Periodic table blocks
The s, p, d and f blocks are a map of which type of atomic subshell is being filled across the periodic table. Their widths and positions are not decorative: they emerge from orbital capacities and the order in which subshell energies become available.
Block definition and subshell capacity
The s block corresponds mainly to filling ns orbitals; the p block to np, the d block to (n−1)d and the f block to (n−2)f. One s subshell holds 2 electrons, p 6, d 10 and f 14, exactly matching the characteristic block widths.
s block
Groups 1–2 plus helium by electron configuration. Typical outer pattern ns¹–².
p block
Groups 13–18 except He. Typical outer pattern ns²np¹–⁶.
d block
Transition elements. Filling (n−1)d orbitals; chemistry often involves both ns and d electrons.
f block
Lanthanoids and actinoids. Filling (n−2)f orbitals; usually drawn below to keep the table compact.
Construction of the periodic-table layout
Period 1 contains only 1s and therefore two elements. Periods 2 and 3 fill ns then np, giving 8 elements each. Periods 4 and 5 insert a ten-element d block between s and p, giving 18. Periods 6 and 7 include the 14-element f block and can contain 32 elements.
The detached f rows are a typography choice. In the full long-form table, the lanthanoids and actinoids belong between the s and d portions of periods 6 and 7; moving them below prevents the table from becoming extremely wide.
Reading configurations from position
Block position gives the final subshell pattern quickly. An element in the second column of the p block has np²; one in the fifth column has np⁵, together with the preceding ns² for main-group elements.
For example, sulfur lies in period 3 and the fourth column of the p block, giving [Ne]3s²3p⁴. Calcium is period 4, second column of s, giving [Ar]4s². Scandium begins the 3d series after 4s²: [Ar]3d¹4s².
The block map is useful, but not infallible
The block classification follows electron structure, yet individual ground-state configurations include exceptions. Cr and Cu do not follow the simplest 4s²3dⁿ prediction, and heavier d- and f-block elements show further irregularities because neighboring subshells are close in energy.
Helium is another instructive case: its configuration is 1s², electronically an s-block pattern, but it is placed with group 18 because its closed shell and chemical behavior align with the noble gases.
So the periodic table simultaneously encodes electron configuration and chemical similarity. The two principles reinforce each other but are not always identical classification rules.
Period-by-period construction
| Period | Subshells filled across the period | Maximum elements |
|---|---|---|
| 1 | 1s | 2 |
| 2 | 2s, 2p | 8 |
| 3 | 3s, 3p | 8 |
| 4 | 4s, 3d, 4p | 18 |
| 5 | 5s, 4d, 5p | 18 |
| 6 | 6s, 4f, 5d, 6p | 32 |
The pattern shows why period number and the n value of the subshell being filled are not always identical. The d block of period 4 is 3d, and the f block of period 6 is 4f.
Blocks connect position to chemistry
s-block metals generally have one or two easily removed ns electrons. p-block chemistry ranges from metals through metalloids to strongly electronegative nonmetals because the np subshell fills across six columns. d-block elements show variable oxidation states and coordination chemistry because several d and s states lie close in energy.
The f block displays subtler changes across the lanthanoids because 4f electrons shield poorly while remaining relatively contracted. That poor shielding drives the lanthanide contraction, which in turn affects sizes of later transition metals.
Helium and the limits of a single classification
Electronically, He is 1s² and would fit an s-block definition. Chemically, its closed shell makes it unmistakably a noble gas, so it is placed over Ne in group 18. This is a useful reminder that the periodic table encodes both orbital filling and recurring chemistry.
Similar debates about the exact placement of some f-block endpoints reflect the same fact: block labels are explanatory tools, while real electronic structures can lie close to classification boundaries.
Group numbers and block position
For the p block, the modern group number can be converted into the number of outer s and p electrons: groups 13 through 18 correspond broadly to ns²np¹ through ns²np⁶. This is why C and Si, both group 14, share an ns²np² valence pattern even though their principal shell numbers differ.
In the d block the relation is less direct because ns and (n−1)d occupancies can rearrange. Group number still tracks broad electron count and chemistry, but oxidation states and actual configurations need to be checked element by element.
The delayed d-block sequence
The first d subshell is 3d, but it does not begin filling until period 4 because 4s becomes energetically accessible first. Likewise 4d belongs to period 5 and 5d to period 6. The period number therefore names the highest principal shell being occupied at the start and end of the row, not every subshell filled in between.
The f block and the lanthanide contraction
Across the 4f series, added f electrons shield the increasing nuclear charge poorly. The outer electron cloud is pulled inward progressively, producing the lanthanide contraction. That effect reaches beyond the f block: Hf ends up close in size to Zr, influencing chemistry throughout the 5d transition series.
So the f block is not a detached appendix. Its electronic structure alters the dimensions and chemistry of elements that follow it in the main body of the table.
Exercises
Period 4 length
Why does period 4 contain 18 elements?
Solution
It fills 4s² + 3d¹⁰ + 4p⁶ = 18 electrons across the period.
Selenium
Se is in period 4, fourth column of the p block. Give its outer configuration.
Solution
4s²4p⁴, preceded by the filled 3d¹⁰ and [Ar] core.
f block
Why is the f block 14 elements wide?
Solution
f has ℓ = 3, so 2ℓ + 1 = 7 orbitals; 7 × 2 = 14 electrons.