Atomic Structure — Class 9 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 9 Science, Chemistry — Chapter 11. Atoms are the building blocks of chemistry. Understanding subatomic particles (protons, neutrons, electrons), Bohr's atomic model, and electron configurations explains elements' chemical reactivity, valency, and isotopic behaviors.

1. About this chapter

This chapter covers atomic models, subatomic particles, valency, electron configuration, and isotopes.

2. Subatomic Particles

• Proton: Discovered by Goldstein/Rutherford, positive charge ($+1.6\times 10^{-19}$ C).
• Electron: Discovered by J.J. Thomson, negative charge ($-1.6\times 10^{-19}$ C), negligible mass.
• Neutron: Discovered by Chadwick, no charge, mass equal to proton.

3. Atomic Models and Bohr Postulates

• Rutherford Model: Discovered nucleus, proposed planetary model.
• Bohr Model: Electrons revolve in fixed non-radiating orbits (shells K, L, M, N).
• Shell capacity: $2n^2$. K shell ($n=1$) holds 2, L shell ($n=2$) holds 8.

4. Atomic and Mass Numbers

• Atomic Number (Z): Number of protons or electrons in neutral atom.
• Mass Number (A): Sum of protons and neutrons in nucleus.
• Isotopes: Same element with same atomic number, different mass numbers (e.g. ${}^{12}\text{C}$ and ${}^{14}\text{C}$).
• Isobars: Different elements with same mass number, different atomic numbers (e.g. ${}^{40}\text{Ar}$ and ${}^{40}\text{Ca}$).