4.1.2 s and p Blocks in the Periodic Table

This topic explains how the periodic table is divided into blocks — specifically the s-block and p-block — based on where the valence electrons are located.

Definition

s-Block Elements Group 1 and Group 2 elements contain their valence electrons in the s sub-shell. Therefore, these elements are called s-block elements.

p-Block Elements Elements in groups 13 to 18 (except He) are known as p-block elements because their valence electrons are located in the p sub-shell.

Simple explanation: The block of an element tells you which type of sub-shell its valence electrons are in.

Main Idea

The periodic table can be divided into blocks based on the sub-shell that is being filled:

• s-block: Groups 1 and 2
• p-block: Groups 13 to 18 (except He)
• d-block: Groups 3 to 12 (transition elements)
• f-block: Lanthanides and actinides

Simple explanation: The periodic table is like a map divided into four regions (blocks) — s, p, d, and f — depending on where the last electron goes.

Core Concept

s-Block Elements

• Groups: 1 and 2
• Valence electrons: in the s sub-shell
• General valence configuration: ns¹ (Group 1) and ns² (Group 2)

• Li = 1s², 2s¹ → valence electron is in s sub-shell → s-block
• Be = 1s², 2s² → valence electron is in s sub-shell → s-block
• Na = 1s², 2s², 2p⁶, 3s¹ → valence electron is in s sub-shell → s-block

p-Block Elements

• Groups: 13 to 18 (except He)
• Valence electrons: in the p sub-shell
• General valence configuration: ns² np¹ to ns² np⁶

• C = 1s², 2s², 2p² → valence electron is in p sub-shell → p-block
• N = 1s², 2s², 2p³ → valence electron is in p sub-shell → p-block
• O = 1s², 2s², 2p⁴ → valence electron is in p sub-shell → p-block
• F = 1s², 2s², 2p⁵ → valence electron is in p sub-shell → p-block

Helium — A Special Case

Helium (He) is in Group 18 but its electronic configuration is 1s² — its valence electrons are in the s sub-shell, not p. Therefore, helium is an s-block element, even though it is placed in Group 18.

Simple explanation: Helium is the exception — it sits with the noble gases but is actually an s-block element.

d-Block Elements (Transition Elements)

Lanthanides and actinides are known as f-block elements since their valence electrons lie in the f sub-shell.

Note: d-block elements (Groups 3–12) are also called transition elements. The book mentions them here briefly but covers them in detail in Section 4.5.

Detailed Explanation

How to Identify the Block of an Element

Step 1: Write the electronic configuration.

Step 2: Identify the valence sub-shell (the sub-shell where the last electron goes).

Step 3: The block is determined by this valence sub-shell:

Examples:

Position of Blocks in the Periodic Table

Intermediate Level

Why Blocks Are Important

The block of an element tells you:

• Where the valence electrons are — in s, p, d, or f sub-shells.
• What kind of chemistry the element will have — s-block metals are reactive; p-block includes metals, non-metals, and metalloids; d-block elements are transition metals with variable oxidation states.
• How to predict the element's properties — based on its position in the periodic table.

s-Block vs p-Block

Helium — The Exception

Helium is placed in Group 18 (noble gases) but its configuration is 1s². Since its valence electrons are in the s sub-shell, it is technically an s-block element.

Simple explanation: Helium is the only element that is in Group 18 but belongs to the s-block.

Important Points

• - s-block elements: Groups 1 and 2; valence electrons in the s sub-shell.
• - p-block elements: Groups 13 to 18 (except He); valence electrons in the p sub-shell.
• - d-block elements: Groups 3 to 12; valence electrons in the d sub-shell.
• - f-block elements: Lanthanides and actinides; valence electrons in the f sub-shell.
• - Helium is an s-block element despite being in Group 18.
• - Examples:
• - Li = 1s² 2s¹ → s-block
• - C = 1s² 2s² 2p² → p-block
• - Na = 1s² 2s² 2p⁶ 3s¹ → s-block
• - Cl = 1s² 2s² 2p⁶ 3s² 3p⁵ → p-block

Remember

• - s-block: Groups 1, 2 (Li, Na, Mg, Ca).
• - p-block: Groups 13–18 except He (C, N, O, F, Cl).
• - d-block: Groups 3–12 (transition metals).
• - f-block: Lanthanides and actinides.
• - Helium = s-block, even though in Group 18.
• - Block is determined by the valence sub-shell.

Common Mistakes

• 1. Thinking helium is a p-block element. It is an s-block element.
• 2. Confusing s-block and p-block. s-block is on the left; p-block is on the right.
• 3. Thinking all Group 18 elements are p-block. Helium is the exception.
• 4. Forgetting the f-block. Lanthanides and actinides are f-block elements.
• 5. Thinking block depends on the group number only. It depends on the valence sub-shell.
• 6. Confusing blocks with periods or groups. Blocks are regions based on valence sub-shell; periods are rows; groups are columns.

Quick Revision

• s-block elements: Groups 1 and 2; valence electrons in s sub-shell.
• p-block elements: Groups 13 to 18 (except He); valence electrons in p sub-shell.
• d-block elements: Groups 3 to 12; transition elements.
• f-block elements: Lanthanides and actinides; valence electrons in f sub-shell.
• Helium is an s-block element even though it is placed in Group 18.
• Examples:
• Li = 1s² 2s¹ → s-block
• C = 1s² 2s² 2p² → p-block
• Na = 1s² 2s² 2p⁶ 3s¹ → s-block
• Cl = 1s² 2s² 2p⁶ 3s² 3p⁵ → p-block

60-Second Revision

• s-block: Groups 1, 2.
• p-block: Groups 13–18 (except He).
• d-block: Groups 3–12.
• f-block: Lanthanides and actinides.
• **Helium** = s-block.
• Block = based on valence sub-shell.
• Warning: helium is not a p-block element even though it is in Group 18.