JEE Main · 2026 · 05 Apr 2026 · Shift 2 · Chemical Thermodynamics

Molar heat capacity reflects the number of ways a mole of a substance can store thermal energy. A monatomic gas such as helium stores energy only in three translational modes, giving the smallest molar heat capacity (near $\frac{3}{2}R$, or $\frac{5}{2}R$ at constant pressure). A crystalline solid such as copper gains vibrational contributions and approaches the Dulong--Petit value of about $3R\approx 25\mathrm{J}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-1}$. Liquid bromine, whose molecules additionally access rotational and extra vibrational and configurational modes, has the largest molar heat capacity, about $75\mathrm{J}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-1}$. The correct order is Bromine (l) $>$ Copper (s) $>$ Helium (g).