Page 96 - Chemistry ICSE Class IX

4 Atomic Structure and

Chemical Bonding

Atoms are too small to be seen even with the most powerful microscope and
CONTENTS too light to be weighed even on the most sensitive balance. The history of
the discovery of the structure of an atom is a fascinating, but a complicated
ͻ ůĞĐƚƌŝĐĂů ŶĂƚƵƌĞ ŽĨ ŵĂƩĞƌ͗ subject.
ŝƐĐŚĂƌŐĞ ƚƵďĞ ĞdžƉĞƌŝŵĞŶƚƐ
ͻ ĂƚŚŽĚĞ ƌĂLJƐ It is not possible to introduce to you the whole story of the development
ͻ :͘ :͘ dŚŽŵƐŽŶ͛Ɛ ĞdžƉĞƌŝŵĞŶƚ ĂŶĚ ƚŚĞ of the atomic structure at this stage. Therefore, in this chapter, we shall
ĚŝƐĐŽǀĞƌLJ ŽĨ ĞůĞĐƚƌŽŶ describe only the most important contributions which helped us in shaping
ͻ WŽƐŝƟǀĞ ƌĂLJƐ ;ĂŶŽĚĞ ƌĂLJƐ Žƌ ĐĂŶĂů our ideas about the structure of atom.
ƌĂLJƐͿ
ͻ dŚĞ ƉƌŽƚŽŶ Electrical Nature of Matter: Discharge Tube
ͻ dŚĞ ŶĞƵƚƌŽŶ
ͻ dŚĞ ƐƚƌƵĐƚƵƌĞ ŽĨ ĂƚŽŵ Experiments
ͻ ZƵƚŚĞƌĨŽƌĚ͛Ɛ ŶƵĐůĞĂƌ ŵŽĚĞů ŽĨ ĂƚŽŵ The rst important clue to the electrical nature of matter came in 1833 as
ͻ ƂŚƌ͛Ɛ ŵŽĚĞů ŽĨ ĂƚŽŵ a result of Faraday’s electrolysis experiments. He showed that the ow
ͻ EƵĐůĞĂƌ ĐŽŵƉŽƐŝƟŽŶ͗ dŚĞ ĂƚŽŵŝĐ
ŶƵĐůĞƵƐ of electricity is due to the ow of charged particles. G. J. Stoney (1874)
ͻ sĂůĞŶĐĞ ĞůĞĐƚƌŽŶƐ ĂŶĚ ǀĂůĞŶĐLJ rst suggested the name electron for the fundamental electrical particle.
ͻ /ƐŽƚŽƉĞƐ However, it was J. J. Thomson who showed the existence of electrons as an
ͻ &ƌĂĐƟŽŶĂů ĂƚŽŵŝĐ ŵĂƐƐĞƐ essential constituent of all matter.
ͻ ŚĞŵŝĐĂů ƌĞĂĐƟǀŝƚLJ ĂŶĚ ĞůĞĐƚƌŽŶŝĐ
ĐŽŶĮŐƵƌĂƟŽŶ What is meant by the electrical discharge through gases
ͻ KĐƚĞƚ ƌƵůĞ Electrical discharge through gases is studied by using a specially designed
ͻ ůĞĐƚƌŽǀĂůĞŶƚ ŽŶĚ glass tube commonly called a discharge tube. It consists of a cylindrical
ͻ ^ƚƌƵĐƚƵƌĞƐ ŽĨ ƐŽŵĞ ĞůĞĐƚƌŽǀĂůĞŶƚ
ĐŽŵƉŽƵŶĚƐ glass tube having a side tube and two metallic electrodes, one at each end.
ͻ ŽǀĂůĞŶƚ ŽŶĚ These electrodes can be connected to the respective terminals of a high
ͻ ^ƚƌƵĐƚƵƌĞƐ ŽĨ ƐŽŵĞ ĐŽǀĂůĞŶƚ tension power supply. Air from inside the tube can be pumped out by
ĐŽŵƉŽƵŶĚƐ ĂŶĚ ŵŽůĞĐƵůĞƐ connecting the side tube to a vacuum pump and a desired pressure can be
maintained inside the tube.
KŶůLJ ŶŝŶĞƚLJ LJĞĂƌƐ ĂŐŽ͕ ƐĐŝĞŶƟƐƚƐ It was found that gases do not conduct electricity even when an
ďĞůŝĞǀĞĚ ƚŚĂƚ ĂƚŽŵƐ ǁĞƌĞ ƐŽůŝĚ͕ electrical potential of about 10,000 volts was applied. But, it was discovered
ŝŶĚĞƐƚƌƵĐƟďůĞ ƉĂƌƟĐůĞƐ͘ ^ŝŶĐĞ ƚŚĞŶ͕ by William Crookes that gases could conduct electricity at low pressures.
ŵĂŶLJ ŐƌĞĂƚ ƐĐŝĞŶƟƐƚƐ ĐŽŶƚƌŝďƵƚĞĚ When the pressure inside the discharge tube was reduced gradually, the
ďƌŝůůŝĂŶƚůLJ ƚŽ ŐŝǀĞ ƵƐ ƚŽĚĂLJ͛Ɛ ŵŽĚĞů ŽĨ following points were noted:
ĂŶ ĂƚŽŵ͘

William Crookes Fig. 4.1 At a pressure of about 10 atm, the tube is ¿lled with a glow
–3
;ϭϴϯϮʹϭϵϭϵͿ
–2
ƒ At about 10 atm pressure, a glow surrounding the cathode (negative
electrode) leaves the electrode surface and little space is left between
'ĂƐĞƐ ĂƌĞ ŽƌĚŝŶĂƌŝůLJ ƉŽŽƌ ĐŽŶĚƵĐƚŽƌƐ
ŽĨ ĞůĞĐƚƌŝĐŝƚLJ͘ it and the electrode. This is called Crooke’s dark space. At this stage,
electric current begins to ow from one electrode to the other.