Chapter 3 — Classification of Elements and Periodicity in Properties
Open PDFReads in your browser→Summary
The Periodic Table 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.
Classification of Elements and Periodicity in Properties explores the historical development of the Periodic Table from Dobereiner's Triads through Mendeleev's groundbreaking predictions to the Modern Periodic Law. The chapter explains how elements arranged by atomic number display periodic trends: atomic radii decrease across periods but increase down groups, ionization enthalpies generally increase across periods and decrease down groups, and elements are classified into s-, p-, d-, and f-blocks based on their electronic configurations. Understanding these periodic trends reveals why chemical reactivity peaks at period extremes and why elements in the same group share similar chemical behavior.
Key points & formulas
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
Frequently asked questions
01Why did Mendeleev leave gaps in his Periodic Table?
Mendeleev recognized that some elements were still undiscovered and left gaps in his table (under aluminum and silicon, which he called Eka-Aluminum and Eka-Silicon). He predicted not only that gallium and germanium would be discovered later, but also described their general physical properties with remarkable accuracy. When these elements were discovered, they matched his predictions so closely that his Periodic Table became famous.
02What is the difference between Mendeleev's Periodic Law and the Modern Periodic Law?
Mendeleev's original law stated that the properties of elements are a periodic function of their atomic weights. However, Henry Moseley's 1913 work with X-ray spectra showed that atomic number, not atomic mass, is the more fundamental property. The Modern Periodic Law states that physical and chemical properties of elements are periodic functions of their atomic numbers, which is why elements like iodine (with lower atomic weight than tellurium) are placed with halogens based on their similar properties.
03Why do elements in the same group have similar chemical properties?
Elements in the same vertical column (group) have the same number and same distribution of electrons in their outermost orbitals, giving them similar valence shell electronic configurations. For example, all Group 1 alkali metals have ns1 outermost electronic configuration, making them all highly reactive metals that readily lose one electron to form 1+ ions. This similarity in outer electron arrangement directly causes their similar chemical behavior.
04Is the NCERT Class 11 Chemistry Chapter 3 PDF free to download?
Yes, the NCERT Class 11 Chemistry Chapter 3 PDF is available free to download. NCERT textbooks are published by India's National Council of Educational Research and Training as official curriculum materials and are freely accessible to all students.
More chapters in Chemistry Part I
This is the complete Chemistry Part I Chapter 3 as published by NCERT — every diagram, solved example, and exercise included, free. Browse all NCERT Class 11 textbooks.
Read offline with notes, solutions & mock tests
CBSE Prepmaster — free on iOS & Android