A-Level — chemistryMedium5 min read

Periodicity and Trends

Explore Period 3 elements and oxides, trends in properties across periods and down groups for A-Level Chemistry.

Tutor AI Team2026-02-25T12:47:11.047Z

# Periodicity and Trends

Periodicity refers to the repeating patterns in physical and chemical properties of elements across periods of the periodic table. At A-Level, you need detailed knowledge of Period 3 elements and their oxides, including their structures, bonding, and reactions with water.

1. Trends Across Period 3

1.1 Atomic Radius

Decreases across the period: Na > Mg > Al > Si > P > S > Cl > Ar

Reason: More protons → greater nuclear charge → stronger attraction for outer electrons → electrons pulled closer → smaller radius. Shielding remains similar (same number of inner shells).

1.2 First Ionisation Energy

Generally increases across Period 3 with two anomalies:

General trend: More protons → greater nuclear charge → harder to remove electron
Anomaly at Al: $1s^2 2s^2 2p^6 3s^2 3p^1$ — the 3p electron is higher energy and easier to remove than the 3s² of Mg
Anomaly at S: $3p^4$ — the paired electron in the 3p orbital experiences extra repulsion, making it easier to remove than the unpaired 3p³ electrons in P

1.3 Electronegativity

Increases across the period (excluding noble gases). Fluorine is the most electronegative element.

1.4 Melting Points

The trend across Period 3 is distinctive:

Element	Na	Mg	Al	Si	P₄	S₈	Cl₂	Ar
Structure	Giant metallic	Giant metallic	Giant metallic	Giant covalent	Simple molecular	Simple molecular	Simple molecular	Monatomic
MP trend	Low	Higher	Higher	Very high	Low	Low	Very low	Very low

Na, Mg, Al: Metallic bonding; MP increases (more delocalised electrons: 1→2→3, and decreasing ionic radius)
Si: Giant covalent — very strong covalent bonds → very high MP
P₄, S₈, Cl₂, Ar: Simple molecular — weak intermolecular forces → low MPs

2. Period 3 Oxides

2.1 Formulae and Structures

Oxide	Formula	Bonding	Structure
Sodium oxide	Na₂O	Ionic	Giant ionic
Magnesium oxide	MgO	Ionic	Giant ionic
Aluminium oxide	Al₂O₃	Ionic (with covalent character)	Giant ionic
Silicon dioxide	SiO₂	Covalent	Giant covalent
Phosphorus(V) oxide	P₄O₁₀	Covalent	Simple molecular
Sulfur trioxide	SO₃	Covalent	Simple molecular
Dichlorine monoxide	Cl₂O	Covalent	Simple molecular

2.2 Reactions with Water

Ionic oxides (Na₂O, MgO) → form alkaline solutions:

$\text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH} \quad (\text{pH} \approx 13-14)$

$\text{MgO} + \text{H}_2\text{O} \rightarrow \text{Mg(OH)}_2 \quad (\text{pH} \approx 9-10, \text{slightly soluble})$

Covalent oxides (P₄O₁₀, SO₃) → form acidic solutions:

$\text{P}_4\text{O}_{10} + 6\text{H}_2\text{O} \rightarrow 4\text{H}_3\text{PO}_4 \quad (\text{pH} \approx 1-2)$

$\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4 \quad (\text{pH} \approx 0-1)$

Aluminium oxide (Al₂O₃) is amphoteric — it reacts with both acids and bases:

$\text{Al}_2\text{O}_3 + 6\text{HCl} \rightarrow 2\text{AlCl}_3 + 3\text{H}_2\text{O}$ $\text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} \rightarrow 2\text{NaAl(OH)}_4$

SiO₂ doesn't react with water (giant covalent, insoluble) but reacts with bases: $\text{SiO}_2 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SiO}_3 + \text{H}_2\text{O}$

2.3 Acid-Base Character Summary

$\text{Na}_2\text{O} \xrightarrow{\text{basic}} \text{MgO} \xrightarrow{\text{basic}} \text{Al}_2\text{O}_3 \xrightarrow{\text{amphoteric}} \text{SiO}_2 \xrightarrow{\text{weakly acidic}} \text{P}_4\text{O}_{10} \xrightarrow{\text{acidic}} \text{SO}_3 \xrightarrow{\text{acidic}}$

Trend: basic → amphoteric → acidic across the period.

3. Period 3 Chlorides

Chloride	Formula	Bonding
NaCl	Ionic	Dissolves; neutral pH 7
MgCl₂	Ionic	Dissolves; slightly acidic pH 6.5
AlCl₃	Covalent (when anhydrous)	Vigorous reaction; acidic (pH ~3); HCl fumes
SiCl₄	Covalent	Vigorous hydrolysis; very acidic; HCl fumes
PCl₅	Covalent	Vigorous; very acidic; HCl fumes

$\text{SiCl}_4 + 2\text{H}_2\text{O} \rightarrow \text{SiO}_2 + 4\text{HCl}$

$\text{PCl}_5 + 4\text{H}_2\text{O} \rightarrow \text{H}_3\text{PO}_4 + 5\text{HCl}$

Ionic chlorides dissolve; covalent chlorides undergo hydrolysis (react with water to form HCl).

Worked Example: Melting Points

Problem

Question: Explain why silicon has a much higher melting point than phosphorus.

Solution

Silicon has a giant covalent structure with many strong Si-O covalent bonds extending throughout the structure. A large amount of energy is needed to break these bonds. Phosphorus (P₄) has a simple molecular structure with weak London dispersion forces between molecules, requiring little energy to overcome.

Worked Example: Oxide Reactions

Problem

Question: Describe what happens when phosphorus(V) oxide is added to water.

Solution

P₄O₁₀ reacts vigorously with water to form phosphoric acid (H₃PO₄). The solution is acidic (pH ~1-2). Universal indicator would turn red.

5. Practice Questions

1. Sketch a graph of first ionisation energy across Period 3. Explain the general trend and the anomalies at Al and S.
1. Explain the trend in melting points from Na to Ar.
1. Write equations for the reactions of Na₂O and SO₃ with water.
1. Why is Al₂O₃ described as amphoteric? Write equations for its reactions with HCl and NaOH.
1. Compare the reactions of NaCl and SiCl₄ with water.

Want to check your answers and get step-by-step solutions?

6. Exam Tips

Know the structures (giant vs simple molecular) — this explains melting points
The oxide acid-base trend is one of the most-tested topics
Al₂O₃ is amphoteric — learns its reactions with both acids AND bases
Covalent chlorides undergo hydrolysis (not just dissolving)
IE anomalies: think about subshells (s vs p) and electron pairing

Summary

Across Period 3: atomic radius ↓, IE ↑ (with anomalies), electronegativity ↑
Melting points: metals rise (Na<Mg<Al), Si very high (giant covalent), then low (simple molecular)
Oxides: basic (Na₂O, MgO) → amphoteric (Al₂O₃) → acidic (SiO₂, P₄O₁₀, SO₃)
Chlorides: ionic dissolve neutrally; covalent undergo hydrolysis → acidic + HCl

Tags:

#a-level#chemistry#inorganic#periodicity

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