Question

Rogue element in the periodic table is ______.
A. oxygen
B. hydrogen
C. astatine
D. rhenium

Answer 1

A rogue element refers to the element which tends to show different behaviour as compared to other elements of its kind. In simple words, it is the word which is used for those elements whose position or properties are not satisfactory or uncertain.

Complete answer:
In modern periodic table, the position of hydrogen is always considered as ambiguous because of the following facts:
Hydrogen has only one electron in its outermost shell due to which it resembles to the alkali metals in group one which consist of lithium, sodium, potassium, rubidium, caesium and francium. Apart from the common valence electrons, it also exhibits similar chemical properties like affinity for non-metals, reducing properties, etc. But hydrogen is not an alkali metal and instead it is a non-metal.
Moreover, we can also say that it has one electron less to complete its s orbital. Thus, it has a tendency to accept an electron to show $- 1$ oxidation state due to which it resembles with the elements belonging to group $17$ i.e., halogen family but hydrogen is not a halogen either.
Thus, the hydrogen atom is the element which is considered as a rogue element in the modern periodic table.
So, option (B) is the correct answer.

Note:
Remember that there are several dissimilarities also among the properties of hydrogen with that of the alkali metals and halogens such as oxides of alkali metals are basic in nature and oxides of halogens are acidic in nature but the oxides of hydrogen are neutral.