Page 99 - Chemistry ICSE Class IX

Atomic Structure and Chemical Bonding 87

Is electron an essential constituent of all matter
In 1897, an English physicist, J. J. Thomson, studied the effect of an electric
eld on cathode rays.
The bending of cathode rays towards the positive plate showed that
cathode rays are constituted of negatively charged particles. The de ection
suffered by the beam of cathode rays (or beam of electrons) in Thomson’s
experiment depends upon the strength of the electric eld applied across
the electrodes. J.J. Thomson
The charge-to-mass (e/m) ratio of an electron ;ϭϴϱϲͲϭϵϰϬͿ
&Žƌ LJŽƵ͕ ǁĞ ǁŝƐŚ ƚŽ ƉŽŝŶƚ ŽƵƚ ŚĞƌĞ
J. J. Thomson studied the combined effect of electric and magnetic elds ƚŚĂƚ ƚŚĞ ĚĞǀĞůŽƉŵĞŶƚ ŽĨ ƐĐŝĞŶĐĞ
on the cathode rays. The two elds were applied in such a way that the ĚĞƉĞŶĚƐ ŶŽƚ ĞŶƟƌĞůLJ ƵƉŽŶ ƚŚĞ
de ection due to the electric eld is cancelled out by the de ection due to ‘direct vision͛ ŽĨ ĐĞƌƚĂŝŶ ƚŚŝŶŐƐ͕ ďƵƚ
the magnetic eld, so that the net de ection was zero. ĂůƐŽ ƐŽŵĞƟŵĞƐ ƵƉŽŶ ƚŚĞ ĐŽůůĞĐƟŽŶ
ŽĨ ĐĞƌƚĂŝŶ ĞǀŝĚĞŶĐĞ͘ tŚĞŶ ƚĂŬĞŶ
From the strengths of the electric and magnetic elds required to ƚŽŐĞƚŚĞƌ͕ ƚŚŝƐ ĞǀŝĚĞŶĐĞ ĐŽŶƐƟƚƵƚĞƐ
balance the de ections, the ratio of charge to mass (e/m) of the particles in ǁŚĂƚ ǁĞ ĐĂůů ͚seeing͛͘ EŽ ĚŽƵďƚ
cathode rays was found out. It was found that the ratio of charge (e) to ƚŚĂƚ ĚĞƚĂŝůĞĚ ŝŶĨŽƌŵĂƟŽŶ ĐĂŶ ŽŶůLJ
mass (m) of particles in cathode rays is the same and does not depend on ďĞ ŚĂĚ ĨƌŽŵ ĚŝƌĞĐƚ ǀŝƐŝŽŶ ƚLJƉĞ ŽĨ
the nature of the gas used in the discharge tube. ĞdžƉĞƌŝŵĞŶƚƐ͘
The e/m value for the particles in the cathode rays was found to have a dŚĞ ƐĂŵĞ ƉĂƌƟĐůĞ ŝƐ ƉƌĞƐĞŶƚ ŝŶ
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constant value of 1.76 × 10 C/kg, i.e., ĐĂƚŚŽĚĞ ƌĂLJƐ ĞǀĞŶ ǁŚĞŶ ĚŝīĞƌĞŶƚ
ŐĂƐĞƐ ĂŶĚ ĞůĞĐƚƌŽĚĞƐ ŽĨ ĚŝīĞƌĞŶƚ
e = 1.76 × 10 C/kg = constant ŵĂƚĞƌŝĂůƐ ĂƌĞ ƵƐĞĚ ŝŶ ĚŝƐĐŚĂƌŐĞ
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m ƚƵďĞ ĞdžƉĞƌŝŵĞŶƚƐ͘ These negaƟvely
From this experiment, Thomson concluded that all atoms contain the charged parƟcles were given
same type of negative particles called electrons. the name electron͘ ^Ž͕ ĞůĞĐƚƌŽŶƐ
ŵƵƐƚ ďĞ ƉƌĞƐĞŶƚ ŝŶ Ăůů ŵĂƩĞƌ͕
The charge and mass of an electron ŝ͘Ğ͕͘ ĞůĞĐƚƌŽŶƐ ĂƌĞ ƚŚĞ ĞƐƐĞŶƟĂů
ĐŽŶƐƟƚƵĞŶƚƐ ŽĨ ŵĂƩĞƌ͘
In 1909, an American physicist, Robert A. Millikan, accurately determined
the charge on an electron by performing his well-known oil-drop
experiment. He studied the motion of charged oil-drops in the space
between two charged metal plates. From the experiment, he found that the
charge on the oil-drops was always a multiple of an elementary charge of
1.6 × 10 –19 coulombs. From these results and those obtained from the studies
of cathode rays, it was found that the charge on the cathode ray particle was
one unit of elementary charge. This quantity of electric charge was called
electronic charge, e. Thus,
Charge on an electron, e = –1.6 × 10 –19 C
C is the symbol of coulomb unit.
From the value of e/m and e for an electron, mass of an electron was
calculated as
e 1.6 × 10 –19 C
Mass of electron, m = = = 9.1 × 10 –31 kg
e
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e/m 1.76 × 10 C/kg
What are the characteristics of an electron
Electron is a negatively charged particle having very small mass.
1
ƒ Mass of an electron (m ). The mass of an electron is about 1840 times
e
of that of a hydrogen atom. Its absolute mass is,
m = 9.108 × 10 –31 kg = 9.108 × 10 –28 g
e
It is a very light particle and therefore, it makes very little contribution
to the total mass of the atom in which it is contained.