neither. It's a nuclear property. Physical properties have to do with things like phase changes, temperature changes, etc. that involve energies on the level of forming and breaking intermolecular forces. read more
It is a physical property, as it concerns the composition and properties of the nucleus, which is a bundle of protons and neutrons. Study of the nucleus began with the discovery of radioactivity, which is a manifestation of the instability of the nucleus. read more