Many broad trends in nuclear binding can be modeled by treating the nucleus as a nearly incompressible drop of nuclear matter. The liquid-drop model does not resolve individual nucleon orbitals; it approximates collective contributions to the binding energy.

A common form is the semi-empirical mass formula $$B(A,Z)\approx a_vA-a_sA^{2/3}-a_c\frac{Z(Z-1)}{A^{1/3}}-a_a\frac{(A-2Z)^2}{A}+\delta(A,Z).$$ Each term has a physical interpretation.

The volume term $a_vA$ reflects short-range attractive nuclear interactions: each interior nucleon binds to a roughly fixed number of neighbors. The surface term subtracts binding because nucleons near the surface have fewer neighbors. The Coulomb term subtracts energy because protons repel one another. The asymmetry term penalizes large imbalance between proton and neutron numbers, a consequence of quantum state filling. The pairing term $\delta$ gives extra stability to paired proton and neutron configurations.

The competition explains why binding energy per nucleon rises for light nuclei but eventually declines for very heavy nuclei: the attractive volume contribution scales roughly as $A$, while proton repulsion becomes increasingly costly.

The model also gives a qualitative picture of fission. Deforming a heavy nucleus can increase surface energy while reducing Coulomb repulsion between separated charge regions. For sufficiently heavy nuclei, the balance creates a pathway toward splitting into more tightly bound fragments.

The liquid-drop model captures smooth collective trends; the nuclear shell model explains discrete corrections such as magic numbers. Real nuclei display both aspects, so the two models are complementary rather than competing descriptions.