The liquid drop model is a model in nuclear physics which treats the nucleus as a drop of incompressible nuclear fluid. The fluid is made of nucleons, and is held together by the strong nuclear force.
This is a crude model that does not explain all the properties of nuclei, but does explain the spherical shape of most nuclei.
Mathematical analysis of the theory delivers an equation which attempts to predict the binding energy of a nucleus in terms of the numbers of protons and neutrons it contains. This equation has five terms on its right hand side. These correspond to the cohesive binding of all the nucleons by the strong nuclear force, the electrostatic mutual repulsion of the protons, a surface energy term, an asymmetry term (derivable from the protons and neutrons occupying independent quantum momentum states) and an exchange term.