Why do atoms in solids vibrate?

Atoms in solids vibrate because all solids contain some thermal energy. This thermal energy comes from the kinetic energy associated with the random motion of the atoms and molecules within the solid. Even at temperatures above absolute zero, atoms possess a certain amount of vibrational energy, which causes them to oscillate around their fixed positions in the crystal lattice structure of the solid.

The level of this vibration depends on the temperature: as the temperature increases, thermal energy increases, leading to greater vibration of the atoms. The vibrations do not mean that the atoms are moving freely; instead, they are constrained to oscillate about a mean position within the solid. Thus, the presence of thermal energy is fundamental to the vibrational motion of atoms in solids, regardless of the specific temperature.