(1) Hydrogen is placed in group 1 and in group 17 as it resembles alkali metals as well as halogens. Thus, no fixed position was given to hydrogen in Mendeleev’s periodic table.
(2) On the other hand, hydrogen easily donates the electron and forms a stable cation (H+), but it does not easily form a stable anion (H-), hydride ion. Hence, it is better placed in group 1 rather than in group 17 in the modern periodic table.
