Are all forms of electromagnetic radiation capable of ionization?

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Ionization happens when a photon carries enough energy to knock an electron completely out of an atom. The energy of a photon is E = hν, so you only get ionization if the photon’s energy exceeds the binding energy of the electrons, which for most atoms is tens of electron volts or more. Photons from visible, infrared, or radio waves have far less energy and typically don’t eject electrons; they may cause excitation or heating instead. Ultraviolet light can ionize some atoms and molecules, but to ensure ionization across common materials, you need much higher energy photons. X-ray and gamma-ray photons have keV to MeV energies, easily surpassing those thresholds, so they reliably ionize. Because of this energy requirement, not all electromagnetic radiation ionizes—primarily the high-energy end of the spectrum does, which is why ionizing radiation in dental contexts is associated with X-rays and gamma rays.