Position of Hydrogen in the Periodic Table

Hydrogen is the first element of the periodic table. Its atomic number is 1, which indicates the presence of only one electron in the atom of hydrogen. This electron is present in its first shell. Hydrogen resembles both alkali metals (group I A) as well as halogens (group VII A) therefore its position is said to be anomalous.

Some Basic Features of Hydrogen

Symbol	H
Molecular formula	H2
Atomic number	1
Mass number	1
Atomic mass	1.008
Molecular mass	2
Electronic configuration	1
Valency	+ 1 or - 1
Isotopes of Hydrogen	11H Protium
	21H Deuterium
	31H Tritium

 Remember :

Isotopes are atoms of the same element having the same atomic number, but different mass number.

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Properties of Hydrogen that Resemble Alkali Metals

Electronic configuration

Hydrogen atom has one electron in its valence shell like alkali metals.

Valency

Hydrogen generally shows + 1 valency like alkali metals.

Metallic character

It readily loses its electron to form a positive ion like other alkali metals.

Combination with non-metals

Hydrogen, like the alkali metals, combines readily with non-metals like halogens, oxygen, sulphur, etc.

Reducing agent

Hydrogen is a good reducing agent like other alkali metals.

Properties of Hydrogen that Resemble Halogens

Non-metallic character

Hydrogen is a non-metal like halogens.

Atomicity

Like halogens, hydrogen is diatomic (H₂) whereas metals are monoatomic.

Valency

Like halogens, hydrogen may also show 1 valency by accepting an electron. Example: NaH (Sodium hydride)

Nature of compounds

Hydrogen combines with non-metals like carbon, silicon, etc. to from covalent compound like halogens.

Examples: CH₄ and CCl₄.

Remember :

In 1766, Henry Cavendish, an English chemist, discovered hydrogen, by treating iron with dilute sulphuric acid. He also proved that when hydrogen is burnt in air, water is the only product obtained. It was "hydrogen", meaning "water producer" (Hydro = water,

gen = generator or producer). Cavendish called this gas "Inflammable gas". Hydrogen has three isotopes namely protium, deuterium and tritium.

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Form the above points it is clear that hydrogen is a unique element, which exhibits the properties of both alkali metals and the halogens. Thus it is difficult to assign a definite position to it in the periodic table.

Isotopes of Hydrogen

All isotopes of the same element have the same number of protons, but a variable number of neutrons. The isotopes of hydrogen have the following structures (Fig.9.1).

Protium has an atomic number 1, and mass number 1 .

Deuterium, has an atomic number 1, and mass number 2.

Tritium has an atomic number 1, and mass number 3