Question

The colour produced by electronic transitions within the d-orbitals of a transition metal ion occur frequently in everyday life. In emerald, $Cr^{x+}$ ions occupy some voids in the mineral beryl $Be_3Al_2Si_mO_n$. Select the correct statements.

• A. The value of x = 3 and m = 6
• B. The value of x = 2 and n = 18
• C. Emerald transmit light in the green region
• D. $Cr^{x+}$ ions occupy octahedral void in beryl

Answer 1

Answer: (A), (C), (D)

A, C, and D

Answer 2

The general formula for beryl is $Be_3Al_2(SiO_3)_6$, which expands to $Be_3Al_2Si_6O_{18}$. Thus, $m=6$ and $n=18$. Chromium in emerald is $Cr^{3+}$, so $x=3$. This makes statement A correct and statement B incorrect. The green color of emerald is due to $Cr^{3+}$ absorbing red/blue light and transmitting green light via d-d transitions in an octahedral field, making statement C correct. $Cr^{3+}$ replaces $Al^{3+}$ which are in octahedrally coordinated sites, meaning $Cr^{x+}$ occupies octahedral voids, making statement D correct.