Why can't we predict when one nucleus will decay?
Radioactivity comes from unstable atomic nuclei that transform spontaneously and emit radiation. It is a nuclear phenomenon, distinct from ordinary chemical reactions involving electrons.
The short answer
Radioactivity comes from unstable atomic nuclei that transform spontaneously and emit radiation. It is a nuclear phenomenon, distinct from ordinary chemical reactions involving electrons.
What is really happening?
Radioactivity comes from unstable atomic nuclei that transform spontaneously and emit radiation. It is a nuclear phenomenon, distinct from ordinary chemical reactions involving electrons.
The same phenomenon helps us understand nuclei, date materials and build important medical techniques.
Half-life is the time required for the expected number of undecayed nuclei, or equivalently the activity of a large sample, to fall to one half of its initial value.
A useful mental picture
Radioactive decay is probabilistic for an individual nucleus. Half-life describes the statistical behaviour of a large population, not a countdown timer attached to each atom.