What are the two main characteristics of a free radical?

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Free radicals have an unpaired electron, which makes them highly reactive because they constantly seek to pair that electron with something else. That same unpaired electron also makes them energetically unstable, so they react quickly with nearby molecules and don’t stay around long. In radiology, when ionizing radiation splits water in tissues, these radicals form and rapidly attack essential biomolecules like DNA, proteins, and lipids, driving cellular damage. So the defining traits are their high reactivity and instability. Saying they are stable and nonreactive contradicts their very nature; expecting direct DNA contact as necessary isn’t the defining feature since radicals can damage DNA indirectly; and claiming they’re always short-lived and harmless ignores their potential to initiate damaging reactions before they dissipate.