Central part of atoms consisting of protons and neutrons.

Atomic nuclei contain the largest part of the atomic mass. The number of (positively charged) protons in a given atomic nucleus determines the number of (negatively charged) electrons in the atomic sheath and thereby the respective chemical element. Atomic nuclei differing in the number of neutrons but with identical numbers of protons are belonging to different isotopes of the same chemical element. Atomic nuclei are only stable when protons and neutrons are present in a certain relation to each other and when the atomic mass is not to big.