Here's why:

* Radioactive isotopes: Only certain isotopes of elements are radioactive. Isotopes are atoms of the same element with different numbers of neutrons. Radioactive isotopes have unstable nuclei that decay, emitting radiation in the process.

* Stable isotopes: Most atoms in the universe are stable isotopes. They have stable nuclei and do not emit radiation.

Examples:

* Carbon-14: A radioactive isotope of carbon used in carbon dating. It decays over time, emitting radiation.

* Carbon-12: The most common isotope of carbon, it is stable and does not emit radiation.

Key point: Radiation is not a property of all atoms, only those with unstable nuclei.