Editing Semi-empirical mass formula (section)

==Liquid-drop model==
[[File:Liquid drop model.svg|thumb|452x452px|Illustration of the terms of the semi-empirical mass formula in the liquid-drop model of the atomic nucleus]]
The liquid-drop model was first proposed by [[George Gamow]] and further developed by [[Niels Bohr]], [[John Archibald Wheeler]] and [[Lise Meitner]].<ref>{{Cite book |last=Sartori |first=E.|title= Histoire des femmes scientifiques de l'Antiquité au XXe siècle|date=2006 |isbn= 2-259-20288-8 |edition=Plon |location= Paris |pages=326–328}}</ref> It treats the [[atomic nucleus|nucleus]] as a drop of [[Incompressible flow|incompressible fluid]] of very high density, held together by the [[Nuclear force#The nuclear force as a residual of the strong force|nuclear force]] (a residual effect of the [[strong nuclear force|strong force]]): there is a similarity to the structure of a spherical liquid drop. While a crude model, the liquid-drop model accounts for the spherical shape of most nuclei and makes a rough prediction of binding energy.

The corresponding mass formula is defined purely in terms of the numbers of protons and neutrons it contains. The original Weizsäcker formula defines five terms:
* ''Volume energy'', when an assembly of nucleons of the same size is packed together into the smallest volume, each interior nucleon has a certain number of other nucleons in contact with it. So, this nuclear energy is proportional to the volume.
* ''Surface energy'' corrects for the previous assumption made that every nucleon interacts with the same number of other nucleons. This term is negative and proportional to the surface area, and is therefore roughly equivalent to liquid [[surface tension]].
* ''[[Coulomb's law|Coulomb]] energy'', the potential energy from each pair of protons. As this is a repelling force, the binding energy is reduced.
* ''Asymmetry energy'' (also called [[Wolfgang Pauli|Pauli]] energy), which accounts for the [[Pauli exclusion principle]]. Unequal numbers of neutrons and protons imply filling higher energy levels for one type of particle, while leaving lower energy levels vacant for the other type.
* ''Pairing energy'', which accounts for the tendency of [[even and odd atomic nuclei|proton pairs and neutron pairs]] to occur. An even number of particles is more stable than an odd number due to [[J-coupling|spin coupling]].