Arranging elements in order of increasing atomic number reveals repeating trends in their physical and chemical properties. This phenomenon is known as periodicity. These trends are directly linked to the electronic structures of the atoms.
Periodicity is the repeating pattern of physical and chemical properties of elements across different periods of the periodic table, which is directly related to the repeating pattern of their outer shell electron configurations.
Classification into s, p, and d Blocks
The periodic table is divided into distinct blocks. The block to which an element belongs is determined by the sub-shell (s, p, d, or f) that contains its highest energy electron.
Key Principle: Highest Energy Electron
The block an element belongs to is determined by the type of sub-shell that is being filled with the highest energy electron:
- s-block: Outermost electron(s) are in an s sub-shell.
- p-block: Outermost electron(s) are in a p sub-shell.
- d-block: Outermost electron(s) are in a d sub-shell.
This classification splits the periodic table into three main blocks that you must know for A-Level Chemistry:
- s-block elements: These include Group 1 (alkali metals) and Group 2 (alkaline earth metals) elements, plus Hydrogen and Helium. Their outer electron configurations are either \(ns^1\) or \(ns^2\).
- p-block elements: These include Groups 13 to 18 (except Helium). Their outer electron configurations range from \(ns^2 np^1\) to \(ns^2 np^6\). These elements are mostly non-metals and metalloids.
- d-block elements: These are the transition metals (Groups 3 to 12). Their outermost electrons are located in d sub-shells, meaning they have configurations ending in \((n-1)d^{1-10} ns^2\) (or similar).
Do not confuse "blocks" with "groups". The s-block contains Group 1 and Group 2. The p-block contains Groups 13 to 18 (historically referred to as Group 3 to Group 0). Helium has the configuration 1s², meaning it belongs to the s-block, despite sitting in Group 18 (Group 0) due to its chemical inertness.
Electron Configurations and Block Placement
A d-block element is an element that has its highest energy electron in a d sub-shell. This includes elements in Groups 3 to 12 of the periodic table.
You must be able to write the electron configuration of any element up to Krypton (\(Z = 36\)) and use it to justify its position in the periodic table blocks.
The period number corresponds to the principal quantum number (\(n\)) of the outermost shell. The block corresponds to the highest occupied energy sub-shell.
Examples:
- Sodium (\(Z = 11\)): 1s²2s²2p⁶3s¹ or [Ne]3s¹. The outermost electron is in a 3s sub-shell. It is in Period 3, Group 1, s-block.
- Sulfur (\(Z = 16\)): 1s²2s²2p⁶3s²3p⁴ or [Ne]3s²3p⁴. The outermost electrons are in a 3p sub-shell. It is in Period 3, Group 16, p-block.
- Iron (\(Z = 26\)): 1s²2s²2p⁶3s²3p⁶3d⁶4s² or [Ar]3d⁶4s². The highest energy sub-shell being filled is the 3d sub-shell. It is in Period 4, Group 8, d-block.
Why Periodicity Occurs
Periodicity occurs because electron configurations repeat in a periodic manner as atomic number increases. Since elements in the same group share the same number of valence electrons in the same type of sub-shell, they display similar chemical behaviours and trends. Each new period corresponds to the start of filling a new principal energy level.
- Element A: [Ne]3s²3p⁴
- Element B: [Ar]3d⁵4s¹
Element A:
- Block: p-block. The highest energy sub-shell containing electrons is the 3p sub-shell.
- Period: 3. The highest principal quantum number containing electrons is \(n = 3\).
- Group: Group 16 (or Group 6). It has 6 outer shell electrons (3s²3p⁴). Add 10 to account for d-block transition metals, yielding Group 16.
Element B:
- Block: d-block. The highest energy sub-shell being filled is the 3d sub-shell.
- Period: 4. The highest principal quantum number containing electrons is \(n = 4\) (the 4s¹ orbital).
- Group: Group 6. It has 6 electrons in its outermost shells (3d⁵4s¹). Note that this configuration is a transition metal exception: it promotes a 4s electron to the 3d orbital to create a stable, half-filled d sub-shell.
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