Hydrogen

Occurrence of Hydrogen:

Hydrogen is most abundant element in the universe. It is about 70% of total mass of universe and is principal element in the solar atmosphere. Jupiter and Saturn mostly consist of hydrogen. On earth, it occurs in free state in small traces in volcanic and natural gases. It is third most abundant element on the earth’s surface. However, in combined state it is widely distributed as water. Hydrogen is an important constituent of organic compounds present in petroleum, animal and vegetable matter and constitutes about 15.4% by mass of earth’s crust and oceans. It is ninth element in order of abundance.

Position of hydrogen in the periodic table:

Hydrogen is the first element in the periodic table having atomic number 1 and electronic configuration 1s¹. Hydrogen resembles both alkali metals and halogens and therefore, its position in the periodic table is anomalous.

Similarities with alkali metals:

• Alkali metals and hydrogen have one electron each in their valence shell. For examples,

H (Z=1) = 1s¹

Li (Z=3) = 1s²2s¹

Na (Z=11) = 1s²2s²2p⁶3s¹, etc.

• Hydrogen like alkali metals is electropositive because both can lose one valence electron to form monopositive ion as-

                                    H  —–> H⁺   + e^–

                                    Na —–> Na⁺ + e^–

                                     K  ——> K⁺   + e^–

• Like alkali metals hydrogen also shows oxidation state of +1 in their compounds. For examples,

H⁺Cl^–           Na⁺Cl^–

• Like alkali metals hydrogen combines with electronegative elements such as halogen, oxygen, sulphur, etc to form compounds with similar formulae.

HCl                 H₂O                 H₂S

NaCl                Na₂O               Na₂S

• Hydrogen is very good reducing agent like alkali metals because of their tendency to lose valence electron.

CuO  +  H₂ ——>  Cu  + H₂O

B₂O₃ +  6K  ——->  2B  + K₂O

Difference Hydrogen from alkali metals:

The properties of hydrogen in which it differs from alkali metals are –

• Ionization enthalpy is very high (1312 kJ/mol) compared to alkali metals.
• Size of hydrogen is smaller than alkali metals.
• Hydrogen halides are different from the halides of alkali metals although they have similar molecular formula. For example HCl is covalent while NaCl is ionic.

Similarities of hydrogen with halogens:

• Like halogens, hydrogen needs one electron to attain nearest noble gas configuration i.e. stable electronic configuration.
• Hydrogen, like halogens exists in a diatomic state. The atomicity of hydrogen and halogens is two.
• Halogens and hydrogen exhibit an oxidation state of-1 in some of its compounds such as metal hydrides.
• Like halogens, hydrogen also combines with alkali metals to form salts with similar formulae. NaH, LiH, CaH₂ and NaCl, LiCl, CaCl₂.
• Like halogens, hydrogen react with non- metals such as carbon, silicon, germanium, etc., to form covalent compounds. Halides: CCl₄ SiCl₄ GeCl₄; Hydrides: CH₄ SiH₄ GeH₄.
• Ionization enthalpy of hydrogen is comparable to the ionization enthalpies of halogens.
  For example, H (1312kJ/mol), F (1681kJ/mol), Cl (1255kJ/mol), Br (1121kJ/mol).

Difference of hydrogen from Halogen:

Some of the characteristics of hydrogen in which it differs from halogens are:

• Although hydrogen forms hydride ion (H^–) like halogens, yet its tendency to form hydride ion is very less in comparison with the halogens.
• There is no unshared pair of electron in hydrogen molecule (H₂) whereas halogen molecules have six unshared electron pairs.
• The oxides of halogens are acidic in nature whereas oxide of hydrogen is neutral.

Conclusion:

From the above discussion, it is quite clear that there is a marked resemblance in the properties of hydrogen with alkali metals as well as with halogens. Therefore, it is very difficult to place it either with the elements of group 1 or those of group 17. In other words, the position of hydrogen in the periodic table is anomalous. It is due to this reason hydrogen is sometimes called rogue element.