In most solids (but not all), the molecules have a fixed mean position and orientation, and therefore the only degrees of freedom available are the vibrations of the atoms. Thus the specific heat is proportional to the number of atoms (not molecules) per unit of mass, which is the Dulong–Petit law. Other contributions may come from magnetic degrees of freedom in solids, but these rarely make substantial contributions. and electronic Since each atom of the solid contributes one independent vibration mode, the number of degrees of freedom in n atoms is 6 n. Therefore, the heat capacity of a sample of a solid substance is expected to be 3 RN a, or (24.94 J/K) N a, where N a is the number of moles of atoms in the sample, not molecules. Said another way, the atom-molar heat capacity of a solid substance is expected to be 3 R = 24.94 J⋅K ⋅mol, where "amol" denotes an amount of the solid that contains the Avogadro number of atoms.