Class 11 Classification of Elements and Periodicity in Properties
Class 10 leaves you with the history of the periodic table — Dobereiner, Newlands, Mendeleev and the modern periodic law. The NCERT Class 11 chapter "Classification of Elements and Periodicity in Properties" builds the deeper, quantitative table you will use for the rest of chemistry: the 18-group long form, the four blocks defined by electronic configuration, and numerical periodic trends you are expected to compare and explain, not just recite. This guide covers that Class 11 treatment with real data.
The modern long-form periodic table
The modern periodic table has 18 vertical groups and 7 horizontal periods, with elements arranged strictly by increasing atomic number. Every element is assigned to a block based on the subshell being filled last by its electronic configuration (the "distinguishing electron"):
| Block | Groups | Distinguishing subshell | Character |
|---|---|---|---|
| s-block | 1, 2 | ns | Reactive metals (alkali, alkaline earth) |
| p-block | 13–18 | np | Metals, non-metals, metalloids and noble gases |
| d-block | 3–12 | (n−1)d | Transition metals |
| f-block | Below the main table | (n−2)f | Lanthanoids and actinoids (inner-transition elements) |
Worked example 1 — finding the block from atomic number
Identify the block for iron (Z = 26) and barium (Z = 56).
Iron: [Ar] 3d⁶ 4s² — the last electron enters a 3d subshell, so iron is a
d-block (transition) element.
Barium: [Xe] 6s² — the last electron enters a 6s subshell, so barium is an
s-block element.
IUPAC nomenclature for elements beyond atomic number 100
Elements with very high atomic numbers are given a temporary, systematic IUPAC name built directly from their atomic number using numerical roots, until a permanent name is approved:
Worked example 2 — naming element 120
Give the IUPAC systematic name and symbol for the element with atomic number 120.
Digits: 1, 2, 0 → roots: un, bi, nil
Name = un + bi + nil + ium = Unbinilium
Symbol = first letter of each root = Ubn
Periodic trends — the reasoning, not just the direction
Every trend in this chapter is explained by two competing effects: increasing nuclear charge (more protons pull electrons in more strongly) and increasing shielding/screening by inner-shell electrons (which weakens that pull). Across a period, nuclear charge increases while the number of shells stays the same, so effective nuclear charge rises steadily. Down a group, a new shell is added each time, and the extra shielding this provides outweighs the rise in nuclear charge.
Down a group: atomic radius increases, ionisation enthalpy generally decreases, electronegativity generally decreases
Worked example 3 — comparing atomic radii
Which is larger, sodium or magnesium (same period)? Which is larger, sodium or potassium (same group)?
Na (≈186 pm) is larger than Mg (≈160 pm): both are in Period 3, but Mg has one more proton pulling on the same third shell, so its electrons are held more tightly.
K (≈227 pm) is larger than Na (≈186 pm): potassium has an extra occupied shell (the fourth), and the shielding this adds outweighs potassium's higher nuclear charge.
Worked example 4 — the period-3 ionisation-enthalpy anomalies
First ionisation enthalpies (kJ/mol) across Period 3: Na 496, Mg 738, Al 577, Si 786, P 1012, S 1000, Cl 1251, Ar 1521. The overall trend rises left to right, but Mg > Al and P > S. Explain both anomalies.
Mg > Al: magnesium's configuration is 3s² (a completely filled subshell, which is extra stable); aluminium's is 3s²3p¹, and that single, higher-energy 3p electron — well shielded by the filled 3s² pair — is easier to remove despite Al's higher nuclear charge.
P > S: phosphorus's configuration is 3s²3p³ (a half-filled p subshell, which is extra stable due to symmetric electron distribution and minimal inter-electronic repulsion); sulphur's is 3s²3p⁴, where one 3p orbital now holds a electron pair, and the repulsion between those two paired electrons makes one of them easier to remove.
Common mistakes that cost marks
- Treating ionisation enthalpy as a perfectly smooth trend. The Mg>Al and P>S anomalies (and their analogues in later periods) are specifically tested and must be explained using half-filled/fully-filled stability, not waved away.
- Confusing group number with number of valence electrons for d-block elements — the simple "group number = valence electrons" rule from Class 10 applies cleanly only to s- and p-block (representative) elements.
- Mixing up which factor dominates where. Effective nuclear charge dominates across a period; the addition of a new shell dominates down a group. Reversing this explanation in an answer is a very common error.
- Getting the IUPAC root order wrong. The digits are read left to right exactly as the atomic number is written — for Z = 120 that is 1-2-0, not any other order.
Where this chapter appears in exams
| Exam | Typical use |
|---|---|
| CBSE Class 11 | Trend comparisons with reasoning, block identification, IUPAC naming exercises |
| JEE/NEET | Quantitative comparison of ionisation enthalpy/atomic radius between named elements |
| GATE / CSIR-NET | Effective nuclear charge (Slater's rules) and periodicity of oxidation states in depth |
Explore the periodic table interactively. The free calculator suite's periodic table tool shows every element's block, group and period at a glance while you revise this chapter.
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