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Here is a comprehensive, high-yield Obsidian Markdown note tailored specifically for the Cambridge CIE AS/A-Level Chemistry (9701) syllabus. It synthesizes the syllabus requirements, detailed revision notes, and highly specific examiner insights derived from past paper mark schemes.

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⚛️ Chapter 9: The Periodic Table & Chemical Periodicity

Tags: chemistry alevel cie #9701 periodicity inorganic Syllabus Focus: 9.1 Physical Periodicity (Period 3), 9.2 Chemical Periodicity (Period 3), 9.3 Predicting Properties

What is Periodicity?

Periodicity is the repeating pattern of physical and chemical properties of elements as you move across the periods of the Periodic Table. In A-Level Chemistry, Period 3 (Na to Ar) is our primary model for understanding these trends.

Periodic Table

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📉 1. Physical Properties of Period 3 Elements (9.1)

Physical properties are dictated entirely by the chemical bonding and macromolecular structure of the elements.

A. Atomic Radius

• Trend: Decreases across the period.
• Reason: The nuclear charge (number of protons) increases, pulling the electrons in more tightly. The shielding effect remains roughly constant because the extra electrons are added to the same principal quantum shell.

Atomic Radius Graph

B. Ionic Radius

• Trend for Cations (Na⁺ to Si⁴⁺): Decreases. They are much smaller than their parent atoms (loss of the outer shell = less shielding). The increasing nuclear charge pulls the remaining shell in tighter.
• Trend for Anions (P³⁻ to Cl⁻): Decreases. They are larger than their parent atoms (added electrons increase electron-electron repulsion, causing the cloud to expand). Across the non-metals, nuclear charge increases, so the radius shrinks from P³⁻ to Cl⁻.

Ionic Radius Graph

C. Melting Point

Melting points depend on the type of structure and bonding.

• Na, Mg, Al (Giant Metallic): Melting point increases.
  • Why? The charge on the metal ion increases (Na⁺, Mg²⁺, Al³⁺) and the number of delocalised electrons increases. This results in a stronger electrostatic attraction (stronger metallic bonding).
• Si (Giant Molecular/Covalent): Highest melting point in the period.
  • Why? Massive network of strong covalent bonds must be broken to melt the solid.
• P, S, Cl, Ar (Simple Molecular): Melting points drop significantly and depend on weak London dispersion forces (instantaneous dipole-induced dipole forces).
  • Order: .
  • Why? Sulfur exists as rings (more electrons = stronger London forces), while Phosphorus is . Argon exists as single atoms.

Melting Point Graph

D. Electrical Conductivity

• Increases from Na to Al (more delocalised electrons per atom available as charge carriers).
• Drops heavily at Si (metalloid/semiconductor – no free electrons, but can conduct under certain conditions).
• Zero for P to Ar (non-metals, simple molecules, electrons are localized in covalent bonds).

Examiner Trick

When asked why aluminium has a higher melting point/conductivity than sodium, you must mention two factors for full marks: (1) Higher charge on the cation ( vs ) AND (2) A greater number of delocalised electrons in the “sea”.

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⚡ 2. Electronegativity & Bonding Variations (9.2)

Electronegativity Definition

The power of an atom to attract the pair of electrons in a covalent bond towards itself.

• Trend: Increases from Na to Cl.
• Reason: Increasing nuclear charge with the same shielding pulls bonding electrons closer.

Electronegativity Graph

This explains the shift across Period 3:

• Na to Al: Low electronegativity form Giant Metallic lattices.
• Si: Intermediate forms Giant Covalent lattice.
• P to Cl: High electronegativity form Simple Molecular structures (covalent bonds within, weak forces between).

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🔥 3. Reactions of Period 3 Elements

A. Reaction with Oxygen

Elements react to form oxides with their highest stable oxidation states (except Sulfur, which mostly forms ).

Element	Equation	Reaction	Flame Colour	Product Obs.
Na		Vigorous	Bright yellow	White solid
Mg		Vigorous	Bright white	White solid
Al		Fast (powder)	Bright white	White powder
Si		Slow (high heat)	Bright sparkles	White powder
P		Vigorous	Yellow/White	White clouds
S		Gentle	Blue	Toxic gas
reaction: vixen’s vulva feels so very good
flame: Bright (yellow white white sparkles) yellow blue
product: white (solid solid powder powder clouds) toxic gas

B. Reaction with Chlorine

Element	Equation	Reaction	Product
Na		Vigorous, heat	white solid
Mg		Vigorous, heat	white solid
Al		Vigorous, heat	pale yellow solid
Si		Slow, heat	colourless liquid
P		Slow, excess
reaction: vigorous vigorous vigorous slow slow

C. Reaction with Water (Na and Mg only)

• Sodium (Cold Water): Vigorous. Melts into a ball, fizzes, hydrogen gas given off.
  • (Strongly alkaline, pH 14)
• Magnesium (Cold Water): Extremely slow.
  • (Weakly alkaline, pH 9-11, due to low solubility of )
• Magnesium (STEAM): Vigorous reaction.

