Structure Of Atoms Class 9 Notes CBSE Science Chapter 4
CBSE NCERT Class 9 Science Revision Notes has chapter 4 on Structure of Atoms, the topics covered in the textbook are part of the revision notes for class . Class 9th Science Revision Notes include an overview of the Main points are given of each concept and topic in Class 9 NCERT Science Textbook of this chapter. includes in-depth explanations of topics/concepts for all topics in the book such as definition of molecules, definition of atoms and molecules, study notes on atoms and molecules, definition of Laws of chemical combination, Dalton's atomic theory · Atoms, symbols of elements, writing chemical formulas, molars. concept and molecular mass.
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Structure Of Atoms
Dalton considered atom to be an indivisible particle but this concept is rejected at the end of
19th Century. When Scientists contributed in revealing the presence of charge particle in an
atom.
These particles were called the Sub-Atomic particle.
• Discovery of Electrons
J.J Thomson
• Discovery of Protons
Rutherford
• Discovery of Neutron
Chadwick
• Discovery of Canal Rays
E. Goldstein
Particle
Charge
Mass
𝑒−
−1.6 × 10−19𝐶
9.1 × 10−31 𝐾𝑔
𝑝+
+1.6 × 10−19𝐶
1.67 × 10−27 𝐾𝑔
𝑛
No charge
1.67 × 10−27 𝐾𝑔
Canal Ray (Or Anode Rays)
Stream of Positively charged particles, which move towards negatively charge electrode.
Structure of an Atom
Thomson’s Model of an Atom
• Compared to Christmas Pudding: Electrons are dry fruits of Christmas pudding, inside
the positive sphere.
• Compared with Watermelon: Red part of watermelon is positive part of Atom and
electrons are studded in positive part in the water melon.
Postulates of Thomson
• An atom consists of a positively charged sphere and the electrons and embedded in it.
• The negative and positive charges are equal in Magnitude, so atom as a whole is
electrically Neutral.
“Thomson’s Model explained that atom are electrically neutral”
Rutherford’s Atomic Model
Rutherford is famous for -Ray scattering experiment. Fast moving -particles were made to
fall on a thin gold foil.
Observation made by Rutherford in his experiment.
• Most of -particles passed via gold foil undeflected.
• Some of the -particles deflected by foil by small angle.
• One out of every 12000 particles appeared to Rebound
Conclusion
• Atom consists of empty space as most of -particles passed through gold Foil
undeflected.
• Since, some of the -particles were deflected by the foil by small angle and very few
bounced back this lead to conclusion that most of the space in an atom is empty.
• Atom contain nucleus in the center of Atom (Carrying positively charged particles) i.e.
one in 12000 bounced back.
• Size of nucleus was about 10−5
times the size of the atom.
Features of Rutherford proposed Model of Atom
• There is positively placed nucleus in an atom. Nearly all the mass present in Nucleus
(Proton + Neutrons)
Eg.
No. of Neutrons present in the Nucleus = 6
No. of Neutrons present in the Nucleus = 6
Mass No. = 6 + 6 = 12
• Electrons Revolve around the Nucleus in well defined orbit.
• Size of Nucleus is very small as compared to the size of atom.
Drawback of Rutherford’s Model
Rutherford’s model could not explain the stability of the atom
this is because according to Rutherford’s model an atom consist
of a small heavy positively charged nucleus in the centre and
the electrons are revolving around it. These revolving electrons
would lose energy as they are charged particles and due to
acceleration charged particle would Radiate energy. Ultimately
the e− should fall into the Nucleus
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