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Asked by week20sat to Sally, Mmboyi, Mike, Michael, Jacinta, Gliday, Elkana, Edna, Arnold on 17 Jul 2025.
Question: Why does hydrogen appear twice in the periodic table
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Jacinta Nzilani answered on 17 Jul 2025:
Hey,
Why Hydrogen Can Appear Twice in the Periodic TableHydrogen 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 chlorineSome 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 .
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Michael Kimwele answered on 21 Jul 2025:
Hydrogen appears to fit in two places on the periodic table because it has properties that resemble both alkali metals in Group 1 and halogens in Group 17. Like alkali metals, hydrogen has one electron in its outer shell, which allows it to form +1 ions, but unlike metals, it is a non-metal gas. It also shares similarities with halogens because it can gain an electron to form a -1 ion, like fluorine or chlorine. Due to this unique behavior, hydrogen is often placed separately at the top of the periodic table or above Group 1.
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