Q.Explain why hydrogen is not placed with the halogen in the periodic table.

Step 1. Hydrogen can, like a halogen, gain one electron to complete a stable duet (matching helium) and form the hydride ion $H^-$ -- superficially similar to how a halogen gains one electron to complete an octet and form $X^-$.

Step 2. But hydrogen's electron affinity is much smaller than a halogen's, so this tendency is weak: $\tfrac12 H_2 + e^- \rightarrow H^-$ has $\Delta H = +36$ kcal mol⁻¹ (energy is CONSUMED), while $\tfrac12 Br_2 + e^- \rightarrow Br^-$ has $\Delta H = -55$ kcal mol⁻¹ (energy is RELEASED) -- halogens form their anion far more readily than hydrogen does.

Step 3. In the overwhelming majority of its actual compounds, hydrogen instead exists in the +1 oxidation state (forming H⁺-type bonds), the opposite pattern to the halogens, whose characteristic, dominant oxidation state is -1.

Step 4. Since hydrogen's real chemical behaviour is governed by +1 far more than by any halogen-like -1 tendency, it is not grouped with the halogens.

Compare hydrogen's actual dominant oxidation state (+1) and weak electron affinity against the halogens' defining -1, electron-hungry character.

• Focusing only on the fact that H- CAN form (like a halide) without weighing how weak and rare that tendency actually is compared to hydrogen's dominant +1 behaviour.