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Hydrogen- Placement in the periodic table, Isotopes, Preparation, Physical and chemical properties, Uses, Practice problems, FAQs.

Do you know which is the most abundant element in the universe? The answer may come as a shock to the lot of you. 

It's actually “Hydrogen”. Named after the Greek word hydro for "water" and genes for "forming," hydrogen makes up more than 90% of all of the atoms, which equals three-quarters of the mass of the universe, The element also occurs in the stars and powers the universe through the proton-proton reaction and carbon-nitrogen cycle. Stellar hydrogen fusion processes release huge amounts of energy as they combine hydrogen atoms to form helium. Pure hydrogen gas is scarce in Earth's atmosphere and any hydrogen that actually enters the atmosphere rapidly escapes Earth's gravity, according to the Royal Society. On our planet, hydrogen occurs mainly in combination with oxygen and water, as well as in organic matter such as living plants, petroleum and coal.

Table of contents

Chemistry of Hydrogen

Hydrogen has the simplest atomic structure among all the elements around us in nature. In the atomic form, it consists of only one proton, one electron, and zero neutrons. The electronic configuration of H is 1s2. In the elemental form, it exists as a diatomic (H2) molecule and is known as dihydrogen.


Position of Hydrogen in the periodic table

Hydrogen is the very first element in the periodic table. There is still an ongoing debate over its proposed  placement due to its tendency to both gain and as well as lose an electron.


Hydrogen is placed in group 1 because of the following reasons:

  • Hydrogen has an electronic configuration of 1S2, i.e., one electron in its valence shell.
  • It forms a unipositive ion H+ ( Protinium ion) like other group 1 elements (LI+ and Na+).
  • It has a valency of 1.
  • It forms oxides, halides, and sulphides like other alkali metals.
  • It is a good reducing agent (in atomic as well as molecular states) like other alkali metals.

Hydrogen also Resembles Halogens.

  • It is a diatomic molecule (H2) like halogens (F2, Cl2, etc).
  • It also forms a uni negative ion H-1 like halogen.
  • Like halogens, it is short by one electron to the corresponding noble gas configuration, which is  helium (1s2).
  • The ionisation energy of H is very high (1312 KJ mol-1), which is of the same order as that of the halogens.

Isotopes of Hydrogen

Isotopes are the different forms of the same element, which have the same atomic number (Z) but different mass numbers (A). Hydrogen has three isotopes: protium 11 H or H), deuterium(  21Hor D), and tritium (      31 or T ).


Name Symbol Atomic number Mass number Relative abundance on earth (%)

Protium or


11H 1 1 99.985
Deuterium 12H 1 2 0.0156
Tritium 13H 1 3 10-15

Preparation of Dihydrogen 

1. By the action of water with metals: 
(a) At room temperature (b) By heating (c) Over steam
(A) At room temperature: Metals like Na and K react with water at room temperature to give dihydrogen and metal


(B) By heating: Heating less active metals like Zn, Mg, Alwith water gives dihydrogen and metal oxide.


(C) Over steam: Metals like Fe, Ni, Co, Sn can react only when steam is passed over their red hot metals.


2. By the reaction of metals like Zn, Al with alkalis (NaOH or KOH)
Zn, Al, Sn, Pb react with NaOH or KOH to give hydrogen gas.

3. Laboratory method: By the action of granulated zinc with dilute HCl
Hydrogen is prepared in the laboratory by the action of dilute  HCl on granular (commercial) zinc.


4. Preparation of pure dihydrogen:

By the electrolysis of warm aq. Ba(OH)2 between Ni electrodes
Dihydrogen of high purity (> 99.95%) is obtained by the process of electrolysis. Electrolysis is defined as a process of breaking ionic compounds into their elements by passing a direct electric current through the aqueous or molten state of that compound.

The reactions occurring at the anode and the cathode are as follows:


Physical properties of dihydrogen: 

  • It is a colourless, tasteless and odourless gas at room temperature.
  • It is insoluble in water. 
  • It is highly combustible.

