Position of hydrogen in the periodic table

Where hydrogen belongs in the periodic table has long been debated, because its single-electron configuration, 1s¹, can be read two different ways. Written as ns¹, it looks like the general outer configuration of the group 1 alkali metals, which is the traditional reason hydrogen is drawn above group 1. But 1s¹ also resembles the group 17 (halogen) pattern ns²np⁵ in a different sense: adding one electron to hydrogen's 1s¹ gives 1s², the configuration of the inert gas helium, in exactly the same way that adding one electron to a halogen's ns²np⁵ gives the next inert gas's ns²np⁶ configuration. So hydrogen shares group 1's single valence electron but shares group 17's one-electron-short-of-a-noble-gas relationship — some of hydrogen's chemical properties genuinely resemble the alkali metals, and others resemble the halogens. What makes hydrogen unique is that the cation formed by removing its one electron, H⁺, is simply a bare proton, and a bare proton is far too reactive to exist freely — it is always found attached to another molecule, for example combining with water to give the hydronium ion, H⁺ + H2O → H3O⁺. Because a free H⁺ cannot exist the way a free Na⁺ or Cl⁻ can, hydrogen is conventionally placed on its own, separately above group 1, rather than being folde …