Question

why is position asigned to hydrogen in periodic table considered anomalous

Answer 1

Because,Hydrogen has a oxidation number of 1, just as do the alkali metals, and so it is placed there. Hydrogen can also have an oxidation number of -1, as do the halogens, and so it is sometimes placed above F and right before He. The periodic table on my wall (in my classroom) is from Fisher Scientific and has hydrogen in both places.

Answer 2

Hydrogen has electric configuration 1s1. On one hand, its electronic configuration is similar to  the outer electronic configuration of alkali metals, which belong to the  first group of the periodic table. On the other hand, like halogens, it is short by one electron to the corresponding noble gas configuration helium. Hydrogen therefore has resemblance to alkali metals, which lose one electron to form unipositive ions, as well as with halogens, which gain one electron to form uninegative ion. In spite of the fact that hydrogen, to a certain extent resembles both with alkali metals and halogens, it differs from them as well.

Loss of the electron from hydrogen atom results in nucleus (H+) of ~1.5*10-3 pm size. This is extremely small as compared to normal atomic sizes of 50 to 200pm. As a consequence, H+ does not exist freely and is always associated with other atoms or molecules. Thus, it is unique in behavior and is, therefore, best placed separately in the periodic table.

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