Hydrogen is the first element of the periodic table as its atomic number is one, which means it has only one electron in its atom and thus only one electron is present in its outermost shell.The placement of elements in the periodic table is based on their electronic configuration. This structure is similar to that of alkali metals (ns1) which have 1 electron in their outer most shell. It can attain the noble gas configuration of helium, by accepting one electron. This character is very much similar to that of halogen family (ns2 np5) which are also short of one electron to complete the octet of electrons in their shells. When hydrogen loses an electron and forms a cation, it resembles alkali metals but when it gains an electron and becomes a uni-negative ion it shows similarity to halogens. Looking at these properties the position of hydrogen in the periodic table was a big question.
Moving on to the compound formation, hydrogen forms oxides, halides and sulphides resembling the alkali metals, but unlike the alkali metals it has a very high ionization enthalpy, and so it lacks metallic characteristics under normal conditions. When we look in terms of ionization enthalpy, it is found that hydrogen resembles more to halogens than alkali metals. For example, ΔiH of lithium is 520 kJ mol-1, fluorine is 1680 kJ mol-1 and for hydrogen, it is 1312 kJ mol-1. It exists as a diatomic molecule like that of halogens (for example chlorine Cl2), there is a single hydrogen bond when H2 molecule is formed.
Though hydrogen shows a lot of resemblance to halogens and alkali metals, it is very different from both. So a great thought has to be given for the position of hydrogen in the periodic table. When hydrogen loses electron, the size of its nucleus decreases and becomes almost 1.5 × 10-3pm, which is very small as compared to the atomic sizes of normal metals and hence hydrogen ion does not exist freely in nature. This unique behavior of this atom is the reason for it being placed separately in the periodic table. To know more about hydrogen, its properties and the unique characteristics associated with it go for Byjus-The Learning App.