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# 3: Nuclear Masses

• 3.1: Experimental Facts
Each nucleus has a (positive) charge Ze , and integer number times the elementary charge e . This follows from the fact that atoms are neutral! Nuclei of identical charge come in different masses, all approximate multiples of the “nucleon mass”.  Nuclei of identical charge (chemical type) but different mass are called isotopes. Nuclei of approximately the same mass, but different chemical type, are called isobars.
• 3.2: Interpretation
We conclude that the nucleus of mass m≈AmN contains Z positively charged nucleons (protons) and N=A−Z neutral nucleons (neutrons). These particles are bound together by the “nuclear force”, which changes the mass below that of free particles
• 3.3: Deeper Analysis of Nuclear Masses
The quantity of most interest in understanding the mass is the binding energy, defined for a neutral atom as the difference between the mass of a nucleus and the mass of its constituents. This seems to show that to a reasonable degree of approximation the mass is a function of A alone, and furthermore, that it approaches a constant. This is called nuclear saturation.
• 3.4: Nuclear mass formula
The nuclear sturcture can be modeled in terms of bulk energy, surface energy, Pauli energy and Coulombic energy.
• 3.5: Stability of Nuclei
We can see that typically the nuclei that are most stable for fixed A have more neutrons than protons, more so for large A increases than for low A . This is the “neutron excess”.
• 3.6: Properties of Nuclear States
Nuclei are quantum systems, and as such must be described by a quantum Hamiltonian. Fortunately nuclear energies are much smaller than masses, so that a description in terms of non-relativistic quantum mechanics is possible. Such a description is not totally trivial since we have to deal with quantum systems containing many particles. Rather then solving such complicated systems, we often resort to models.

Thumbnail: A model of the atomic nucleus showing it as a compact bundle of the two types of nucleons: protons (red) and neutrons (blue). Image used wtih permission (Cc BY-SA 3.0; Marekich).