Ch 3 covers the modern periodic table — its structure (s, p, d, f blocks), and periodic trends in atomic/ionic radii, ionisation enthalpy, electron gain enthalpy, and electronegativity.
Modern periodic law: properties are periodic functions of atomic number. 7 periods (rows), 18 groups (columns). s-block (Groups 1-2 + He): reactive metals. p-block (Groups 13-18): metals, metalloids, non-metals. d-block (Groups 3-12): transition metals. f-block: inner transition (lanthanides and actinides). Diagonal relationship: Li~Mg, Be~Al, B~Si.
Atomic radius: decreases across period (more nuclear charge, same shell), increases down group (more shells). Ionisation enthalpy (IE): energy to remove electron — increases across period, decreases down group. Electron gain enthalpy: energy change on gaining electron — becomes more negative across period (halogens). Electronegativity (Pauling scale): tendency to attract shared electrons — F is most electronegative.
Download: https://ncert.nic.in/textbook/pdf/kech103.pdf | Part I: https://ncert.nic.in/textbook/pdf/kech1ps.zip
Nitrogen has a half-filled 2p³ configuration (extra stability). Removing an electron from the stable half-filled subshell requires more energy. Oxygen has 2p⁴ — one orbital has a paired electron which experiences inter-electron repulsion, making it easier to remove. So IE: N > O despite O having more nuclear charge.
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