Hydrogen is the lightest element and its atom has the simplest structure among atoms of all

elements.

Resemblance with Alkali Metals.

Resemblance with Halogens.

Unique Position of Hydrogen in Periodic Table. Hydrogen can be placed in group 1 along with

alkali metals. It can also be placed along with halogens in group 17 because many of its

properties are common with those of halogens.

Hydrogen has dual nature but it does not completely resemble with either alkali metals or

halogens. Due to its distinct properties, hydrogen is sometimes referred to as 'rogue element’. It is

allotted a special position in the periodic table and is not associated with any particular group. It is

usually shown on the left side of the periodic table on the top of the alkali elements (group 1), but

separated from them to indicate its distinctive character.

Occurrence of Hydrogen. Hydrogen is the most abundant element in the universe. Spectral

analysis of light emitted by stars indicates that most of them are predominantly hydrogen.

Isotopes of Hydrogen. Natural hydrogen has three isotopes: protium, deuterium and tritium.

(i) Protium has 1 proton in the nucleus and 1 electron in the extra - nuclear part. It is highly stable

and non-radioactive. It constitutes 99.85% of hydrogen gas.

(ii) Deuterium has one proton and one neutron in the nucleus and one electron in the extranuclear

part. It is also stable and non-radioactive. Deuterium is also called heavy hydrogen and constitutes

0.015% of hydrogen gas

(iii) Tritium has one proton and two neutrons in the nucleus and one electron in the extranuclear

part. It is unstable and radioactive in nature. Tritium occurs in traces and constitutes 1 part in

1017 of the hydrogen gas. The atomic properties of the three isotopes indicate that they can be

used in isotope studies, radioactive tracer studies and NMR spectroscopy.