Question
Classify the elements of the periodic table into s, p, d, and f blocks based on their electron configurations. What are the general properties of each block?
(CBSE 11 boards ask block classification; NEET tests electron configuration of d-block elements)
Solution — Step by Step
The block of an element is determined by the subshell in which the last (differentiating) electron enters.
- Last electron enters s subshell → s-block
- Last electron enters p subshell → p-block
- Last electron enters d subshell → d-block
- Last electron enters f subshell → f-block
Outer configuration: (Group 1, alkali metals) or (Group 2, alkaline earth metals). Hydrogen and Helium are also s-block.
Properties: highly electropositive, form ionic compounds, low ionisation energies, strong reducing agents. Metals are soft, low melting points.
Outer configuration: . Contains metals, metalloids, and non-metals — the most diverse block.
Properties: wide range of ionisation energies, includes noble gases (Group 18). Electronegativity increases left to right. Most non-metals and all metalloids are here.
Outer configuration: . The (n-1)d subshell is being filled while electrons are also in the ns subshell.
Properties: variable oxidation states, coloured ions, catalytic activity, form complex compounds, high melting and boiling points, paramagnetic behaviour.
Outer configuration: . The (n-2)f subshell is being filled — two shells below the outermost.
Lanthanoids (4f): similar properties, +3 is the most common oxidation state. Actinoids (5f): radioactive, variable oxidation states.
flowchart TD
A[Where does the last electron enter?] --> B{Which subshell?}
B -->|s subshell| C["s-Block<br/>Groups 1-2<br/>ns¹ or ns²"]
B -->|p subshell| D["p-Block<br/>Groups 13-18<br/>ns²np¹⁻⁶"]
B -->|d subshell| E["d-Block<br/>Groups 3-12<br/>(n-1)d¹⁻¹⁰ ns⁰⁻²"]
B -->|f subshell| F["f-Block<br/>Lanthanoids + Actinoids<br/>(n-2)f¹⁻¹⁴"]
C --> G["2 groups, 14 elements<br/>Most reactive metals"]
D --> H["6 groups, 36 elements<br/>Metals + Nonmetals"]
E --> I["10 groups, 40 elements<br/>Transition metals"]
F --> J["28 elements<br/>Inner transition metals"]Why This Works
The block classification directly mirrors the order in which orbitals are filled according to the Aufbau principle (filling order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, …). Each block corresponds to one type of orbital, and the number of groups in each block equals the maximum number of electrons that orbital can hold (s=2, p=6, d=10, f=14).
This is why the periodic table has its characteristic shape — the s-block is 2 columns wide, p-block is 6, d-block is 10, and f-block is 14.
Alternative Method
Quick location trick: If the element is in Groups 1-2, it is s-block. If it is in Groups 13-18, it is p-block. If it is in Groups 3-12, it is d-block. If it is in the two rows below the main table (lanthanoids/actinoids), it is f-block. No need to write the full electron configuration for this classification.
Common Mistake
Helium (He) has the configuration , making it an s-block element by electron configuration. But it is placed in Group 18 (p-block) because its properties match noble gases — it has a completely filled outermost shell. This is a very common NEET trap question. Similarly, students forget that Group 12 elements (Zn, Cd, Hg) are technically d-block but not true transition metals because they have a completely filled d-subshell () in their common oxidation states.