This bar chart shows electron affinity - the energy change when an atom gains an electron. Positive values (exothermic) mean the atom wants electrons; negative values mean it resists them.

Chlorine has the highest electron affinity, not fluorine, because fluorine's tiny size creates electron-electron repulsion in its compact orbitals. Noble gases have near-zero or negative values since their complete shells resist additions. This property explains why halogens form negative ions (Cl⁻, Br⁻) while metals form positive ones (Na⁺, K⁺).