The big idea: The periodic table lists every element in order of increasing atomic number (Z — the number of protons). Because Z controls the electron configuration, elements with similar outer electrons end up in the same place — so the table's layout mirrors electron structure.
The table is organised in three ways at once:
- Periods — the horizontal rows (period number = the highest occupied main energy level, n). - Groups — the vertical columns (elements in a group have the same number of outer electrons). - Blocks — s, p, d and f regions named after the sublevel the outer electrons are filling.
The table is split into four blocks by which sublevel the outer electrons fill: s (blue), p (green), d (amber) and f (violet).
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Order = atomic number: Elements are placed by atomic number Z (protons), not by relative atomic mass. Reading left to right along a period, Z goes up by one each step, and one electron is added each step.
A period is a row. The period number tells you the highest main energy level (n) that the element's electrons occupy. Sodium is in period 3, so its outer electrons are in the third main energy level (n = 3).
A group is a column. Elements in the same group have the same number of outer (valence) electrons, which is why they have similar chemistry.
A group is a vertical column. Group 1 (highlighted) is the first column — every element in it has one outer electron.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Reading config off the table: For a main-group element you can read its outer shell straight from its position:
- Period number = the value of n for the outer shell. - Group number gives the number of outer electrons (group 1 → 1; group 2 → 2; groups 13–18 → the units digit, e.g. group 17 → 7).
Example: chlorine is period 3, group 17 → outer shell is 3s² 3p⁵ (7 outer electrons).
| Element | Position | Outer shell | Outer electrons |
|---|---|---|---|
| Na | period 3, group 1 | 3s¹ | 1 |
| Mg | period 3, group 2 | 3s² | 2 |
| Cl | period 3, group 17 | 3s² 3p⁵ | 7 |
| Ar | period 3, group 18 | 3s² 3p⁶ | 8 (full) |
Feeling unprepared for exams?
Get a clear study plan, practice with real questions, and know exactly where you stand before exam day. No more guessing.
Each block is named after the sublevel that the highest-energy electrons are filling. This is the single characteristic that defines the block structure of the table.
s-block
- groups 1 and 2 (plus H, He)
- outer electrons fill the s sublevel
- the reactive metals + H/He
p-block
- groups 13–18
- outer electrons fill the p sublevel
- non-metals, metalloids, noble gases
d-block
- groups 3–12 (the centre)
- electrons fill the d sublevel
- the transition metals
The d-block sits in the centre of the table (groups 3–12) — these are the transition metals.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
And the f-block: The f-block is the two separate rows below the main table — the lanthanides and actinides. Their highest-energy electrons fill the f sublevel.
The f-block is the two detached rows at the bottom (the lanthanides and actinides).
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Block from the last electron: Look at the sublevel the last electron enters:
- Na is 1s² 2s² 2p⁶ 3s¹ → last electron in s → s-block. - Cl is …3s² 3p⁵ → last electron in p → p-block. - Fe is …3d⁶ 4s² → highest sublevel filled is d → d-block.
How this is tested: S3.1.1 turns up as a quick Paper 1A MCQ — 'which region is the d-block?', 'which element is in the f-block?', or 'what characteristic groups the table into blocks?'
The trickier version asks you to deduce the block of an undiscovered or unfamiliar element from its electron configuration — work out where the next electron would go, then read off the block.
The scoring move: Write (or extend) the electron configuration, find the sublevel the outermost electron enters, and name that block. s → s-block, p → p-block, d → d-block, f → f-block.
IB-style question — deduce the block of element 119
Element 119 has not yet been made. By considering its electron configuration, deduce the block of the periodic table it would occupy. [2]
How to score the marks
- Mark 1 — find where the next electron goes. Element 118 (the noble gas at the end of period 7) has a full outer shell. The next electron in element 119 starts a new period (period 8) and enters the lowest available sublevel, which is 8s: configuration … 8s¹.
- Mark 2 — name the block. The outermost electron is in an s sublevel, so element 119 would be in the s-block (it would sit below francium in group 1).
Final answer
The s-block — its outer electron occupies the 8s sublevel (group 1, period 8).
IB-style question — block from a configuration
An element has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d³ 4s². State the block it belongs to and explain your answer. [2]
How to score the marks
- Mark 1 — identify the highest-energy sublevel being filled. Beyond the noble-gas core, electrons are entering the 3d sublevel (3d³).
- Mark 2 — name the block. Because the d sublevel is filling, the element is in the d-block (it is a transition metal — this is vanadium, Z = 23).
Final answer
The d-block, because its highest-energy electrons are filling the 3d sublevel.