class9 ch 4 class best notes

This chapter explains how scientists discovered the internal structure of the atom. It begins with the discovery of subatomic particles – electron (J.J. Thomson, 1897), proton (Rutherford, 1911), and neutron (Chadwick, 1932). Different atomic models were proposed: Thomson’s Model (Plum Pudding Model) – the atom is a sphere of positive charge with electrons embedded in it. Rutherford’s Model – based on the gold foil experiment, he concluded that the atom has a small, dense, positively charged nucleus at the center with electrons revolving around it. Limitation: it could not explain electron stability. Bohr’s Model – electrons revolve in fixed circular orbits (energy levels), and can jump between levels by absorbing or emitting energy. The chapter also defines atomic number (Z) as the number of protons, and mass number (A) as the sum of protons and neutrons. It explains isotopes (same Z, different A) and isobars (same A, different Z). Finally, it discusses electronic configuration, the arrangement of electrons in shells (K, L, M, N), following the 2n² rule (maximum electrons in nth shell = 2n²). In short, this document gives a clear explanation of the structure of atoms, discovery of subatomic particles, atomic models, isotopes, isobars, and electronic configuration, which form the basis of modern atomic theory.