CIE AS Chemistry – Topic 9 : Periodic Table & Chemical Periodicity (Part 1 & 2)

Vocabulary flashcards summarising key terms, trends and reactions connected with physical and chemical periodicity of Period 3 elements for CIE AS Chemistry.

Periodicity

The repeating, predictable trends in physical and chemical properties of elements as atomic number increases across periods or down groups.

Atomic Radius

Half the distance between the nuclei of two bonded atoms of the same element; decreases across Period 3 due to increasing nuclear charge and constant shielding.

Ionic Radius

Size of an ion; cations shrink relative to atoms, anions expand; across Period 3 cationic radii decrease Na⁺→Si⁴⁺, while anionic radii decrease P³⁻→Cl⁻.

Nuclear Charge

The total positive charge of the nucleus (number of protons); increases one unit at each step across a period.

Shielding Effect

Repulsion by inner-shell electrons that reduces attractive pull of nucleus on valence electrons; remains almost constant across a period.

Effective Nuclear Charge (Z*)

The net attraction exerted by the nucleus on valence electrons after shielding; rises across a period, pulling electrons closer.

First Ionisation Energy (IE₁)

Energy needed to remove one mole of electrons from one mole of gaseous atoms; generally increases across Period 3 with small dips at Al and S.

Spin-Pair Repulsion

Extra electron-electron repulsion when two electrons occupy the same orbital; explains IE₁ drop from P to S.

s-block

Portion of the Periodic Table where the outermost electrons occupy an s orbital; Groups 1 and 2.

p-block

Section where the highest‐energy electrons occupy p orbitals; Groups 13–18 (except He).

d-block

Set of transition elements whose last electron enters a d orbital.

f-block

Lanthanides and actinides where the last electron enters an f orbital.

Metallic Bonding

Electrostatic attraction between a lattice of positive metal ions and a ‘sea’ of delocalised electrons.

Giant Metallic Lattice

Three-dimensional array of metal cations surrounded by mobile electrons; gives metals high conductivity and variable melting points.

Number of Delocalised Electrons

Key factor in metallic bond strength; increases Na (1) < Mg (2) < Al (3), raising melting point and conductivity accordingly.

Giant Covalent (Macromolecular) Structure

Network where atoms are joined by covalent bonds throughout the lattice, e.g. SiO₂, giving very high melting points.

Van der Waals (id-id) Forces

Weak instantaneous dipole–induced dipole attractions between molecules; dominate in simple molecular solids like P₄, S₈, Cl₂, Ar.

Cation

Positively charged ion formed by electron loss; smaller than parent atom and has higher charge density.

Anion

Negatively charged ion formed by electron gain; larger than parent atom due to added electron–electron repulsion.

Melting-Point Trend (Period 3)

Rises Na→Al (stronger metallic bonds), peaks at Si (giant covalent), then falls sharply P→Ar (simple molecules with weak id-id forces).

Electrical Conductivity Trend (Period 3)

Increases Na < Mg < Al (more delocalised electrons), drops drastically at Si (semiconductor) and is negligible for P, S, Cl, Ar.

Basic Oxide

Oxide that reacts with water to give alkaline solution or with acids to form salts; e.g. Na₂O, MgO.

Acidic Oxide

Oxide that reacts with water to form acids or with bases to form salts; e.g. P₄O₁₀, SO₂, SO₃, SiO₂ (with hot alkali).

Amphoteric Oxide

Oxide that reacts with both acids and bases; Al₂O₃ is amphoteric, insoluble in water but dissolves in acid or hot alkali.

Oxidation Number Trend (Period 3 Oxides)

Increases from +1 in Na₂O to +6 in SO₃, mirroring the number of valence electrons available for bonding.

Hydrolysis

Reaction of a compound with water, often breaking covalent bonds and forming acidic or basic solutions (e.g. SiCl₄ + H₂O).

Hydrated Ion

Ion surrounded by a coordinated shell of water molecules, e.g. [Al(H₂O)₆]³⁺, stabilising ions in aqueous solution.

[Al(H₂O)₆]³⁺ Complex

Highly charged, small aluminium ion surrounded by six water ligands; undergoes partial hydrolysis releasing H⁺ and giving acidity.

Na + H₂O Reaction

2 Na (s) + 2 H₂O (l) → 2 NaOH (aq) + H₂ (g); vigorous, produces strongly alkaline solution (pH ≈ 14).

Mg + Cold Water

Mg (s) + 2 H₂O (l) → Mg(OH)₂ (aq) + H₂ (g); slow, yields weakly alkaline solution (pH ≈ 11).

Mg + Steam

Mg (s) + H₂O (g) → MgO (s) + H₂ (g); vigorous red-hot reaction producing magnesium oxide.

Silicon Dioxide (SiO₂)

Giant covalent network of tetrahedral Si each bonded to four O atoms; hard, high-melting and acidic toward hot alkali.

Aluminium Oxide (Al₂O₃)

Ionic‐covalent lattice, amphoteric, very high melting point (~2850 °C); unreactive with water.

Phosphorus(V) Oxide (P₄O₁₀)

Simple molecular solid; reacts violently with water to give phosphoric(V) acid, showing strong acidity.

Sodium Chloride in Water

Ionic lattice dissociates: NaCl (s) → Na⁺ (aq) + Cl⁻ (aq); pH ~7 (neutral).

Magnesium Chloride in Water

MgCl₂ (s) → Mg²⁺ (aq) + 2 Cl⁻ (aq); solution slightly acidic only if Mg²⁺ hydrolyses minimally.

Aluminium Chloride Dimer (Al₂Cl₆)

Covalent dimer in the solid or gaseous state; in water it dissociates, ions hydrate, and hydrolysis yields acidic solution.

Silicon Tetrachloride Hydrolysis

SiCl₄ (l) + 2 H₂O (l) → SiO₂ (s) + 4 HCl (g); vigorous, produces misty HCl fumes and acidic mixture.

Phosphorus(V) Chloride Hydrolysis

PCl₅ (s) + 4 H₂O (l) → H₃PO₄ (aq) + 5 HCl (g); rapid, exothermic, strongly acidic products.

Trend of Cationic Radii (Period 3)

Na⁺ > Mg²⁺ > Al³⁺ > Si⁴⁺ because increasing nuclear charge pulls fewer electron shells closer.

Trend of Anionic Radii (Period 3)

P³⁻ > S²⁻ > Cl⁻; radii decrease as nuclear charge rises while electron number stays constant within the anion series.

Argon (Ar)

Monatomic noble gas at end of Period 3; full valence shell makes it chemically inert with extremely low melting and boiling points.

Hydroxide Basicity Trend

NaOH is strongly basic and very soluble; Mg(OH)₂ is sparingly soluble, weakly basic; Al(OH)₃ shows amphoteric behaviour.

Effective Nuclear Charge & Atomic Size

A greater Z* pulls valence electrons closer, resulting in smaller atomic radii and higher IE₁ across a period.

Position of Silicon in Melting-Point Graph

Highest peak across Period 3 owing to its giant covalent lattice requiring extensive energy to break.