Chapter 3 — Classification of Elements and Periodicity in Properties

Chapter 3 of the Class 11 Chemistry NCERT textbook, "Classification of Elements and Periodicity in Properties", organizes the 118 known elements in order of atomic number into 7 periods and 18 groups, revealing periodic trends in physical and chemical properties such as atomic radius, ionization enthalpy, electron gain enthalpy, and electronegativity.

• How the Periodic Table came to be — The chapter shows the historical journey from Dobereiner's triads through Mendeleev's predictive table to the Modern Periodic Law, where properties are periodic functions of atomic number rather than atomic weight.
• Structure of periods, groups, and blocks — It explains how electronic configuration organises elements into seven periods and eighteen groups, and into s-, p-, d-, and f-blocks, so an element's position predicts its valence behaviour.
• Periodic trends and why they occur — Atomic radius, ionization enthalpy, electron gain enthalpy, and electronegativity vary systematically across periods and down groups because of competing nuclear charge and added electron shells.
• Reactivity linked to position — The chapter connects trends to behaviour: metals at the left lose electrons easily and non-metals at the right gain them, explaining why reactivity peaks at period extremes and groups behave alike.

1. 01The Modern Periodic Law states that physical and chemical properties of elements are periodic functions of their atomic numbers, replacing Mendeleev's original atomic weight-based classification.
2. 02Seven periods and eighteen groups organize elements by electronic configuration: period number equals the highest principal quantum number (n), while group determines valence electron configuration.
3. 03Atomic radius decreases left-to-right across periods (increasing nuclear charge) but increases down groups (additional electron shells). Ionization enthalpies follow the opposite trend, with noble gases showing maximum values and alkali metals showing minima.
4. 04Elements classified into four blocks—s-block (Groups 1-2), p-block (Groups 13-18), d-block (Groups 3-12), and f-block (lanthanoids/actinoids)—based on which orbitals receive the final electrons during aufbau.
5. 05Chemical reactivity is highest at period extremes: alkali metals (left) lose electrons readily (low ionization enthalpy), while halogens (right) gain electrons readily (high electron gain enthalpy). Center elements show lowest reactivity and form amphoteric oxides.
6. 06Electronegativity increases across periods and decreases down groups, inversely correlating with metallic character. Electrons gain enthalpy becomes more negative across periods, measuring atomic ability to accept electrons.