Question

Potassium dichromate belongs to which crystal system?

• A. Tetragonal
• B. Orthorhombic
• C. Triclinic
• D. Hexagonal
• E. Monoclinic

Hint:

Potassium dichromate ($K_2Cr_2O_7$) belongs to the Triclinic system. While the prefixes don't match numerically, associating the "D" in dichromate with the next number "T" in triclinic is a common mnemonic for students!

Answer 1

The Correct Option is C

Concept: Crystalline solids are classified into seven crystal systems based on their unit cell parameters (axial lengths $a, b, c$ and axial angles $\alpha, \beta, \gamma$).
• Triclinic System: The most unsymmetrical system where all sides are unequal ($a \neq b \neq c$) and all angles are unequal and not equal to 90$^\circ$ ($\alpha \neq \beta \neq \gamma \neq 90^\circ$).
• Common Examples: $K_2Cr_2O_7$ (Potassium dichromate), $CuSO_4 \cdot 5H_2O$ (Blue vitriol), and $H_3BO_3$ (Boric acid).

Step 1: Recall crystal data for K\textsubscript{2}Cr\textsubscript{2}O\textsubscript{7}. Potassium dichromate forms orange-red crystals. Crystallographic analysis shows that its unit cell has the parameters $a \neq b \neq c$ and $\alpha \neq \beta \neq \gamma \neq 90^\circ$.

Step 2: Categorize the system. These specific parameters define the Triclinic crystal system. Therefore, $K_2Cr_2O_7$ belongs to the Triclinic system.