Page 74 - ICSE Chemistry 8

• The e/m value depends on the gas being used in the
Thirst for Knowledge
discharge tube.
• The mass of a proton is almost equal to the mass of an A proton is represented by the
atom of hydrogen, i.e., 1.672 × 10 –24 g. symbol +1 p . The superscript 1
1
• The charge on a proton is equal in magnitude to represents 1 mass and the subscript
–19
the charge on an electron, i.e., 1.602 × 10 C +1 represents one unit posi ve
but opposite in nature and sign. charge.

The number of electrons in an atom is equal to the number
of protons in the atom which makes the atom electrically neutral.
Thomson’s Model of an Atom
positively
charged –
The discovery of protons and electrons led to a very important ques on –
matter – –
which was s ll unanswered. The ques on was about the arrangement of –
electrons and protons in an atom. It was considered necessary to know negatively – – – –
charged
how the electrons and protons are arranged within an atom. electrons – –
So in 1904, J.J. Thomson proposed the fi rst atomic model. This model Fig. 4.8: Thomson’s plum pudding
model
was called the plum pudding model. In this model, only electrons and
protons were present. According to Thomson, each atom was a sphere of posi ve charge with electrons of
nega ve charge embedded in it. The model resembled a plum pudding, where a proton was the pudding
and electrons were embedded in it like plums. That is why the name plum pudding model was used. This
model failed to explain many experimental observa ons about the atom and so it was not accepted.

Discovery of Nucleus
Alpha Par cles
In 1911, Ernest Rutherford, a New Zealand-born Bri sh physicist, carried
Alpha par cles are posi vely
out his now famous gold foil experiment to fi nd the arrangement
charged par cles emi ed by
of electrons and protons in an atom. In his experiment, Rutherford radioac ve substances like
bombarded a thin sheet of gold of thickness 0.00004 cm with fast-moving uranium, radium, etc.
alpha () par cles, in an evacuated chamber.

Rutherford’s Observations

On the basis of his experiment, Rutherford radioactive sample emits beam
made the following observa ons. of alpha particles
• Most of the alpha par cles passed
straight through the foil without any
defl ec on from their path.
lead block shield
• Few par cles were defl ected at very
gold foil
large angles.
zinc sulphide screen
• A small frac on of the par cles were
defl ected from their path at small most alpha particles
angles. hit here some alpha particles are defl ected
Fig. 4.9: Gold foil experiment

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