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Position in Periodic Table
The hydrogen has the electronic configuration of 1s1 which resembles with ns1 general valence shell configuration of alkali metals and shows similarity with them as follows:
1. It forms unipositive ion (H+) like alkali metals (Na+, K+, Cs+)
2. It forms halides (HX), oxides (H2O), peroxides (H2O2) and sulphides (H2S) like alkali metals (NaX, Na2O, Na2O2, Na2S)
3. It also acts as a reducing agent.
However, unlike alkali metals which have ionization energy ranging from 377 to 520 kJ mol-1, the hydrogen has 1,314 kJ mol-1 which is much higher than alkali metals.
Like the formation of halides (X-) from halogens, hydrogen also has a tendency to gain one electron to form hydride ion (H-) whose electronic configuration is similar to the noble gas, helium. However, the electron affinity of hydrogen is much less than that of halogen atoms. Hence, the tendency of hydrogen to form hydride ion is low compared to that of halogens to form the halide ions as evident from the following reactions:
½ H2 + e- → H- ∆H = +36 kcal mol-1
½ Br2 + e- → Br- ∆H = -55 kcal mol-1
Since, hydrogen has similarities with alkali metals as well as the halogens; it is difficult to find the right position in the periodic table. However, in most of its compounds hydrogen exists in +1 oxidation state. Therefore, it is reasonable to place the hydrogen in group 1 along with alkali metals as shown in the latest periodic table published by IUPAC.
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