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The Periodic Table

Periodic trends, Period 3 chemistry and Group 2 and 17 patterns.

  • GCE A-Level H2 Chemistry 9476-2027
  • 6 lessons
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Before you begin

Periodic trends, Period 3 chemistry and Group 2 and 17 patterns.

Learning goals
  • Periodic Trends (Radius, IE, EN)
  • Period 3 Elements: Melting Point and Conductivity
  • Period 3 Oxides and Chlorides
  • Group 2 Chemistry Trends
  • Group 17 Chemistry Trends
  • Deducing Unknown Elements from Data
Syllabus statements covered
  • recognise variation in the electronic configurations across a Period and down a Group
  • describe and explain qualitatively the general trends and variations in atomic radius, ionic radius, first ionisation energy and electronegativity: — across a Period in terms of shielding and nuclear charge
  • describe and explain qualitatively the general trends and variations in atomic radius, ionic radius, first ionisation energy and electronegativity: — down a Group in terms of increasing number of electron shells, shielding and nuclear charge
  • interpret the variation in melting point and in electrical conductivity across a Period in terms of structure and bonding in the elements (metallic, giant molecular, or simple molecular)
  • state and explain the variation in the highest oxidation number of the elements in oxides (for Na2O; MgO; Al2O3; SiO2; P4O10; SO3) and chlorides (for NaCl; MgCl2; AlCl3; SiCl4; PCl5)
  • state and explain the variation in bonding in oxides and chlorides in terms of electronegativity (with the exception of AlCl3)
  • describe the reactions of the oxides with water (for Na2O; MgO; Al2O3; SiO2; P4O10; SO3)
  • describe and explain the acid/base behaviour of oxides (for Na2O; MgO; Al2O3; SiO2; P4O10; SO3) and hydroxides (for NaOH; Mg(OH)2; Al(OH)3), including, where relevant, amphoteric behaviour in reaction with sodium hydroxide (only) and acids
  • describe and explain the reactions of the chlorides with water (for NaCl; MgCl2; AlCl3; SiCl4; PCl5)
  • suggest the types of structure and bonding present in the oxides and chlorides from observations of their chemical and physical properties
  • describe and deduce from E⦵ values the relative reactivity of elements of: — Group 2 as reducing agents
  • describe and explain the trend in thermal stability of: — Group 2 carbonates in terms of the charge density of the cation and the polarisability of the large anion
  • describe and explain the trend in volatility of the Group 17 elements in terms of instantaneous dipole-induced dipole attraction
  • describe and deduce from E⦵ values the relative reactivity of elements of: — Group 17 as oxidising agents
  • describe and explain the trend in thermal stability of: — Group 17 hydrides in terms of bond energies
  • predict the characteristic properties of an element in a given Group by using knowledge of chemical periodicity
  • deduce the nature, possible position in the Periodic Table, and identity of unknown elements from given information of physical and chemical properties

Lessons

Work through them in order.

  1. Periodic Trends (Radius, IE, EN)Explain trends in atomic radius, ionisation energy and electronegativity, and compare ionic radii.
  2. Period 3 Elements: Melting Point and ConductivityExplain physical-property patterns using structure and bonding.
  3. Period 3 Oxides and ChloridesExplain the acid–base behaviour of Period 3 oxides and the reactions of their chlorides with water.
  4. Group 2 Chemistry TrendsExplain Group 2 reducing strength from electrode potentials and the thermal stability of carbonates.
  5. Group 17 Chemistry TrendsExplain Group 17 volatility, oxidising strength and hydrogen-halide stability.
  6. Deducing Unknown Elements from DataIdentify an unknown element from data, moving from evidence to inference to conclusion.

Practise and check

Recommended nextThe Periodic Table topic check

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Topic reference

This topic covers physical periodicity, Period 3 chemistry, Group 2 and Group 17 trends, and deducing unknown elements from data. The lessons build the trend drivers first, then apply them across Period 3 and down Groups 2 and 17.

Be comfortable with:

The Group 2 and Group 17 lessons introduce the electrode-potential reasoning they need, so you do not need to complete Electrochemistry first.

Quick Reference

A Level periodic-table focus regionsThree vertically arranged panels identify the exact Period 3, Group 2 and Group 17 element ranges and the direction of the principal trends.Topic 5 focus regionsAcross Period 3Na → Mg → Al → SiP → S → ClElectrons enter the third shell.Nuclear charge increases.Down Group 2MgCaSrBaouter configuration: ns²radius and shielding increasereducing strength increasescarbonate stability increasesDown Group 17ClBrIouter configuration: ns²np⁵volatility decreasesoxidising strength decreasesH–X thermal stability decreasesRanges match the 2027 9476 syllabus.
The 9476 periodicity focus: sodium to chlorine across Period 3, magnesium to barium down Group 2, and chlorine to iodine down Group 17.
  • Explain a trend: state the change in Z, shielding and distance; link the resulting attraction to the property.
  • Compare ionic radii: check electron count first; within an isoelectronic series, higher Z means smaller radius.
  • Classify an oxide or hydroxide: state basic, amphoteric or acidic, then prove it with the requested reaction.
  • Use E⦵ data: remember that tables show reductions. More positive means the left-hand oxidant is reduced more readily; more negative E⦵(M²⁺/M) means the metal is oxidised more readily.
  • Deduce an element: quote the clue, make one inference, cross-check it, then state the conclusion.

What You Must Memorise

  • Across a period: Z increases; electrons added to same shell; shielding changes little → attraction increases.
  • Down a group: more shells → distance and shielding increase → attraction decreases.
  • Isoelectronic ions: same electrons; higher Z → smaller radius.
  • Oxide acidity across Period 3: basic → amphoteric → acidic (support with equations).
  • Highest oxidation number: rises from +1 in Na₂O/NaCl to +6 in SO₃; the specified chlorides stop at +5 in PCl₅.
  • Thermal stability: Group 2 carbonates increase down the group; hydrogen halides decrease from HCl to HI.

Common Exam Traps

  • Writing “it increases” with no driver (nuclear charge / shielding / distance) and no attraction link.
  • Treating shielding as the main across-period driver (it changes little across a period).
  • Mixing atomic radius vs ionic radius rules (cation smaller; anion larger; isoelectronic → higher Z is smaller).
  • Period 3 oxides: claiming acidic/basic/amphoteric without a supporting reaction.
  • Group 2: treating a more negative E⦵(M²⁺/M) as a weaker reducing metal, or omitting charge density and polarisation for carbonates.
  • Group 17: treating a less positive E⦵(X₂/X⁻) as a stronger oxidising halogen, or confusing H–X bond strength with intermolecular forces.
  • Halogens are diatomic: write Cl₂, Br₂, I₂ (not Cl).
  • Displacement ionic equations must conserve atoms and charge (and show the correct electron transfer logic).