Periodic table blocks — s, p, d, f block properties and electron configuration

Question

How is the periodic table divided into s, p, d, and f blocks? What determines which block an element belongs to, and what are the general properties of each block?

Solution — Step by Step

An element’s block is determined by the subshell in which the last electron enters (differentiating electron):

Last electron enters s subshell $\to$ s-block
Last electron enters p subshell $\to$ p-block
Last electron enters d subshell $\to$ d-block
Last electron enters f subshell $\to$ f-block

This is the only rule we need. No exceptions for block classification.

Configuration: $ns^1$ (Group 1, alkali metals) and $ns^2$ (Group 2, alkaline earth metals). Helium ( $1s^2$ ) is technically s-block but placed in Group 18 due to noble gas behaviour.

Properties: Highly electropositive, form ionic compounds, low ionisation energy, strong reducing agents. Compounds are mostly ionic. Flame colours are characteristic (Li = crimson, Na = golden yellow, K = violet).

Configuration: $ns^2np^{1-6}$ . Includes metals, metalloids, and non-metals.

Properties: Diverse — includes reactive non-metals (halogens), noble gases, metalloids (B, Si, Ge), and some metals (Al, Sn, Pb). Electronegativity generally increases across a period. These elements show variable oxidation states due to inert pair effect (in heavier elements like Tl, Pb, Bi).

Configuration: $(n-1)d^{1-10}ns^{0-2}$ . The penultimate d-subshell is being filled.

Properties: Variable oxidation states, coloured ions (due to d-d transitions), form complex compounds, act as catalysts, show paramagnetism. High melting points and densities compared to s and p block.

Configuration: $(n-2)f^{1-14}(n-1)d^{0-1}ns^2$ . The ante-penultimate f-subshell is being filled.

Properties: Lanthanoids (4f) show +3 as the most common oxidation state, lanthanoid contraction. Actinoids (5f) show variable oxidation states, most are radioactive.

graph TD
    A[Periodic Table Blocks] --> B[s-Block: Groups 1-2]
    A --> C[p-Block: Groups 13-18]
    A --> D[d-Block: Groups 3-12]
    A --> E[f-Block: Lanthanoids + Actinoids]
    B --> B1["ns1-2, electropositive"]
    C --> C1["ns2np1-6, diverse"]
    D --> D1["n-1 d1-10 ns0-2, coloured ions"]
    E --> E1["n-2 f1-14, lanthanoid contraction"]

Why This Works

The block system maps directly onto the Aufbau principle. As we move across a period, electrons fill orbitals in order of increasing energy: $1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p...$

Each block in the periodic table corresponds to one type of orbital being filled. The s-block is 2 elements wide (s holds 2 electrons), p-block is 6 wide, d-block is 10 wide, and f-block is 14 wide.

Block	Groups	Width	Elements per period	Last electron enters
s	1-2	2	2	$ns$
p	13-18	6	6	$np$
d	3-12	10	10	$(n-1)d$
f	Lanthanoids/Actinoids	14	14	$(n-2)f$

Alternative Method

For MCQ questions asking “which block does element X belong to?”, use the atomic number shortcut:

Write the electronic configuration using Aufbau order
Check the last subshell being filled
That subshell name = the block

For example, Fe (Z=26): $[\text{Ar}] 3d^6 4s^2$ . Last electron entered $3d$ $\to$ d-block.

Common Mistake

Students place Zn, Cd, and Hg in d-block (which is correct based on configuration) but then call them transition metals. Strictly, transition metals must have an incomplete d-subshell in at least one stable oxidation state. Zn ( $3d^{10}4s^2$ ), Cd, and Hg have completely filled d-orbitals in their ground state and common +2 oxidation state — so they are d-block elements but NOT transition metals. NEET has tested this distinction as an assertion-reason question.

Question

Solution — Step by Step

Why This Works

Alternative Method

Common Mistake

Try These Next