Page 102 - Chemistry ICSE Class IX

90 ICSE Chemistry – 9
values, it was discovered that the mass of helium atom (to be precise, that
of a helium ion), is four times the mass of a hydrogen atom. To explain
this, Rutherford, in 1920, predicted the presence of another kind of particle
in atoms. He further predicted that such a particle should be electrically
neutral and have a mass equal to that of a proton.
What are the characteristics of a neutron
dŝůů ϭϵϮϬ͕ ĂŶ ĂƚŽŵ ǁĂƐ ĐŽŶƐŝĚĞƌĞĚ
ƚŽ ĐŽŶƐŝƐƚ ŽĨ ŽŶůLJ ĞůĞĐƚƌŽŶƐ ĂŶĚ ƒ Mass of a neutron. The mass of a neutron is equal to that of a proton.
ƉƌŽƚŽŶƐ͘ ůĞĐƚƌŽŶƐ ŚĂǀĞ ŶĞŐůŝŐŝďůĞ Thus,
ŵĂƐƐ͘ ^Ž͕ ƚŚĞ ǁŚŽůĞ ŵĂƐƐ ŽĨ ĂŶ ĂƚŽŵ Mass of a neutron = 1.676 × 10 –27 kg = 1.676 × 10 –24 g
ǁĂƐ ĐŽŶƐŝĚĞƌĞĚ ƚŽ ďĞ ŽŶůLJ ĚƵĞ ƚŽ ƚŚĞ
ƉƌŽƚŽŶƐ ƉƌĞƐĞŶƚ ŝŶ ŝƚ͘ ƒ Charge on a neutron. Neutrons do not carry any electrical charge. So,
>ĂƚĞƌ͕ ŝŶ ϭϵϯϮ͕ ŚĂĚǁŝĐŬ ĚŝƐĐŽǀĞƌĞĚ neutrons are neutral particles.
ƚŚĞ ƉƌĞƐĞŶĐĞ ŽĨ ŶĞƵƚƌĂů ƉĂƌƟĐůĞƐ
ŝŶƐŝĚĞ ƚŚĞ ĂƚŽŵƐ͘ ^ŝŶĐĞ͕ ƚŚĞƐĞ How do electrons, protons and neutrons compare with
ƉĂƌƟĐůĞƐ ĂƌĞ ŶĞƵƚƌĂů͕ ŚĞŶĐĞ ƚŚĞƐĞ each other
ǁĞƌĞ ŶĂŵĞĚ neutrons.
Electrons, protons and neutrons are the most important subatomic particles
for chemists. Their properties are compared in Table 4.1.

Table 4.1 Characteristics of an electron, proton and a neutron
Property Electron Proton Neutron
Mass The mass of an electron is about The mass of a proton is equal to The mass of a neutron is equal
1/1840 of that of a hydrogen atom. that of a hydrogen atom. Its mass to that of a proton.
Mass of an electron, is about 1840 times the mass of Mass of a neutron
m = 9.1 × 10 –31 kg an electron. = 1.676 × 10 –27 kg
e
= 9.1 × 10 –28 g Mass of a proton = 1.676 × 10 –24 g
= 1.676 × 10 –27 kg
= 1.676 × 10 –24 g
Charge An electron carries a unit negative It carries a charge equal in It carries no charge, i.e., it is a
charge. magnitude but opposite in sign neutral particle.
Charge on an electron to that carried by an electron.
e = –1.602 × 10 –19 coulombs Thus, it possesses a unit positive
charge.
Charge on a proton
= 1.602 × 10 –19 coulombs

Illustrative Questions

Q.1. What are the different stages observed when a discharge tube is connected to a high voltage source and the
pressure of the air in it is reduced gradually?
Ans. The various stages observed when a discharge tube is connected to a high voltage and the pressure of the air in
it is gradually reduced are:
Stage 1: Flickering light emission.
Stage 2: Entire tube is lled with nearly uniform glow. With air in the tube, colour of the glow is magenta-red.
Stage 3: The uniform glow is intercepted by dark bands, which are perpendicular to the axis of the tube.
Stage 4: The discharge vanishes and the tube appears dark. The end of the tube opposite to the cathode emits
greenish-yellow light. This is the uorescence of the glass. The radiation emitted by the cathode are called
cathode rays.
Q.2. What happens when cathode rays are passed through an electric eld between two parallel plates? Can one
determine the nature of charge of the particles constituting the cathode rays from this experiment? If so how?
Ans. The cathode rays when passed through an electric eld, get de ected towards the anode (positively charged
plate).