Radiation attacks cancer at the level of the atom, whereas chemo is attacking at the cellular level of a cell.
Atoms & Cells
An atom is the smallest living unit of matter. A cell is the smallest unit of living things. A cell has plenty of atoms combined into molecules -- like DNA.
It's one of the waves of energy that travel through our world; in fact, it's powerful enough to dislodge an electron from its atomic orbit, leaving the atom damaged and in an "ionized" state.
What Happens When Atoms Are Damaged?
When atoms (that are part of the DNA Molecule) get damaged by radiation the result is that chromosomes cannot properly replicate themselves. Unable to divide and grow, the cancer cell (before long) dies off.