Question

The element with controversial placement in modern periodic table is __.

Answer 1

Hydrogen can gain one electron and can become $H^{-}$ ion just like halogens (group 17) form $X^{-}$ ion. Also, it can lose an electron and can become $H^{+}$ ion just like alkali metals (group 1) form $M^{+}$ ion. Hence, the position of hydrogen is controversial in the modern periodic table.