it cannot be placed in carbon family. because it is entirely different from carbon family like electronic configuration,valency....
The 1st element in the periodic table is hydrogen. Hydrogen exhibits dual behaviour because it has only 1 electron on its one ‘S’ shell.(i.e.,) hydrogen resembles both halogens and alkali metals.
Electronic configuration of hydrogen = [1s1]
Hydrogens resemblance with alkali metals:
Hydrogen has 1 valence electron on its valency shell like alkali metals.
[He] 2s 1 – Li
1s1 – H
[Ne] 3s1 – Na
Therefore, to form a uni positive ion, it can lose one of its electron.
To form halides, oxides and sulphides, it combines with electro –ve elements which is same as alkali metals
Hydrogens resemblance with halogens:
Only 1 electron is required to complete their respective octets for both the halogen and hydrogen.
H : 1s 1
F : 1s 2 2s 2 2p 5
Cl : 1s 2 2s 3 2p 6 3s 2 3p 5
It forms several covalent compounds and diatomic molecule like halogens. Even though hydrogen has certain similarity among both halogen and alkali metal, it differs from them. Hydrogen wont possess metallic characteristics; it possesses higher ionization enthalpy and reacts less than halogens.
Due to these reasons, hydrogen cant be replaced with alkali metal of 1st group or with the halogens of 2nd group. Therefore, it is best to place hydrogen separate in the periodic table.