Classification of Elements and Periodicity in Properties

This chapter tells the story of how chemists organised the 118 known elements into the modern Periodic Table — from Dobereiner's triads and Newlands' octaves to Mendeleev's brilliant predictions and the modern periodic law based on atomic number. You'll learn how the electronic configuration of an atom decides its position in the s-, p-, d-, or f-block, and then dive deep into the five big periodic trends — atomic radius, ionic radius, ionization enthalpy, electron gain enthalpy, and electronegativity. The chapter closes with periodicity in chemical properties (valence, oxide character, anomalous behaviour of the second period, and diagonal relationships) — a high-yield area for both Board exams and JEE/NEET.