Sodium reacting with water

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🧪 4. Period 3 Oxides & Hydroxides

Acid / Base Nature of Oxides

The nature of the oxides shifts from Basic Amphoteric Acidic as bonding shifts from Ionic Covalent.

Oxide	Structure/Bonding	Acid/Base Nature	Reaction with Water	pH of solution
	Giant Ionic	Basic		12 - 14
	Giant Ionic	Basic		8 - 10
	Giant Ionic (covalent char.)	Amphoteric	Insoluble	-
	Giant Covalent	Acidic	Insoluble	-
	Simple Molecular	Acidic		1 - 2
	Simple Molecular	Acidic		1 - 2
	Simple Molecular	Acidic		1

Past Paper Trap: Amphoteric Aluminium Oxide

You must know the equations for acting as both an acid and a base. This comes up frequently!

• As a base (reacting with acid):
• As an acid (reacting with hot conc. base): (Note: The product is sodium aluminate).

Past Paper Trap:

is insoluble in water. However, it is classified as an acidic oxide because it reacts with hot, concentrated alkali:

Hydroxides

• : Strong base.
• : Weak base (used in indigestion remedies).
• : Amphoteric.
  • With Acid:
  • With Base:

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🌫️ 5. Period 3 Chlorides & Water

The behaviour of chlorides with water depends entirely on their bonding.

Chloride	Bonding	Structure	Reaction with Water	pH	Observations
	Ionic	Giant Ionic	Dissolves (hydration)	7.0	White solid dissolves colourless sol.
	Ionic	Giant Ionic	Dissolves (slight hydrolysis)	6.5	White solid dissolves colourless sol.
	Covalent	Simple Molec.	Hydrolyses highly exothermic	3.0	White solid dissolves, white fumes of HCl
	Covalent	Simple Molec.	Hydrolyses rapidly	2.0	White fumes of HCl, off-white ppt ()
	Covalent	Simple Molec.	Hydrolyses violently	2.0	White fumes of HCl

The Aluminium Chloride Anomaly

Aluminium chloride is unique. At room temperature, solid is a giant ionic lattice. But upon heating, it sublimes and forms covalent dimers () featuring coordinate (dative) bonds.

Aluminium Chloride Dimer

Why does make water acidic? When is added to water, the high charge density of the ion severely distorts the electron cloud of surrounding water molecules. It causes a water molecule to lose a hydrogen ion (), turning the solution acidic.

Aluminium Hydration

Hydrolysis of Silicon(IV) Chloride

The forms a white precipitate, and the gas escapes as steamy white fumes.

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🔮 6. Predicting Chemical Periodicity (9.3)

Examiners love giving you an unknown element (e.g., Selenium, Germanium, Gallium) and asking you to predict its properties based on its position in the periodic table.

Flowchart for predicting elements

How to approach these questions:

1. Identify the Group: Tells you the number of valence electrons and likely oxidation states.
   • Example: Selenium is in Group 16. It will behave like Sulfur.
2. Identify the block/type: Metal, Metalloid, or Non-metal?
   • Example: Se is a non-metal. It will likely form a simple molecular structure (like ), have a low melting point, and form acidic oxides ().
3. Reactions: Apply the known period 3 element template.
   • Reaction with water: Simple molecules don’t react with water.
   • Reaction with oxygen: Forms covalent oxides.
   • Reaction with chlorine: Forms simple covalent chlorides () that will hydrolyse in water to give white fumes of .

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🧠 Expert Examiner Tips & Common Traps

Having analysed decades of past papers (2009-2022), here are the exact places students lose marks:

1. Dissolving vs. Hydrolysing:
   • and dissolve in water (physical process, ions simply get hydrated).
   • and hydrolyse (chemical reaction, bonds break, gas is evolved). Do not use the word “dissolve” for covalent chlorides!
2. Missing State Symbols:
   • In periodicity equations, state symbols are often required for the marks. Always include them, especially for the formation of or .
3. Phosphorus vs. Sulfur Melting Points:
   • You must know that Sulfur () has a higher melting point than Phosphorus (). Both are simple molecular, but is a larger molecule with more electrons, meaning it has stronger instantaneous dipole-induced dipole forces (London forces).
4. Silicon Dioxide ():
   • Do NOT write . It does not react with water. It is a giant covalent structure (like sand/quartz). Its acidic nature is only proven by reacting it with hot, concentrated .
5. Oxidation Numbers Check:
   • When asked to state oxidation numbers across period 3 oxides, ensure you include the ”+” sign. (e.g., +1, +2, +3, +4, +5, +6).
6. Aluminium Chloride Bonding:
   • Solid = Ionic (with covalent character).
   • Gaseous = Covalent (dimer with dative/coordinate bonds).
7. “Suggest the type of bonding…”
   • If an unknown chloride melts at a low temperature and reacts violently with water to give pH 2, it is simple molecular / covalent. If it melts at and forms a pH 7 solution, it is giant ionic.