Correct Answer: Hydrogen

Solution : The correct answer is Hydrogen .

There is an unusual occurrence in the Modern Periodic table concerning the position of Hydrogen (H). During the first period, Hydrogen can be found in either Group 1 or Group 17. Group 1, known as the alkali metals, is formed when Hydrogen reacts with halogens. However, to be included in Group 17 - the halogen group, Hydrogen must also be considered a diatomic molecule because it can form ionic compounds with metals.