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1.4.3Physical Chemistry

Ionic Trends — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 12 of 42

📚 Edexcel AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 4Ionic Radius versus Atomic Radius

Ion size determines how closely ions can pack, which affects the strength of the electrostatic attraction.

Cations
  • Cations A metal atom that loses electrons has fewer shells and the same number of protons pulling fewer electrons,
    • so the positive ion is smaller than the atom.
Anions
  • Anions A non-metal atom that gains electrons has more repulsion between electrons with unchanged nuclear charge, so the negative ion is larger.
Down a group
  • Down a group Ions in a group have the same charge but more shells lower down, so ionic radius increases.
Isoelectronic ions
  • Isoelectronic ions Ions with the same number of electrons shrink as nuclear charge rises, so N³⁻ > O²⁻ > F⁻ > Na⁺ > Mg²⁺ > Al³⁺.
Group 1 example
  • Group 1 example Li⁺ is smaller than Na⁺, which is smaller than K⁺, because each ion has one more full shell than the one above it.

Part 2 of 4Charge Density

Charge Density
Definition
  • Definition Charge density is the ionic charge compared with the ionic radius, so small, highly charged ions have the highest values.
Attraction
  • Attraction Ions with high charge density attract oppositely charged ions more strongly.
Greater charge
  • Greater charge Doubling the charge on both ions roughly quadruples the attraction at the same separation.
Smaller radius
  • Smaller radius Smaller ions can approach more closely, increasing the force.
Lattice link
  • Lattice link Small, highly charged ions pack closely, giving strong attraction through the whole lattice and a high melting point.
CompoundIonsMelting point / °C
NaClNa⁺, Cl⁻801
MgOMg²⁺, O²⁻2852
CaOCa²⁺, O²⁻2613

Part 3 of 4Applying the Trends

1
Compare the charges on the ions: higher charges give much stronger ionic bonding.
2
Compare the ionic radii: smaller ions give stronger bonds when charges are equal.
3
MgO melts far higher than NaCl because both of its ions have double the charge and the radii are smaller.
4
CaO melts a little lower than MgO because Ca²⁺ is larger than Mg²⁺, which weakens the attraction.

Part 4 of 4Exam Checklist & Tips

1.4.3 – Key Points for the Exam

  • Isoelectronic Having the same number of electrons, for example O²⁻, F⁻, Na⁺ and Mg²⁺.
  • Ionic radius The size of an ion, measured from its nucleus to the edge of its electron cloud.
  • Charge density Charge divided by size, showing how concentrated an ion's charge is.
  • Electrostatic attraction The force between oppositely charged particles, stronger at higher charge and smaller separation.

Common Mistakes

  • Saying a cation is larger than its parent atom; it is smaller.
  • Ordering isoelectronic ions by number of electrons, which is identical, instead of by nuclear charge.
  • Ignoring the charges and comparing melting points by radius alone.
  • Describing the force between ions as a covalent attraction.

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