Nuclei, isotopes and stability

A nucleus contains protons and neutrons, collectively nucleons. In the notation ZXA​,A is mass number and Z is atomic number. Proton number is Z and neutron number is N=A−Z. A neutral atom has Z electrons; an ion’s electron count changes without changing its nucleus. Most atomic mass is concentrated in the much smaller nucleus.

Isotopes share Z but differ in neutron number. Their similar electron arrangements give similar ordinary chemistry, while nuclear and some other physical properties can differ. The attractive nuclear interaction is very short-range, whereas protons repel electrically. Stable neutron-to-proton patterns change with nuclear size; heavy stable nuclei generally need more neutrons than protons. Composition alone does not assign an exact half-life.

Watch out: Different proton numbers define different elements. Isotopes of one element have different neutron numbers.