In describing the problem of nuclear structure we ended up asking ourselves a question: Could a nucleus of mass A consist of a number A of protons? The short answer is No.
If this were the case, the charge on the nucleus would be A units, but, with the exception of hydrogen, we know that the nuclear charge Z is always less than A, generally less than A / 2. To get around this difficulty, it was assumed from the beginning that, in addition to protons, atomic nuclei contained enough electrons to cancel the positive charge of the additional protons; that is, they were supposed to contain (A-Z) electrons. After all, nuclei emit electrons when they decay, so apparently electrons must exist within the nucleus. These electrons would contribute only a small amount to the mass of the nucleus, but together with the protons they would make the net charge equal to Z units, as was necessary.
It seemed plausible, then, to consider the atom to consist of a nucleus made up of A protons and (AZ) electrons, with additional Z electrons outside the nucleus to result in an electrically neutral atom. For example, an oxygen-16 atom would have a nucleus with 16 protons and 8 electrons, with an additional 8 electrons outside the nucleus. This model of the nucleus is known as the proton-electron hypothesis of nuclear composition . 
The proton-electron hypothesis seemed to be consistent with the emission of alpha and beta particles by atoms of radioactive substances. By including electrons in the nucleus the explanation for beta decay was not a problem: when the nucleus reaches a certain state it simply expels one of its electrons. It also seemed reasonable that an alpha particle could be formed in the nucleus by combining four protons and two electrons; an alpha particle could exist already preformed in the nucleus, or be formed at the moment of emission.
Although the proton-electron hypothesis was satisfactory in some aspects, the development of quantum mechanics, among other problems, forced it to be discarded. One of the most serious difficulties arises from Heisenberg's uncertainty principle and Einstein's theory of relativity : the confinement of an electron in a space as small as the nucleus would give rise to the circumstance that sometimes the speed of the electron would be greater than the speed of light, which is not possible according to the special theory of relativity.
How could one explain the fact that electrons cannot be confined in the nucleus, but emerge from the nucleus in disintegration? Heisenberg told the following anecdote:
One day he and his assistants were discussing this problem while having a coffee in front of the building that housed the city's indoor swimming pool. The movement of people in and out of it suggested to Heisenberg a possible new approach to the problem. “You see people coming into the building fully dressed, and you see them coming out fully dressed. But does that mean that she also swims fully dressed? " That is, you see electrons leaving the nucleus, and at other times you see electrons that are captured by the nucleus, but that does not mean that they remain as electrons while they are in the nucleus. Perhaps electrons were created in the process of emission from the nucleus. A completely new model was necessary.
 The proton-electron hypothesis is similar to an earlier idea suggested by the English physician William Prout in 1815. Based on the small number of atomic masses known at the time , Prout proposed that all atomic masses are multiples of the atomic mass of hydrogen and that therefore, all elements could be formed by hydrogen. Prout's hypothesis was discarded when, at the end of the 19th century, it was found that the atomic masses of some elements were, without any doubt, fractional, specifically, those of chlorine (35.46 units) and copper (63, 54 units). However, with the discovery of isotopes, it was discovered that the fractional atomic masses of chlorine and copper, like that of neon, arise because these elements are mixtures of isotopes, and each isotope does have an atomic mass close to an integer.
The article The proton-electron hypothesis of nuclear composition has been written in Cuaderno de Cultura Científica .