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NCERT Solutions for Class 9th Science Chapter 8 Journey Inside the Atom

Updated on 2026-09-19

About this chapter

An atom is not indivisible. Thomson's cathode-ray work (1897) showed that every atom contains electrons — negatively charged particles of charge –1.602 × 10 –19 C, taken as –1 by convention, and far lighter than the atom itself. The gold foil experiment (Geiger and Marsden, 1911) settled the atom's shape. Most α-particles went straight through, so the atom is mostly empty; a few bounced back, so all the positive charge and nearly all the mass sit in a tiny, dense nucleus . Diameter of atom ≈ 10 –10 m, of nucleus ≈ 10 –15 m — a factor of 10 5 . Rutherford's planetary model could not explain stability : an electron in a circular orbit is accelerating, so it should radiate energy and spiral into the nucleus. Bohr (1913) fixed this by postulating stationary states — fixed shells K, L, M, N (n

  • Chapter opener
  • .1 Rediscovering the Roots of Atomic Theory
  • .2.1 Thomson's model of an atom
  • .2.2 A. Rutherford's model of an atom
  • .2.2 C. Discovery of the proton
  • .2.3 Bohr's model of the atom
  • .3 What Components Contribute to the Mass of an Atom?
  • .3.1 Discovery of the Neutron
  • .4 Symbols of Elements
  • .5 Atomic Number
Quick revision
TermWhat it meansKey number or ruleWhere it appears
Electron (e–)Negatively charged subatomic particle outside the nucleusCharge –1.602 × 10–19 C, taken as –1; mass negligible8.2 Thomson's cathode-ray experiment
Proton (p+)Positively charged particle in the nucleusRelative charge +1; mass ≈ 1 u8.2.2 C, discovered by Rutherford
Neutron (n0)Uncharged particle in the nucleusRelative charge 0; mass ≈ 1 u8.3.1, discovered by Chadwick, 1932
NucleusTiny dense centre holding all the positive charge and nearly all the massDiameter ≈ 10–15 m, i.e. 105 times smaller than the atom8.2.2 A, gold foil experiment
Shell / energy levelFixed circular path in which an electron has a definite energyNamed K, L, M, N for n = 1, 2, 3, 48.2.3 Bohr's model
Atomic number (Z)Number of protons in the nucleusEquals the number of electrons in a neutral atom8.5 Atomic Number
Mass number (A)Total number of nucleons (protons + neutrons)A = p+ + n0; written as AZX8.6 Mass Number
Bohr–Bury rulesRules for filling shells with electronsMaximum 2n² per shell; outermost shell ≤ 8; fill K → L → M8.7
Valence electronsElectrons in the outermost (valence) shellDecide chemical behaviour; 8 of them = octet = stable8.8 Valency
ValencyCombining capacity — electrons gained, lost or shared to get an octetNa (2,8,1) → 1; O (2,6) → 2; C (2,4) → 48.8 Valency
IsotopesAtoms of one element with the same Z but different A11H, 21H, 31H; 126C, 136C, 146C8.9.1 Isotopes
IsobarsAtoms of different elements with the same A but different Z4018Ar, 4019K, 4020Ca8.9.2 Isobars
Average atomic massMass of an element averaged over its isotopes by abundanceCl: 35 × 75/100 + 37 × 25/100 = 35.5 u8.9.1 A
Useful isotopesIsotopes put to work in industry and medicine23592U fuel, 6027Co cancer therapy, 13153I thyroid, 146C datingBridging Science and Society, p. 155
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Exercises

  1. Chapter opener — Think It Over Page 140
  2. Chapter opener — In-text Questions Page 140
  3. .1 Rediscovering the Roots of Atomic Theory — In-text Questions Page 1418
  4. .2.1 Thomson's model of an atom — Pause and Ponder Page 1438
  5. .2.2 A. Rutherford's model of an atom — Ready to Go Beyond Page 1448
  6. .2.2 A. Rutherford's model of an atom — Think as a Scientist Page 1448
  7. .2.2 A. Rutherford's model of an atom — Pause and Ponder Page 1448
  8. .2.2 C. Discovery of the proton — Pause and Ponder Page 1458
  9. .2.3 Bohr's model of the atom — In-text Questions Page 1468
  10. .2.3 Bohr's model of the atom — Threads of Curiosity Page 1468
  11. .3 What Components Contribute to the Mass of an Atom? — What if … Page 1478
  12. .3.1 Discovery of the Neutron — Threads of Curiosity Page 1478
  13. .3.1 Discovery of the Neutron — India's Scientific Contributions Page 1488
  14. .4 Symbols of Elements — Pause and Ponder Page 1498
  15. .5 Atomic Number — In-text Questions Page 1508
  16. .6 Mass Number — Pause and Ponder Page 1508
  17. .7 How Are Electrons Distributed in Different Energy Levels? — In-text Questions Page 1518
  18. .7.1 Building up atoms — Pause and Ponder Page 1528
  19. .8 Combining Capacity of an Atom: Valency — In-text Questions Page 1538
  20. .9.1 Isotopes — In-text Questions Page 1548
  21. .9.1 A. Average atomic mass — Pause and Ponder Page 1568
  22. End-of-chapter exercise — Revise, Reflect, Refine Page 158 – 160
  23. Project work — The Journey Beyond Page 161
  24. An open question to carry forward — The Quest Continues … Page 161
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