The s, p, and d Blocks

The periodic table is divided into blocks based on the subshell being filled by the outermost (highest-energy) electrons. This classification explains many patterns in properties and behaviour. Understanding the blocks helps you predict an element's chemistry from its position in the table.

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Block Definitions

The block of an element is determined by the subshell that contains its highest-energy electron:

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: Lanthanides and Actinides]
    B --> B1[Filling s subshell]
    C --> C1[Filling p subshell]
    D --> D1[Filling d subshell]
    E --> E1[Filling f subshell]

Block	Subshell Being Filled	Groups	Examples
s-block	s subshell	1 and 2 (plus He)	Li, Be, Na, Mg, K, Ca
p-block	p subshell	13-18	B, C, N, O, F, Ne, Al, Si
d-block	d subshell	3-12	Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn
f-block	f subshell	Lanthanides, Actinides	La, Ce, Pr... / Ac, Th, Pa...

Note: Helium (1s2) is placed in Group 18 with the noble gases due to its chemical properties, but it is technically an s-block element by electron configuration.

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The s-Block: Groups 1 and 2

The s-block elements have their outermost electron(s) in an s subshell.

Group 1: The Alkali Metals (Li, Na, K, Rb, Cs, Fr)

Configuration pattern: [noble gas] ns1

Key properties:

• One electron in the outer shell -> form M+ ions
• Very low ionisation energies (lowest across their period)
• Very reactive metals -- reactivity increases down the group
• Low melting points and densities for metals
• Soft metals that can be cut with a knife
• Produce strongly alkaline solutions when reacted with water: 2M + 2H2O -> 2MOH + H2

Physical data for Group 1:

Property	Li	Na	K	Rb	Cs
1st IE (kJ/mol)	520	496	419	403	376
Melting point (degrees C)	181	98	63	39	29
Density (g/cm3)	0.53	0.97	0.86	1.53	1.87
Atomic radius (pm)	152	186	227	248	265

Trend down the group: Reactivity increases because:

• Atomic radius increases -> outer electron is further from nucleus
• More shielding from inner shells
• First ionisation energy decreases
• The outer electron is more easily lost

Group 2: The Alkaline Earth Metals (Be, Mg, Ca, Sr, Ba, Ra)

Configuration pattern: [noble gas] ns2

Key properties:

• Two electrons in the outer shell -> form M2+ ions
• Higher ionisation energies than Group 1 (two electrons to remove, higher nuclear charge)
• Still reactive, but less so than Group 1
• Higher melting points and densities than Group 1
• Harder metals

Trend down the group: Reactivity increases for the same reasons as Group 1.

Element	Reaction with Cold Water
Beryllium	No reaction
Magnesium	Very slow (practically no reaction)
Calcium	Steady reaction, fizzes gently
Strontium	Vigorous reaction
Barium	Very vigorous reaction

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The p-Block: Groups 13-18

The p-block elements have their highest-energy electron in a p subshell (except helium).

This block contains a huge diversity of properties because it spans from metals (aluminium, tin, lead) through metalloids (silicon, germanium) to non-metals (carbon, nitrogen, oxygen, fluorine) and noble gases (neon, argon).

Group 17: The Halogens (F, Cl, Br, I, At)

Configuration pattern: [noble gas] ns2 np5

Key properties:

• Seven electrons in the outer shell -> form X- ions or share electrons in covalent bonds
• High electronegativity (highest of all groups)
• Exist as diatomic molecules: F2, Cl2, Br2, I2
• Strong oxidising agents
• Reactivity decreases down the group (opposite to Group 1)

Property	F2	Cl2	Br2	I2
State at 25 degrees C	Pale yellow gas	Green-yellow gas	Brown liquid	Grey-black solid
Electronegativity	4.0	3.2	3.0	2.7
1st EA (kJ/mol)	-328	-349	-325	-295
Bond energy (kJ/mol)	158	242	193	151

The decrease in reactivity occurs because:

• Atomic radius increases -> the nucleus is further from the incoming electron
• More shielding -> lower effective nuclear charge
• Electron affinity becomes less negative -> harder to gain an electron

Note on fluorine: F2 has a surprisingly low bond energy (158 kJ/mol) compared to Cl2 (242 kJ/mol). This is because fluorine atoms are so small that the lone pairs on adjacent F atoms strongly repel each other in the F-F bond, weakening it. Despite this, fluorine is the most reactive halogen because its high electronegativity and small size make it extremely effective at attracting electrons from other atoms.

