NCERT Solutions for Class 9 Science Chapter 4 Structure of the Atom
NCERT Solutions for Class 9 Science Chapter 4 Structure of the Atom covers subatomic particles (electrons, protons, neutrons), Thomson's model, Rutherford's scattering experiment, Bohr's model, electron distribution, valency, atomic number, mass number, isotopes, and isobars.
Overview of Topics & Subtopics (Click to Jump)
1. Charged Particles in Matter & Thomson's Atomic Model
Short Note:
• Subatomic Particles:
- Electron (e-): Discovered by J.J. Thomson; charge is -1, mass is negligible (1/2000 of a hydrogen atom).
- Proton (p+): Discovered by E. Goldstein (canal rays); charge is +1, mass is 1 unit.
• Thomson's Model of Atom (Watermelon Model): An atom consists of a positively charged sphere with electrons embedded in it like seeds in a watermelon. Total positive charge equals total negative charge, making the atom electrically neutral.
2. Rutherford's Scattering Experiment & Bohr's Atomic Model
Short Note:
• Rutherford's Gold Foil Experiment: Fast moving alpha (α) particles were bombarded on thin gold foil.
- Most α-particles passed straight through (most space in an atom is empty).
- Some were deflected by small angles (positive charge occupies small space).
- 1 out of 12,000 rebounded back (all positive charge and mass concentrated in a tiny central nucleus).
• Bohr's Model of Atom: Electrons revolve in special discrete orbits (energy levels labeled K, L, M, N or n = 1, 2, 3, 4) without radiating energy.
3. Neutrons & Distribution of Electrons (Bohr-Bury Scheme)
Short Note:
• Neutron (n): Discovered by J. Chadwick in 1932; neutral subatomic particle present in the nucleus (except hydrogen). Mass is nearly equal to a proton.
• Bohr-Bury Rules for Electron Distribution:
1. Maximum electrons in a shell = 2n2 (K=2, L=8, M=18, N=32).
2. The outermost orbit can accommodate a maximum of 8 electrons.
3. Shells are filled in a step-wise manner.
4. Valency, Atomic Number, Mass Number, Isotopes & Isobars
Short Note:
• Valency: Combining capacity of an atom (Valency = valence electrons if ≤ 4; or 8 − valence electrons if > 4).
• Atomic Number (Z): Total number of protons in the nucleus of an atom.
• Mass Number (A): Total number of protons and neutrons (nucleons) in the nucleus.
• Isotopes: Atoms of the same element with the same atomic number (Z) but different mass numbers (A) (e.g., 1H, 2H, 3H).
• Isobars: Atoms of different elements with different atomic numbers but the same mass number (e.g., 40Ca and 40Ar).
Intext Questions - Page Number: 47
Intext Questions - Page Number: 49
Textbook Chapter End Exercise Questions
• Electron: Charge = −1 unit; Mass = 1/2000 u (negligible); Location = Revolves outside nucleus in shells.
• Proton: Charge = +1 unit; Mass = 1 u; Location = Inside nucleus.
• Neutron: Charge = 0 (Neutral); Mass = 1 u; Location = Inside nucleus.
• Silicon (Z=14): Electronic configuration = 2, 8, 4. Outer shell has 4 valence electrons. Therefore, Valency of Silicon = 4.
• Oxygen (Z=8): Electronic configuration = 2, 6. Outer shell has 6 valence electrons. Valency = 8 − 6 = 2.
(i) Atomic Number (Z): Equal to number of protons = 8 (Oxygen).
(ii) Charge: Since number of protons (8) = number of electrons (8), the net charge on the atom is 0 (Neutral).
Read More & Chapter Resources
These short notes are designed for quick revision. Read the main points once, then use the related material below to practise the same chapter. Revising a concept and checking it with questions can make your preparation more focused.