Distribution of nucleons into the nuclei
In Atomic Physics, the electron is considered as a cloud of probability about the nucleus.
But in Nuclear Physics this is not possible. The nucleons have to be considered as particles, because if we consider them as a cloud of probability there is no way to explain some properties of the nuclei.
For instance, let's consider the oxygen nucleus 8O16.
The physicists discovered that into the nuclei the protons and neutrons are linked together (the nuclei are filled by deuterons).
The 8O16 has null nuclear spin and also null magnetic moment.
If the nucleons into the 8O16 should be a cloud of probability in disorder, its nuclear spin and magnetic moment could not be null.
For the 8O16 to have null nuclear spin and null magnetic moment there is need the following:
- in each instant, for each deuteron A situated in a distance with regard to the center of the nucleus, there is need to have another deuteron B symmetrically placed with regard to the center of the nucleus, so that its spin and magnetic moment are contrary to the deuteron A.
This is shown in fig. 1:

The same must occur between the other pairs of deutrons C-D, E-F, G-H, I-J, K-L, M-N, and so the 8O16 can have a null nuclear spin and null magnetic moment.
The distribution of charges in the nucleus 8O16 is spherical, and this is detected in experiments.
This is measured by the electric quadrupole moment. For the 8O16 it is null
In nuclei with odd number of nucleons the distribution of charges deviates from the spherical shape. They dont have null electric quadrupole moment (EQM).
For instance, the istope 8O17 nas no nulll EQM, because with the addition of one neutron in the structure of 8O16, the resulting 8O17 has a trepidation, and its distribution of charges becomes ellipsoidal (measured by experiments).
The distribution of charges in the nucleus 8O18 is spherical (the two additional neutrons get a symmetrical position, and the nucleus keeps a symmetrical distribution of nucleons).
So, the 8O18 has null EQM.
But when the 8O18 is excited, it gets a non null EQM.
Of course, according to Nuclear Physics, the two neutrons have to keep their opposite positions with regard to the center of the nucleus.
So, according to Nuclear Physics, the 8O18 and the 8O17 have the following distribution of nucleons (of course they are not flat, you have to imagine it spatially):

So, the question is:
what isotope must have stronger trepidation?
The excited 8O18, or the 8O17 ?