Group 18: The Noble Gases (He, Ne, Ar, Kr, Xe, Rn)

Configuration pattern: [noble gas] ns2 np6 (full outer shell)

Key properties:

• Full outer electron shells -> extremely unreactive
• Exist as monatomic gases
• Very low boiling points (weak London forces only)
• Boiling points increase down the group as atomic size (and London forces) increase

Noble Gas	He	Ne	Ar	Kr	Xe
Electrons	2	10	18	36	54
BP (degrees C)	-269	-246	-186	-152	-108

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The d-Block: Groups 3-12

The d-block elements (transition metals) have their highest-energy electrons entering the 3d, 4d, or 5d subshells.

Definition: Transition Metal vs d-Block Element

A transition metal is a d-block element that forms at least one ion with a partially filled d subshell.

This distinction is important:

Element	Config	Common Ion	Ion Config	Transition Metal?
Scandium	[Ar] 4s2 3d1	Sc3+	[Ar] 3d0	No (empty d)
Titanium	[Ar] 4s2 3d2	Ti2+, Ti3+, Ti4+	[Ar] 3d2, 3d1, 3d0	Yes (Ti2+ and Ti3+ have partial d)
Iron	[Ar] 4s2 3d6	Fe2+, Fe3+	[Ar] 3d6, 3d5	Yes (both have partial d)
Copper	[Ar] 4s1 3d10	Cu+, Cu2+	[Ar] 3d10, 3d9	Yes (Cu2+ has partial d)
Zinc	[Ar] 4s2 3d10	Zn2+	[Ar] 3d10	No (full d)

So scandium and zinc are d-block elements but NOT transition metals by the strict definition.

Characteristic Properties of Transition Metals

1. Variable oxidation states: e.g., iron can be +2 or +3; manganese can be +2, +4, +6, or +7. This is possible because the energy gap between 3d and 4s is small, allowing different numbers of electrons to be removed.

Element	Common Oxidation States	Example Compounds
Ti	+2, +3, +4	TiCl4, TiO2
V	+2, +3, +4, +5	VO2+, V2O5
Cr	+2, +3, +6	CrCl3, K2Cr2O7
Mn	+2, +4, +6, +7	MnO2, KMnO4
Fe	+2, +3	FeSO4, FeCl3
Cu	+1, +2	Cu2O, CuSO4

2. Coloured compounds: due to partially filled d orbitals absorbing specific wavelengths of visible light (d-d transitions when ligands split the d orbital energies)

Ion	Colour	d-electrons
Ti3+	Purple	1
V3+	Green	2
Cr3+	Green/violet	3
Mn2+	Very pale pink	5
Fe2+	Pale green	6
Fe3+	Yellow/brown	5
Cu2+	Blue	9

3. Catalytic activity: both as elements (Fe in Haber process, Ni in hydrogenation) and as compounds (V2O5 in Contact process, MnO2 decomposing H2O2). Variable oxidation states allow transition metals to provide alternative reaction pathways with lower activation energies.

4. Complex ion formation: transition metal ions form coordination compounds with ligands (molecules or ions that donate lone pairs to the metal ion)

5. Relatively high melting points and densities (compared to s-block metals) due to strong metallic bonding involving d electrons

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Diagonal Relationships

Elements that are diagonally adjacent in the periodic table sometimes show surprising chemical similarities. The two most important are:

Lithium and Magnesium

• Both form nitrides when burned in nitrogen: 6Li + N2 -> 2Li3N; 3Mg + N2 -> Mg3N2
• Both form oxides (not peroxides) when burned in oxygen
• Both carbonates decompose on heating (unlike other Group 1 carbonates)
• LiF and MgF2 are both sparingly soluble
• Both form covalent organometallic compounds

Beryllium and Aluminium

• Both form amphoteric oxides (BeO and Al2O3)
• Both form covalent chlorides that act as Lewis acids
• Both form complex ions with hydroxide
• BeCl2 and AlCl3 are both covalent and form dimers (or polymers for BeCl2)

graph TD
    subgraph "Diagonal Relationships"
        Li["Li"] --- Mg["Mg"]
        Be["Be"] --- Al["Al"]
    end
    subgraph "Why?"
        A["Moving RIGHT increases<br/>nuclear charge and<br/>polarising power"]
        B["Moving DOWN increases<br/>atomic radius"]
        C["Effects partially cancel,<br/>giving similar charge density"]
    end

Why do diagonal relationships occur? Moving one element right increases nuclear charge (tending to decrease size and increase polarising power). Moving one element down increases atomic radius. These effects partially cancel, giving diagonal neighbours similar charge density and polarising power.

