POSITION OF HYDROGEN IN THE PERIODIC TABLE
Hydrogen is the first element of the periodic table having atomic number 1. It has electronic configuration ls1. It is difficult to give a proper place to hydrogen in the periodic table because of its resemblance with halogens as well as alkali metals. The dual behaviour of hydrogen is attributed to its electronic configuration.
Resemblance with Alkali Metals
• Electronic configuration of hydrogen resembles that of alkali metals.
• By losing one electron hydrogen forms H+ ion and exhibits oxidation state of + 1.
• Like alkali metals, hydrogen is a good reducing agent. Resemblance with Halogens
• Like halogens it can attain noble gas configuration gaining one electron and forming hydride ion, H-.
• It exhibits oxidation state of -1 in metal hydrides. It exists as diatomic molecule, Hz.
• In most of its compounds it forms bond by sharing of electrons.
Due to its resemblance with both alkali metals and halogens it is placed separately as shown in Fig. 6.1.
Period Principal Orbitals being Electrons to be Number of
valence filled up accommodated elements
First n=1 1s 2 2
Second n=2 2s, 2p 2+6 8
Third n=3 3s,3p 2+10+6 8
Fourth n=4 4s,3d.4p 2+10+6 18
Fifth n=5 5s, 4d, 5p 2+ 10+6 18
Sixth n=6 6s. 4f, 5d, 6p 2 + 14 + 10 + 6 32
Seventh n=7 7s. 5f, 6d, 7p 2+14+10+6 32