In physical adsorption (physisorption), the electronic structure of atoms or molecules remains practically unchanged. Typical binding energies are about 10-100 meV. Multilayer adsorption is typical for physisorption, there is no activation barrier, the physisorption process is reversible. Physisorption can be observed only at relatively low temperatures (at room temperatures the thermal energy k_BT is about 25 meV). A typical example of physisorption is inert gas adsorption on metal surfaces at low temperatures.