Reason for placing hydrogen in top of the periodic table

Hydrogen has been placed separately at the top of the periodic table. However its position is still uncertain since it resembles in its characteristics with the member of both the alkali metals and halogen.

          Following are the reasons for placing hydrogen in top of the periodic table:

• Hydrogen has electronic configuration 1s1. On one hand, its electronic configuration is similar to the outer electronic configuration (ns1) of alkali metals. On the other hand, like halogens it is short by one electron to the corresponding noble gas  configuration, helium (1s2).

• Like alkali metals hydrogen loses one electron to form uni positive ions. Similarly like halogens, hydrogen gains one electron to form uni negative ion.   

• Like alkali metals hydrogen forms oxides, halides, and sulphides. It has a very high ionization enthalpy and does not possess metallic characterstics under normal conditions. In fact, in terms of ionization enthalpy, hydrogen resembles more with halogens.

• Like halogens it forms a diatomic molecule, combines with elements to form hydrides and a large number of covalent compounds.

• In terms of reactivity it is very low as compared compound to halogens.      

          Inspite of the fact that hydrogen to a certain extent resembles both with alkali metals and halogens it differ from them as well as. Since the loss of the electron from hydrogen atom results in the nucleus of small size. This is extremely small as compared to normal atomic and ionic size of 50 to 200 pm. Therefore H+ does not exist freely. Thus hydrogen is unique in behaviour and hence it has been placed separately in the top of the periodic table.