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Common Misconception

Misconception: "All d-block elements are transition metals."

Correction: Scandium and zinc are d-block elements but are NOT transition metals. A transition metal must form at least one stable ion with a partially filled d subshell. Sc3+ has an empty d subshell (3d0), and Zn2+ has a full d subshell (3d10), so neither qualifies.

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A-Level Deep Dive: The s, p, and d Blocks

Spec mapping

Edexcel 9CH0 specification Topic 1, sub-topic 1.6 covers the classification of elements into s-, p-, d- and f-blocks based on the subshell of the highest-energy electron; the characteristic chemistry of each block (s-block: ionic compounds, +1/+2 ions, reactive metals; p-block: ranges from metallic to non-metallic, multiple oxidation states, covalent and ionic bonding; d-block: variable oxidation states, coloured compounds, catalytic activity; f-block: lanthanides and actinides, trivalent ions); and the use of block classification to predict element properties (refer to the official Pearson Edexcel specification document for exact wording). Examined principally in Paper 1 (Advanced Inorganic and Physical Chemistry) and re-examined in Topic 15 (Transition metals) and Topic 4 (Inorganic chemistry of group 1, 2, 7), where block classification underpins the structural framework.

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Worked example with full mark scheme

Question (8 marks):

(a) Classify each of the following elements into s-, p-, d- or f-block, justifying your answer with the electron configuration of the highest-energy subshell: K (Z = 19), Br (Z = 35), Fe (Z = 26), Ce (Z = 58). (4) (b) Explain why d-block elements typically show variable oxidation states whereas s-block elements show only one or two characteristic oxidation states. (2) (c) Predict, with reasoning, whether the +3 oxidation state would be more accessible for Fe (a d-block element) or for Ca (an s-block element). (2)

Solution with mark scheme:

(a) B1 — K, Z = 19, [Ar] 4s¹ — highest-energy subshell is 4s → s-block.

B1 — Br, Z = 35, [Ar] 3d¹⁰ 4s² 4p⁵ — highest-energy subshell is 4p → p-block.

B1 — Fe, Z = 26, [Ar] 3d⁶ 4s² — highest-energy subshell is 3d (lower in energy than 4s in the cation, but the characterising subshell is 3d, since 4s² is also present in s-block elements). d-block.

B1 — Ce, Z = 58, [Xe] 4f¹ 5d¹ 6s² — highest-energy subshell includes 4f → f-block (lanthanide). Cerium is sometimes classed as d-block due to the 5d¹, but Edexcel and most periodic tables place it in f-block as the first lanthanide.

Common error: candidates classify Fe as s-block because of the 4s² in its configuration. The defining subshell for d-block is the 3d (or 4d, 5d depending on row), not the s. The presence of 4s² is shared with the alkali metals.

(b) B1 — d-block elements have similar energies for 3d and 4s subshells, and the differences between successive ionisation energies are small once the 4s electrons are removed. So removing 1, 2, 3 or more electrons from a d-block element produces stable cations of multiple oxidation states (e.g. Fe²⁺, Fe³⁺ both exist in aqueous solution).

B1 — s-block elements have a much larger gap between successive IEs once the valence shell is empty: removing the 1 (Group 1) or 2 (Group 2) outer electrons gives the noble gas configuration; further removal would require accessing the next shell at very high energy cost. Hence Group 1 shows only +1; Group 2 shows only +2.

(c) B1 — Fe³⁺ is readily accessible: Fe loses 4s² then one 3d electron to form [Ar] 3d⁵ — half-filled, stable. The third IE of Fe is moderate (~2957 kJ mol⁻¹).

B1 — Ca³⁺ is not accessible: Ca³⁺ would require breaking into the inner [Ar] core (3p⁶ → 3p⁵), at very high energy cost (>4910 kJ mol⁻¹ for IE3 of Ca). So Ca only forms Ca²⁺. The block determines the oxidation states accessible; the 3d subshell provides Fe with extra "stretching room" not available to Ca.

Total: 8 marks (B4 + B2 + B2).

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Specimen question modelled on the Edexcel 9CH0 Paper 1 format