Key Fact
Blocks of periodic table
s-block (groups 1, 2): outermost s; p-block (groups 13-18): outermost p; d-block (groups 3-12): incomplete d; f-block (lanthanides, actinides): incomplete f.
-- NCERT Class 11 Chemistry, Ch. 3, p. 6Blocks of Periodic Table
8 MCQs9-step worked example
Source: NCERT Classification of Elements and Periodicity in PropertiesPYQ coverage: NEET 2021, 2024Official key: NTA-verifiedLast reviewed: May 2026
Lesson
The modern periodic table groups 118 elements into four blocks — s, p, d, and f — named after the subshell that receives the last electron in the ground-state electronic configuration. This classification is the backbone of inorganic chemistry for NEET: once you know the block, you can predict valence, bonding behaviour, and broad chemical trends.
s-block (Groups 1 and 2): The differentiating electron enters the outermost s-orbital. Group 1 elements (alkali metals: Li to Fr) have the configuration ns¹; Group 2 elements (alkaline earth metals: Be to Ba) have ns². Hydrogen and helium technically belong to the s-block by configuration (1s¹ and 1s²), though helium is placed with noble gases due to its filled-shell chemistry (NCERT Class 11 Chemistry Chapter 3, page 6).
p-block (Groups 13–18): The differentiating electron enters the outermost p-orbital (ns²np¹ to ns²np⁶). This block includes metals, metalloids, and non-metals — the widest chemical diversity. Noble gases (Group 18) complete the block with ns²np⁶ (except He: 1s²).
d-block (Groups 3–12): The differentiating electron enters the (n−1)d orbital. General outer configuration: (n−1)d¹⁻¹⁰ ns⁰⁻². These are the transition metals (though Zn, Cd, Hg with d¹⁰ are debated — NEET treats Groups 3–12 as d-block).
f-block (Lanthanoids and Actinoids): The differentiating electron enters the (n−2)f orbital. Lanthanoids: 4f¹⁻¹⁴ 5d⁰⁻¹ 6s². Actinoids: 5f¹⁻¹⁴ 6d⁰⁻¹ 7s². These two rows sit below the main table.
Watch-out for NEET: The question "Which block does element X belong to?" tests one thing — which subshell receives the last electron. Don't confuse the block label with the outermost shell. For d-block, the differentiating electron is in (n−1)d, not ns. For f-block, it is (n−2)f. Getting the "differentiating electron" definition wrong is a common source of lost marks.
Practice MCQs
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
The elements in which the last electron enters the outermost s-orbital belong to which block of the periodic table?
MCQ 2Easy RecallPractice
Which of the following groups constitute the s-block of the periodic table?
MCQ 3Easy RecallPractice
Helium (He) has the electronic configuration 1s². In which block of the periodic table is it placed?
MCQ 4Direct ApplicationPractice
An element has the ground-state electronic configuration [Ar] 3d⁶ 4s². To which block does it belong?
MCQ 5Direct ApplicationPractice
An element has the ground-state configuration [Xe] 4f¹ 5d¹ 6s². The differentiating electron enters which subshell, and to which block does the element belong?
MCQ 6Direct ApplicationPractice
Which of the following correctly represents the general outer electronic configuration of d-block elements?
MCQ 7Concept TrapPractice
A student claims that zinc (Zn, [Ar] 3d¹⁰ 4s²) should not be classified as a d-block element because its d-subshell is completely filled. Which statement best addresses this claim in the NEET context?
MCQ 8Concept TrapPractice
Elements of the f-block are placed separately at the bottom of the periodic table. The differentiating electron in lanthanoids enters the:
Worked Example
- 1
Given
Atomic number Z = 26.
- 2
Required
(a) Block classification. (b) Outer electronic configuration.
- 3
Concept
The block of an element is determined by the subshell into which the differentiating (last) electron enters during the Aufbau filling sequence.
- 4
Approach
Write the full electronic configuration using the Aufbau principle, then identify which subshell receives the final electron.
- 5
Configuration build-up
Z = 26: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s² Core: [Ar] = 1s² 2s² 2p⁶ 3s² 3p⁶ Outer: 3d⁶ 4s²
- 6
Identification
The differentiating electron enters the 3d subshell (the (n−1)d orbital, where the outermost shell is n = 4). This places the element in the **d-block**. The element is **iron (Fe)**, located in Period 4, Group 8.
- 7
Final answer
Element X (Z = 26) is a **d-block element** with outer electronic configuration **[Ar] 3d⁶ 4s²**. Note: Atomic number 26 is an exact counting integer and does not affect any significant-figure considerations.
- 8
Common trap
Students sometimes look at the outermost shell (4s²) and classify this as s-block. The block is determined by the *differentiating* subshell (3d), not the outermost occupied subshell.
- 9
Similar NEET-style question
An element has atomic number 58. Identify its block and write its expected outer configuration. *(Answer: [Xe] 4f¹ 5d¹ 6s² — f-block, lanthanoid.)* ---
Before solving, remember these
Formulas
Ionization energy of hydrogen-like atom
Energy required to ionize an electron from the n-th shell of hydrogen-like atom.
| Symbol | Quantity | SI Unit |
|---|---|---|
| Z | nuclear charge | - |
| n | quantum number | - |
Valid when
- One-electron atom
- Non-relativistic
Exam Traps & Common Mistakes
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Inorganic Exception
Student includes inert-gas radius in atomic-radius trends. But inert gases use van der Waals radius (much larger than covalent), making 'monotonic decrease across period' look broken.
When it triggers
Atomic radius comparison includes a noble gas or trends across period 2/3.
How to avoid
Compare like with like: covalent radii for non-noble gases. Noble gas radii are van der Waals (no covalent bond). Don't compare noble-gas radius directly to halogen.
Category: Inorganic Exception
Student expects monotonic increase in IE across period. Anomalies: Be(s²) > B(s²p¹); N(p³ half-filled) > O(p⁴).
When it triggers
Compare IE values across period 2 (Li, Be, B, C, N, O, F).
How to avoid
Be > B (s² stable; B's p¹ easier to remove). N > O (N has p³ half-filled stability; O loses one to attain p³). Memorise these two anomalies.
Root cause: concept gap
Correction
Be>B (s² stability); N>O (N's p³ half-filled stability). Memorise these two anomalies in period 2.
Root cause: concept gap
Correction
Don't compare different radius types. Noble gases use vdW radius (much larger); halogens use covalent radius. Compare like-with-like.
Past Year Questions
3 questions from NEET 2021, 2024. Answers verified against NTA official keys.
NEET 2024Revised key
1Li < Be < B < C < N
2Li < B < Be < C < N
3Li < Be < C < B < N
4Li < Be < N < B < C
NTA Answer: Option 2(revised_final)
NEET 2024Revised key
1Si < C < N < O < F
2Si < C < O < N < F
3O < F < N < C < Si
4F < O < N < C < Si
NTA Answer: Option 1(revised_final)
NEET 2021
1CO < SiO : Increasing 2 2 < SnO < PbO oxidizing power 2 2
2HF < HCl : Increasing acidic < HBr < HI strength
3H O < H S : Increasing pK 2 2 a < H Se < H Te values 2 2
4NH < PH : Increasing 3 3 < AsH < SbH acidic character 3 3
NTA Answer: Option 3(final)
How NEET usually asks this
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Periodic trends comparison — atomic/ionic radii, ionization energy, electron gain enthalpy, electronegativity.
Direct ApplicationMedium
Common distractors
swapped classes
Tempts surface-level recall.
Sources
NCERT refs: Class 11 Chemistry Chapter 3, p.6
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