Position of Hydrogen in the Periodic Table

Why Hydrogen Is Hard to Place

Hydrogen sits at the very start of the periodic table, but where it truly belongs has long been debated. Elements are arranged by electronic configuration, and hydrogen's single electron in the $1s^1$ orbital gives it a split personality.

Resemblance to alkali metals (Group 1): Like Li, Na, K and the rest (outer configuration $ns^1$), hydrogen can lose its lone electron to form a unipositive ion, $H^+$. It also forms oxides, halides and sulphides the way alkali metals do.

Resemblance to halogens (Group 17): Halogens have configuration $ns^2np^5$ and are one electron short of the nearest noble gas. Hydrogen is exactly one electron short of helium ($1s^2$) in the same way, so it can instead gain an electron to form $H^-$. It also exists as a diatomic molecule ($H_2$) and forms a wide range of covalent hydrides, paralleling halogen chemistry.

Where hydrogen actually differs:

• Its ionisation enthalpy is very high and sits far closer to a halogen's than to an alkali metal's: $\Delta_iH$(Li) $\approx 520\ kJ\ mol^{-1}$, $\Delta_iH$(H) $\approx 1312\ kJ\ mol^{-1}$, $\Delta_iH$(F) $\approx 1680\ kJ\ mol^{-1}$.
• Unlike alkali metals, it shows no metallic character under ordinary conditions.
• Unlike halogens, it is far less reactive as a diatomic gas.

Why it gets its own spot: When hydrogen loses its electron, what remains is a bare proton, $H^+$, with a radius of only about $1.5\times10^{-3}\ pm$ — thousands of times smaller than a typical atom or ion (50–200 pm). A charge that small and bare cannot exist freely; it must always attach to another atom or molecule. Because this behaviour has no real parallel elsewhere in the periodic table, hydrogen is conventionally shown on its own, separate from both Group 1 and Group 17.