Atomic properties of hydrogen

(a) Atomic radius (Pm) = 37 

(b) Ionic radius of H- ion (pm) = 208

(c) Ionisation enthalpy (kj mil-1) = 1312 

(d) Electron affinity (kj mil-1) = 73 

(e) Electronegativity ( as per pauling scale) = 2.1

Chemical properties of dihydrogen:

1. Dihydrogen is quite stable and dissociates into hydrogen atoms only when heated over 2000 K.
2. Action with s-block metals to form corresponding hydrides:   imageWith transition metals (elements of d-block) such as Pd, Ni, Pt etc., dihydrogen forms interstitialhydrides, in which the small molecules of hydrogen occupy the interstitial sites in the crystal lattices of these hydrides.

3. Reaction with non-metals:


 Non-metals want to gain electrons. Here, H goes from 0 to +1 oxidation state.
Haber’s process:

4. Reaction with organic compounds: 


H2 reacts with unsaturated hydrocarbons to give saturated hydrocarbons.

Uses of dihydrogen: 

  • Dihydrogen is used in the hydrogenation of vegetable oils
  • Dihydrogen acts as a  reducing agent.
  • As a rocket fuel in liquid H2 form.
  •  In the preparation of many compounds like urea NH3, which is extensively used as a fertilizer. 
  • Atomic hydrogen and oxy-hydrogen torches are used for cutting and welding purposes. Atomic hydrogen atoms (produced by the dissociation of dihydrogen with the help of an electric arc) are allowed to recombine on the surface to be welded to generate the temperature of 4000 K.
  •  In the manufacturing of synthetic petrol

Practice Problems: 

Q 1.  One commercial production of dihydrogen is
A. By the electrolysis of warm aq. Ba(OH)2 between Ni electrodes
B. By the action of granulated zinc with dilute HCL
C. By the action of water on alkali and alkaline earth metal hydrides
D. By Bosch process
Answer: (D)
Solution: Bosch process involves the use of a considerable quantity of commercial graphite from which by the action of water H2O, hydrogen gas is obtained by this method.


Q 2.  Synthesis gas or syngas is a mixture of 
A. CO and H2
B. CO and H2O
C. H2 and CO2
D. H2O and CO2
Answer: (A)
Solution: The mixture of CO and H2 is known as water gas. As this mixture is used for the synthesis of

methanol and a number of hydrocarbons, it is also known as synthesis gas or syngas.

Q 3. What do you understand by the term Hydrides?
Solution: Dihydrogen combines with a large number of non-metals and metals, except noble gases, under certain suitable reaction conditions to form compounds called hydrides.

Examples: MgH2 (magnesium hydride), B2H6 (Diborane)

Q 4.   The catalysts used in the conversion of Acetylene to  Ethane are: 
A. Ni,pt, pd
B. Ni,Na, Hg
C. Na, Mo, Zn
D. Ni, Pd, Hg
Answer: (A)
Solution: To convert Acetylene to Ethane, we generally use Ni,pt, pd catalysts at around 473 K.

Frequently asked questions:


Q 1.  Who discovered hydrogen?
Answer:  Hydrogen was discovered by Henry Cavendish in 1766.

Q 2.  What is the current application of green hydrogen? 
Answer:  Green hydrogen is employed in a variety of industries. One of the most important is mobility, as green hydrogen is a fuel that only creates water as a by-product. Chemical, petrochemical and steel industries also use it. In addition, progress is being made in terms of its application to in-home energy and heating supplies.

Q 3. Why is hydrogen seen as a long-term fuel?
Answer: The only product emitted by vehicles running on green hydrogen is water, rather than
CO2. Hence we can say that hydrogen is seen as a long-term environmental friendly fuel.

Q 4.  Who proposed the name ‘Hydrogen’?
Answer: The name hydrogen was proposed by Antoine Lavoisier.


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