While there are commonly known "Magic Numbers" in nuclear physics, which correspond to the number of protons or neutrons that result in increased stability of atomic nuclei, there is no specific concept of "Forbidden Numbers" in the same sense. The concept of magic numbers arises from the nuclear shell model, which describes the behavior of nucleons (protons and neutrons) in atomic nuclei.
Magic numbers are values such as 2, 8, 20, 28, 50, 82, and 126, which correspond to complete shells of protons or neutrons in a nucleus. Nuclei with magic numbers of protons or neutrons tend to have increased stability and are often more abundant or exhibit longer half-lives compared to neighboring isotopes.
On the other hand, the absence of stable isotopes with certain specific mass numbers, such as 5 or 8, is not due to a concept of "Forbidden Numbers" but rather a result of nuclear properties and the underlying nuclear forces. The stability of an isotope depends on various factors such as the balance between the strong nuclear force, which holds the nucleus together, and the electromagnetic repulsion between protons.
Isotopes with certain mass numbers may be less stable due to their specific combination of protons and neutrons, which results in an unfavorable nuclear configuration. These isotopes may decay rapidly or have very short half-lives. However, it's important to note that there is no general term or concept of "Forbidden Numbers" used in nuclear physics to describe these specific cases.