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Classification of Elements and Periodicity in Properties

NCERT Class 11 Chemistry Chapter 3

Classification of Elements and Periodicity for Class 11 Chemistry, JEE and NEET: the modern periodic table, s, p, d and f blocks and periodic trends.

7 topics Updated 8 Nov 2026

For: Class 11, JEE and NEET, unless a topic says otherwise

In the syllabus: JEE Class 11 syllabus, NEET Class 11 syllabus

With your textbook

NCERT Chemistry Part I (Class 11), Chapter 3 “Classification of Elements and Periodicity in Properties”, reprint 2026-27.

Open the official NCERT chapter (PDF, 3.4 MB, ncert.nic.in)

Checked on 19 Oct 2026.

Learn in this order

  1. Why do we classify elements?

    Why do we need to classify elements? There are too many to study one by one; a classification organises known facts and predicts new ones, for Class 11.

    Class 11 only NCERT Section 3.1 8 min read 4 solved examples 4 practice questions

  2. Genesis of periodic classification

    Mendeleev’s periodic table and the attempts before it: Döbereiner’s triads, Newlands’ octaves, and Mendeleev’s gaps and predictions, for Class 11.

    Class 11 only NCERT Section 3.2 12 min read 5 solved examples 7 practice questions

  3. Modern periodic law and the periodic table

    How Moseley showed atomic number sets the order, and the modern periodic table’s 7 periods and 18 groups.

    NCERT Section 3.3 12 min read 5 solved examples 8 practice questions

  4. Naming elements with atomic numbers above 100

    Digit roots, “ium”, three-letter symbols like Ubn, and official names up to 118.

    NCERT Section 3.4 9 min read 5 solved examples 7 practice questions

  5. Electronic configuration and the periodic table

    Electronic configuration and the periodic table: why periods hold 2, 8, 8, 18, 18, 32 and 32 elements, and how to find an element’s period and group.

    NCERT Section 3.5 12 min read 5 solved examples 8 practice questions

  6. s, p, d and f blocks of the periodic table

    Outer configurations, properties, the places of H and He, and metals, non-metals and metalloids.

    NCERT Section 3.6 15 min read 4 solved examples 7 practice questions

  7. Periodic trends in properties of elements

    Atomic and ionic radii, ionisation enthalpy, electron gain enthalpy, electronegativity.

    NCERT Section 3.7 20 min read 7 solved examples 14 practice questions

Key formulas in this chapter

Why classify elements?

  • Known elements: 31 (1800), 63 (1865), 118 (today)
  • Same family, same type of compound: NaCl, KCl; NaO, KO

Genesis of periodic classification

  • Triad rule: middle atomic weight ≈ (first + third)/2
  • Li, Na, K: (7 + 39)/2 = 23
  • Octaves: element 8 resembles element 1 (Li → Na → K)
  • Mendeleev: properties = periodic function of atomic weight
  • Mendeleev’s highest oxides, Groups I–VIII: RO, RO, RO, RO, RO, RO, RO, RO

Modern periodic law

  • Modern law: properties = periodic function of atomic number Z
  • Moseley: √ν ∝ (Z − 1); ν ≈ 2.47 × 10 (Z − 1) Hz
  • Period number = principal quantum number n of the outer shell
  • Period lengths: 2, 8, 8, 18, 18, 32, 32 (total 118)

Naming elements above 100

  • Name = roots of the digits in order + “ium”
  • 0 nil, 1 un, 2 bi, 3 tri, 4 quad, 5 pent, 6 hex, 7 sept, 8 oct, 9 enn
  • Symbol = first letter of each root (3 letters)
  • bi/tri + ium → bium/trium; enn + nil → ennil

Configurations and the periodic table

  • Period number usually = n of the outermost occupied shell (except Pd)
  • Elements in a period = 2 × orbitals filled in it
  • Period 4: 4s + 3d + 4p = 1 + 5 + 3 orbitals → 18
  • Period 6: 6s + 4f + 5d + 6p = 1 + 7 + 5 + 3 orbitals → 32

s, p, d and f blocks

  • s-block: ns; p-block: nsnp
  • d-block: (n − 1)dns
  • f-block: typical (n − 2)f(n − 1)dns; exceptions include Th and Lr
  • Columns per block = electrons per subshell: 2, 6, 10, 14
  • Group number: s metals = ns electrons (He is group 18); d = (n − 1)d + ns electrons; p = 10 + ns + np electrons
  • Ionisation: X(g) → X(g) + e (always positive; second > first)
  • Electron gain: X(g) + e → X(g)
  • Electron gain enthalpy = −(electron affinity) − (5/2)RT (NCERT ideal-gas thermal convention)
  • Covalent radius = ½ × bond length (Cl: 198/2 = 99 pm)
  • Valence = outer electrons or 8 − outer electrons

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