Binding energy is the energy required to disassemble an entity into its constituent parts. This corresponds to the mechanical work which must be done in acting against the forces which hold the entity together. The binding energy of an atom is that required to disassemble an atom into free electrons and a nucleus, acting against the electromagnetic force. The nuclear binding energy is that required to disassemble a nucleus into its constituent neutrons and protons, acting against the strong nuclear force. The term is also used in other contexts, for example, for the energy involved in attaching a gaseous molecule to an interstellar dust grain upon collision. In this case, the magnitude depends on both the physical nature of the molecule (e.g., polarizability) and of the grain surface. Binding energies for physisorption (through van der Waals bonding) are generally much lower than those of chemisorption.