Page 101 - Chemistry ICSE Class IX
P. 101
Atomic Structure and Chemical Bonding 89
What are the properties of positive rays
Positive rays consist of positively charged particles.
The nature of these rays depends on the gas used in the discharge
tube.
These rays travel in straight lines.
These rays get de ected by an electrical eld and bend towards
the negative plate. Thus, the de ection of the positive rays is in the
direction opposite to that shown by the cathode rays.
These rays are also de ected by the magnetic elds in a direction
opposite to that of the cathode rays.
These rays can produce mechanical as well as chemical effects.
The ratio of charge (e) to mass (m), i.e., (e/m) for the particles in the
positive rays is not the same for all gases.
The ratio e/m for the positive rays is very low as compared to the e/m
value for cathode rays. This shows that the particles in positive rays
are heavier than the particles in the cathode rays.
The Proton
How was the proton discovered
Goldstein (1886) found that the charge-to-mass (e/m) ratio of the positive
ray particles depends upon the nature of the gas in the discharge tube, i.e., dŚĞ ĐŚĂƌŐĞͲƚŽͲŵĂƐƐ ;e/mͿ ƌĂƟŽ ŽĨ
ƚŚĞ ƉĂƌƟĐůĞƐ ŝŶ ĐĂƚŚŽĚĞ ƌĂLJƐ ;i.e.͕ ŽĨ
e/m values for the positive rays of different gases are different. The e/m ĂŶ ĞůĞĐƚƌŽŶͿ ǁĂƐ ĨŽƵŶĚ ƚŽ ďĞ ϭϴϰϬ
value for positive rays was the highest when hydrogen gas was used in ƟŵĞƐ ůĂƌŐĞƌ ƚŚĂŶ ƚŚĂƚ ĨŽƌ , ŝŽŶ͘ dŚŝƐ
+
the discharge tube. This is because hydrogen atom is the lightest atom of ƐŚŽǁĞĚ ƚŚĂƚ ƚŚĞ ŵĂƐƐ ŽĨ ƚŚĞ ƉĂƌƟĐůĞ
all, i.e., m is the lowest, hence, e/m is the highest. ŝŶ ĐĂƚŚŽĚĞ ƌĂLJƐ ŝƐ ϭͬϭϴϰϬ ƟŵĞƐ ƚŚĞ
The particle in the positive rays when hydrogen gas was used in the ŵĂƐƐ ŽĨ Ă ŚLJĚƌŽŐĞŶ ĂƚŽŵ͘
discharge tube, was given the name proton. A proton is produced when
one electron is removed from a hydrogen atom (H).
H o H + + e –
hydrogen atom proton electron
+
Thus, the proton is a hydrogen ion (H ).
What are the characteristics of a proton ^ŝŶĐĞ͕ ƚŚĞ ŵĂƐƐ ŽĨ ĂŶ ĞůĞĐƚƌŽŶ ŝƐ ǀĞƌLJ
th
ƐŵĂůů ;ϭͬϭϴϰϬ ŽĨ Ă ŚLJĚƌŽŐĞŶ ĂƚŽŵͿ͕
Proton is a positively charged particle. Its physical characteristics are: ƚŚĞƌĞĨŽƌĞ͕ ƚŚĞ ŵĂƐƐ ŽĨ Ă ƉƌŽƚŽŶ
Mass of a proton. The mass of a proton is equal to that of a hydrogen ŝƐ ĂůŵŽƐƚ ĞƋƵĂů ƚŽ ƚŚĞ ŵĂƐƐ ŽĨ Ă
atom. Its mass is about 1840 times that of an electron. ŚLJĚƌŽŐĞŶ ĂƚŽŵ͘
Mass of a proton = 1.676 × 10 –27 kg = 1.676 × 10 –24 g
Charge on a proton. A proton carries a charge equal in magnitude
but opposite in sign to that carried by an electron. Thus, it has a unit
positive charge.
Charge on a proton = +1.602 × 10 –19 coulombs
The Neutron
How was the neutron discovered
In 1920, Rutherford found that except hydrogen atom, the atomic masses
of other atoms could not be explained on the basis of only electrons and
protons. For example, helium atom contains two protons. As a result, the
atomic mass of helium should be double of a hydrogen atom. From the e/m
