1. POSITION OF HYDROGEN IN THE PERIODIC TABLE

Hydrogen (OLD NCERT) | Chemistry

Hydrogen (OLD NCERT)

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Hydrogen

 
Objectives

After studying this unit, you will be able to

• present informed opinions on the position of hydrogen in the periodic table;

• identify the modes of occurrence and preparation of dihydrogen on a small and commercial scale; describe isotopes of hydrogen;

• explain how different elements combine with hydrogen to form ionic, molecular and non-stoichiometric compounds;

• describe how an understanding of its properties can lead to the production of useful substances, and new technologies;

• understand the structure of water and use the knowledge for explaining physical and chemical properties;

• explain how environmental water quality depends on a variety of dissolved substances; difference between ‘hard’ and ‘soft’ water and learn about water softening;

• acquire the knowledge about heavy water and its importance;

• understand the structure of hydrogen peroxide, learn its preparatory methods and properties leading to the manufacture of useful chemicals and cleaning of environment;

• understand and use certain terms e.g., electron-deficient, electron-precise, electron-rich, hydrogen economy, hydrogenation etc.

 

“Hydrogen, the most abundant element in the universe and the third most abundant on the surface of the globe, is being visualised as the major future source of energy.”

 

 NEETprep Audio Note:  

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   NEETprep Audio Note (English):
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    Hydrogen has the simplest atomic structure among all the elements around us in Nature. In atomic form it consists of only one proton and one electron. However, in elemental form it exists as a diatomic (H2) molecule and is called dihydrogen. It forms more compounds than any other element. Do you know that the global concern related to energy can be overcome to a great extent by the use of hydrogen as a source of energy? In fact, hydrogen is of great industrial importance as you will learn in this unit.

9.1 Position of Hydrogen in the Periodic Table

 NEETprep Audio Note: 

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    NEETprep Audio Note (English):  
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  Hydrogen is the first element in the periodic table. However, its placement in the periodic table has been a subject of discussion in the past. As you know by now that the elements in the periodic table are arranged according to their electronic configurations.

Hydrogen has electronic configuration 1s1. On one hand, its electronic configuration is similar to the outer electronic configuration (ns1) of alkali metals , which belong to the first group of the periodic table. On the other hand, like halogens (with ns2np5 configuration belonging to the seventeenth group of the periodic table), it is short by one electron to the corresponding noble gas configuration, helium (1s2). Hydrogen, therefore, has resemblance to alkali metals, which lose one electron to form unipositive ions, as well as with halogens, which gain one electron to form uninegative ion. Like alkali metals, hydrogen forms oxides, halides and sulphides. However, unlike alkali metals, it has a very high ionization enthalpy and does not possess metallic characteristics under normal conditions. In fact, in terms of ionization enthalpy, hydrogen resembles more
with halogens, ∆i H of Li is 520 kJ mol–1, F is 1680 kJ mol–1 and that of H is 1312 kJ mol–1. Like halogens, it forms a diatomic molecule, combines with elements to form hydrides and a large number of covalent compounds. However, in terms of reactivity, it is very low as compared to halogens.

Inspite of the fact that hydrogen, to a certain extent resembles both with alkali metals and halogens, it differs from them as well. Now the pertinent question arises as where should it be placed in the periodic table? Loss of the electron from hydrogen atom results in nucleus (H+) of ~1.5×10–3 pm size. This is extremely small as compared to normal atomic and ionic sizes of 50 to 200pm. As a consequence, H+ does not exist freely and is always associated with other atoms or molecules. Thus, it is unique in behaviour and is, therefore, best placed separately in the periodic table (Unit 3).