Hydrogen

Symbol: H
Atomic number: 1
Molecular formula: H₂
Atomic mass: 1.008 amu
Electronic configuration: 1s¹
Electronegativity: 2.1

Position of hydrogen in periodic table:

The ground state electronic configuration of hydrogen is 1s¹. It resembles the alkali metals (group IA) as well as halogens (group VIIA). Hence the position of hydrogen is not fixed.

Similarities with alkali metals:

1. Electronic configuration: Like alkali metals, hydrogen has one electron in its valence shell.
   H- 1S¹
   Li- 1s²2s¹
   Na- 1s² 2s² 2p⁶ 3s¹
   K- 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹

2. Electropositive nature: Like alkali metals, hydrogen has a tendancy to lose electrons and acquire positive charges.

3. Affinity towards non-metals: Like alkali metals, hydrogen combines with non-metals to form oxides, sulphides, and halides.

Difference from alkali metals:

1. Hydrogen is non-metal but alkali metals are metal.
2. Oxides of hydrogen are neutral but oxides of alkali metals are basic.
3. Hydrogen exists in a gaseous state but alkali metals in solid.

Similarities with halogens:

1. Electronic configuration: Like halogens, a hydrogen atom has one electron short of the nearest inert gas configuration.

2. Electronegative character: Like halogens, hydrogen has the tendency to gain one electron to form hydride.

3. Non-metallic nature and atomicity: Like halogens, hydrogen exists in the diatomic molecule and is non-metallic in nature.

Differences from halogens:

1. Hydrogen has less tendency to gain electrons than halogens.
2. Oxide of hydrogen is neutral but oxides of halogens are acidic.
3. Hydrogen doesn’t have unshared pair of electrons but halogen has.

Different forms of hydrogen:

1. Molecular hydrogen: Ordinary hydrogen consists of diatomic molecule in which two hydrogen atoms are combined together by a covalent bond is known as molecular hydrogen. Due to high bond energy, molecular hydrogen is very stable.

2. Nascent hydrogen: It is newly formed hydrogen in the reaction mixture. It is formed in situ during the chemical reaction but has not been isolated yet.

The nascent hydrogen is a more reactive and powerful reducing agent than molecular hydrogen which can be proved by following chemical reactions.

i. Reaction with acidified potassium permanganate solution:

ii. Reaction with acidified potassium dichromate solution:

iii. Reaction with ferric chloride solution:

3. Atomic hydrogen: It is the hydrogen formed by dissociation of molecular hydrogen at about 1000- 2000°C at low pressure. This can be done by passing hydrogen gas by the electric spark between two tungsten electrodes.

Atomic hydrogen can be isolated and has a half-life of 0.3 sec. It is more powerful reducing agent than nascent hydrogen.

Isotopes of hydrogen:

The atoms of the same element having the same atomic number but a different mass numbers are called isotopes. There are three known isotopes of hydrogen.

1. Protium or ordinary hydrogen (1H_¹ or H)
2. Deuterium or heavy hydrogen (1H² or D)
3. Tritium or radioactive hydrogen (1H³ or T)

Uses of protium:

• Hydrogenation of vegetable oils.
• Synthesis of ammonia by Haber’s process.
• As rocket fuel.

Uses of deuterium:

• As a moderator in the nuclear power plant.
• As a tracer to determine the mechanism of the reaction.

Uses of tritium:

• As radioactive tracer
• To make hydrogen bomb.

Application of hydrogen as fuel:

Hydrogen is used in a fuel cell or in internal combustion engines. Fuel cells are the energy conversion device which can change chemical energy to electrical energy directly. Commercially, fuel cell vehicles such as cars, buses have been begun since few decades. Fuel cells are being preferentially used in spacecraft. The addition of 20% of hydrogen to natural gas can reduce carbon dioxide emissions. Besides, hydrogen also acts as fuel in sun and stars as a key material for nuclear fusion reaction where tremendous energy is generated.

Heavy water:

Deuterium oxide is known as heavy water. It is represented by D₂O
or ₁H²₂O.

Uses:

• Moderator in nuclear reactions.
• Coolant in a nuclear reactor.
• To prepare deuterium.

Some Important Questions

1. Name the isotopes of hydrogen. Which isotope of hydrogen:
   a. does not contain neutron
   b. is radioactive
   c. is heavy?
2. In what respect nascent hydrogen differ from atomic hydrogen?
3. Write the molecular formula of the compounds where hydrogen shows +1 and -1 oxidation state.
4. Show that nascent hydrogen is powerful reducing agent than molecular hydrogen with the help of reactions.
5. What is heavy water? Write its two uses.