Page 73 - ICSE Chemistry 8

Properties of Electrons

• An electron is a nega vity charges par cle having a Thirst for Knowledge
defi nite mass and a defi nite electric charge.
• An electron is an integral part of an atom and all ma er. The charge of an electron was
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found to be 1.602 × 10 coulombs,
• The charge to mass ra o (e/m) of electron was found to
in 1908, by R.A. Millikan.
-1
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be 1.78 × 10 C kg (coulomb per gram).
• The value of e/m is the same irrespec ve of the nature of the gas and the material of the electrodes
used in the discharge tube.
• The mass of an electron was found to be 9.1 × 10 –31 kg, which was about 1/1837 of a hydrogen
atom. Therefore, electrons are considered to have negligible mass.
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• The charge on an electron is one unit nega ve charge, i.e., 1.602 × 10 coulomb.
Discovery of Protons cathode perforated
anode rays cathode red glow
An atom is electrically neutral. This suggests that if
nega vely charged electrons are present in an atom,
there must be some posi vely charged par cles to
neutralise the nega ve charge of the electrons.
anode rays
E. Goldstein, a German scien st, proved the existence
of posi vely charged par cles in an atom. He did this by
performing an experiment in a modifi ed discharge tube,
with a perforated cathode (a cathode with small holes + –
to allow passage of posi ve rays). When a high voltage
Fig. 4.7: Emission of anode rays
of electric current was passed through this cathode with
air inside the discharge tube at a very low pressure, he observed a faint red glow behind the perforated
cathode. During the experiment, he observed another set of rays moving in the direc on opposite to that
of the cathode rays. Since these rays originated from the anode, they were named anode rays. They were
also called canal rays. The anode rays consist of the posi vely charged par cles called protons.
Characteristics of Anode Rays

• Anode rays are produced from the anode in a discharge tube when electric current is passed
through gas kept at a very low pressure.
• Anode rays consist of posi vely charged par cles Thirst for Knowledge
because when an electric fi eld was applied in the path
• Since an atom is electrically
of anode rays, the rays got a racted (or bent) towards
neutral, the number of electrons
the nega vely charged plate.
in an atom is equal to the number
• Anode rays are a stream of fast-moving charged of protons in that atom.
par cles.
• A proton is about 1840 mes
• Anode rays travel in a straight line. heavier than an electron.
Properties of Protons

• A proton is a posi vely charged par cle.
• A proton is an integral part of an atom.
• The charge to mass ra o (e/m) of proton is not fi xed, i.e., it varies.

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