Question 5Compare all the proposed models of an atom given in this chapter.

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Thomson's ModelRutherford's ModelBohr's modelIt was proposed by J.J. Thomson.It was proposed by Ernst Rutherford.It was proposed by Neil Bohr.An atom consists of a positively charged sphere and the electrons are embedded in it.There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.There is very large empty space between nucleus and electrons.While revolving in discrete orbits the electrons do not radiate energy.It could not explain the result of Rutherford's ∝–particle scattering experiment.It could not explain the stability of atom because revolving electron will lose energy and will ultimately fall into the nucleus.This model has overcome the limitations of the Rutherford model by defining energy levels for electrons.

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  Science | Chapter 4: Structure of the AtomWeb Content &mdash; Quick Reference
  
    Chapter 4: Structure of the Atom &mdash; Quick Reference

Thomson model
      Rutherford experiment
      Bohr model
      electron
      proton
      neutron
      isotopes
      isobars
      electronic configuration

Quick Revision Points
      
        Thomson: positive sphere with embedded electrons (plum pudding)
Rutherford: nucleus (dense, positive) + electrons in orbits. Limitation: electron should spiral in
Bohr: electrons in fixed energy levels (shells K, L, M, N)
Max electrons per shell: 2n² (K=2, L=8, M=18, N=32); outermost max 8
Z = protons = electrons; A = protons + neutrons; neutrons = A − Z
Isotopes: same Z, different A. Isobars: different Z, same A
Valency: ≤ 4 → valency = outermost e⁻; > 4 → valency = 8 − outermost e⁻

Exam Tips for Chapter 4
      
        Read the detailed chapter notes for complete coverage of all NCERT topics.
        Practice all NCERT in-text and back exercise questions &mdash; they are frequently asked in exams.
        Focus on comparison tables, diagrams, and definitions &mdash; these are high-scoring areas.
        For numericals (if applicable), practice at least 20 problems of varying difficulty.
        Refer to the practice question bank (200+ questions) for thorough preparation.

Related Resources
      
        Detailed Notes: ch04-structure-of-the-atom.html
        Practice Questions: 100+ questions with answers in 05-practice-questions/
        Chapter Test: 30-mark test paper in 06-tests/chapter-tests-30marks/
        Formula Sheet: Complete formula reference in 03-teacher-aid/formula-sheet.html

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Structure of the Atom — Question 5

Question 5

Chapter 4: Structure of the Atom — Quick Reference

Quick Revision Points

Thomson's Model	Rutherford's Model	Bohr's model
It was proposed by J.J. Thomson.	It was proposed by Ernst Rutherford.	It was proposed by Neil Bohr.
An atom consists of a positively charged sphere and the electrons are embedded in it.	There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.	Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.	There is very large empty space between nucleus and electrons.	While revolving in discrete orbits the electrons do not radiate energy.
It could not explain the result of Rutherford's ∝–particle scattering experiment.	It could not explain the stability of atom because revolving electron will lose energy and will ultimately fall into the nucleus.	This model has overcome the limitations of the Rutherford model by defining energy levels for electrons.