Ace Chemistry with This All-in-One Notes Package!
Struggling to keep up with Chemistry? This Comprehensive Class 9 Chemistry Notes Package is your perfect study companion! Covering the 4 critical lessons — Matter, Is Matter Pure?, Atoms & Molecules, and Structure of Atoms — these notes provide...
CHEMISTRY
CHAPTER - 04
STRUCTURE OF ATOM
1. Atom
An atom is the smallest particle of matter. The existence of different kinds of matter is due to different atoms
constituting them. Many scientists contributed in revealing the presence of charged particles in an atom.
2. Structure of atom
Dalton’s atomic theory suggested that the atom was indivisible and indestructible. But the discovery of two
fundamental particles (electrons and protons) inside the atom, led to the failure of this aspect of Dalton’s atomic
theory.
3. Sub-Atomic Particles
∙ Electron: Electron was discovered by J.J. Thomson in cathode ray experiment, Electron contains negative charge,
it is represented by symbol e–.
∙ Proton: Proton was discovered in anode ray experiment. Anode rays are also called positive rays or canal rays.
Proton was discovered by E. Goldstein. It is represented by symbol p.
∙ Neutron: Neutron was discovered by James Chadwick. It is a neutral particle. It is represented by symbol n.
4. Models of atom
4.1 Thomson’s model of atom:
According to Thomoson’s model, electrons are embedded in the positively charged mass distributed uniformly
throughout the atomic sphere. The negative and positive charges are equal in magnitude. So atom as a whole is
electrically neutral.
4.2 Rutherford’s Model of atom:
∙ According to this model all the positively charged particles are present in a small space in the center of the atom.
This small space is called nucleus. Nearly all the mass of an atom is concentrated in the nucleus.
∙ The electrons revolve around the nucleus just as the planets revolve around the sun.
∙ Electrons (negatively charged) revolve around the nucleus in orbits with a very high speed to overcome the
electrostatic force of attraction due to positively charged particles (protons) present in the nucleus.
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