Ch 4 explores atomic structure through the historical development of models — Thomson, Rutherford, and Bohr. Students learn about subatomic particles, electron shells, valency, isotopes, and isobars.
Thomson (1897): atom as positive sphere with embedded electrons (plum pudding). Rutherford (1911): gold foil experiment → most of atom is empty space, mass concentrated in tiny positive nucleus, electrons orbit around it. Bohr (1913): electrons occupy specific energy levels (shells K, L, M, N).
Proton: +1 charge, mass ≈ 1 u, in nucleus. Neutron: 0 charge, mass ≈ 1 u, in nucleus. Electron: −1 charge, mass ≈ 1/1836 u, in shells. Atomic number (Z) = number of protons. Mass number (A) = protons + neutrons. Atom is neutral: protons = electrons.
Maximum electrons per shell: K=2, L=8, M=18. Valency = combining capacity, determined by outermost shell electrons. Valency = 8 − outermost electrons (if >4) or = outermost electrons (if ≤4). Isotopes: same Z, different A (¹²C, ¹⁴C). Isobars: same A, different Z (⁴⁰Ca, ⁴⁰Ar).
Download: https://ncert.nic.in/textbook/pdf/iesc104.pdf | Complete book: https://ncert.nic.in/textbook/pdf/iesc1ps.zip
Isotopes are atoms of the same element with the same atomic number (same protons) but different mass numbers (different neutrons). Example: Carbon-12 (6p + 6n) and Carbon-14 (6p + 8n). They have the same chemical properties but different physical properties.
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