JEE Main · 2021 · 27 Jul 2021 · Shift 1 · Solid State

A monoclinic unit cell is specified by three unequal edges with two of the interaxial angles equal to $90^{\circ }$ and the third different from $90^{\circ }$. Here $a=2.5$, $b=3.0$ and $c=4.0$ are all different, $\alpha =\gamma =90^{\circ }$ and $\beta =120^{\circ }\ne 90^{\circ }$, which is exactly that pattern. A hexagonal cell also carries a $120^{\circ }$ angle, but it demands $a=b$, and an orthorhombic cell demands all three angles equal to $90^{\circ }$, so neither fits. The substance belongs to the monoclinic crystal system.