Question: Why does hydrogen appear twice in the periodic table

Jacinta Nzilani answered on 17 Jul 2025:

Hey,
Why Hydrogen Can Appear Twice in the Periodic Table

Hydrogen is sometimes shown twice, both in Group 1 (with alkali metals) and Group 17 (with halogens) because of its dual chemical behavior. This placement helps illustrate its unique nature rather than indicating two separate elements.

🔍 Evidence from Scientific Literature
Hydrogen has one electron (1s¹ configuration) and can lose an electron to form H⁺, like alkali metals (Group 1)
Conversely, it can gain an electron to form H⁻ (hydride), similar to halogens (Group 17), and exists naturally as a diatomic gas, just like fluorine and chlorine

Some researchers argue that hydrogen resembles halogens more closely based on compound behavior and stoichiometric similarity.

Other periodic table versions place hydrogen alone (floating or in the center) to show its distinctiveness, since it doesn’t fit perfectly in either group .