is also used to harden the steel. It has a high melting point and that’s why it is used to make molds for bricks. It is used in dyes and paints also.

Complete answer:
Ruby is a gemstone having pink to red color. It is a mineral corundum that is aluminium oxide. It is one of the traditional cardinal gems. The red color of ruby is due to the presence of impurity elements in the stone. That impurity is chromium. Chromium metal is present in ruby as impurity and gives it a red color. The quality of a ruby stone depends on its color, cut and clarity and also its carat weight affects its quality.
When a chromium atom is introduced in ruby then it replaces the occasional aluminium atom. And that chromium loses three electrons and gets converted to $C{r^{3 + }}$ ion. These ions maintain the charge balance of the aluminium oxide crystal. The chromium ions are larger and have electron orbitals in different directions as compared to aluminium. The octahedral arrangement of the oxygen ions that is ${O^{2 - }}$ ions is distorted. The energy levels of the different orbitals of those chromium ions are slightly altered because of the directions to the oxygen ions. Those energy differences correspond to absorption in the ultraviolet, violet, and yellow-green regions of the spectrum.

Hence, option B is correct.

Note:
If one percent of the aluminium ions are replaced by chromium in ruby, the yellow-green absorption results in a red color for the gem. And absorption at any of the above wavelengths stimulates fluorescent emission of $694nm$ wavelength red light, which adds to its red color and perceived luster.