Page 158 - Chemistry ICSE Class IX

Study of the First
6 Element : Hydrogen

Hydrogen
CONTENTS
Atomic symbol H Atomic number 1
ͻ ŝƐĐŽǀĞƌLJ ŽĨ ŚLJĚƌŽŐĞŶ Electronic con guration K Atomic mass 1.008 u
ͻ ZĞƐĞŵďůĂŶĐĞ ǁŝƚŚ ĂůŬĂůŝ ŵĞƚĂůƐ 1
ͻ ZĞƐĞŵďůĂŶĐĞ ǁŝƚŚ ŚĂůŽŐĞŶƐ Valency +1, –1 Atomic radius 37 pm
ͻ WƌŽƉĞƌƟĞƐ ŽĨ ŚLJĚƌŽŐĞŶ ŶŽƚ Molecular formula H Molecular mass 2.016 u
2
ĐŽŵŵŽŶ ƚŽ ďŽƚŚ ƚŚĞ ĂůŬĂůŝ ŵĞƚĂůƐ Density 0.09 g/L Molar mass 2.016 g mol –1
ĂŶĚ ŚĂůŽŐĞŶƐ
ͻ /ƐŽƚŽƉĞƐ ŽĨ ŚLJĚƌŽŐĞŶ Hydrogen in the gaseous state exists as a diatomic species H . The diatomic
2
ͻ WƌĞƉĂƌĂƟŽŶ ŽĨ ŚLJĚƌŽŐĞŶ H molecule is also termed as dihydrogen to distinguish it from the
2
ͻ >ĂďŽƌĂƚŽƌLJ ƉƌĞƉĂƌĂƟŽŶ ŽĨ ŚLJĚƌŽŐĞŶ hydrogen atom.
ͻ DĂŶƵĨĂĐƚƵƌĞ ŽĨ ŚLJĚƌŽŐĞŶ ŐĂƐ
ͻ KdžŝĚĂƟŽŶ
ͻ KdžŝĚŝƐŝŶŐ ĂŐĞŶƚ Discovery of Hydrogen
ͻ ZĞĚƵĐƟŽŶ In the beginning of the 16th century, a German chemist, Paracelsus
ͻ ZĞĚƵĐŝŶŐ ĂŐĞŶƚ obtained a highly in ammable gas when iron was added to sulphuric acid.
ͻ KdžŝĚĂƟŽŶʹZĞĚƵĐƟŽŶ ŝƐ ƌĞĐŝƉƌŽĐĂů
ͻ WŚLJƐŝĐĂů ĂŶĚ ĐŚĞŵŝĐĂů ƉƌŽƉĞƌƟĞƐ ŽĨ Robert Boyle (in 1672) studied the combustibility of this gas.
ŚLJĚƌŽŐĞŶ Henry Cavendish (1766) obtained this gas by the reaction between
ͻ hƐĞƐ ŽĨ ŚLJĚƌŽŐĞŶ zinc and dilute sulphuric acid. He also showed that hydrogen when burnt
gives water as the only product.
Antoine Lavoisier named this gas as hydrogen because it produced
water when burnt (Greek: hydro = water, gennas = producer).

Occurrence of hydrogen
Hydrogen constitutes 0.9% (by mass) of the earth’s crust and is the ninth
element in the order of abundance.
Hydrogen occurs free in the earth’s atmosphere in traces. In combined
state, it occurs as water, acids, organic materials, such as hydrocarbons and
many minerals.
Hydrogen occurs in abundance on the Sun and other bright stars. The
Jupiter and Saturn consist mainly of hydrogen. The source of the Sun’s
energy is the fusion of hydrogen into helium.
All plants and animal tissues contain compounds of hydrogen in
combination with carbon, oxygen, nitrogen, etc.

Position of hydrogen in periodic table
Hydrogen is the rst element of the periodic table. It was assigned rst
group and rst period of the periodic table. This position of hydrogen in
the periodic table was disputed because it resembles both, the alkali metals
as well as halogens. Moreover, although hydrogen resembles both alkali
metals and halogens, it differs from both in certain aspects. Therefore, it
was considered more appropriate and justi ed to allot hydrogen a special
position in the periodic table and not to associate it with any particular
group.