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3.1.3Additional lessons

Periodic Trends — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 39 of 98

📚 IB Chemistry SL 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 6Atomic and Ionic Radius

Trends are explained by nuclear charge, shielding and distance of the valence electrons.

Across a period
  • Across a period Nuclear charge increases while shielding stays similar, so the valence electrons are pulled closer and radius decreases.
Down a group
  • Down a group Extra electron shells increase the distance and shielding, so atomic radius increases.
Cations
  • Cations A metal loses electrons and often an entire shell, so a cation is smaller than its atom, e.g. Na 186 pm and Na⁺ 102 pm.
Anions
  • Anions Extra electrons repel one another, so an anion is larger than its atom, e.g. Cl 99 pm and Cl⁻ 181 pm.

Part 2 of 6Ionisation Energy

1
First ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous atoms: X(g) → X⁺(g) + e⁻.
2
Across a period it generally increases because nuclear charge rises and radius falls, so electrons are held more strongly.
3
There is a small drop from Mg to Al because the 3p electron is higher in energy and slightly shielded by the 3s electrons.
4
There is another drop from P to S because in sulfur two electrons share a 3p orbital and repel each other.
5
Down a group it decreases because the valence electron is further from the nucleus and more shielded.
ElementNaMgAlPSClAr
First IE / kJ mol⁻¹4967385781012100012511521

Part 3 of 6Electronegativity and Melting Point

Electronegativity and Melting Point
Electronegativity across
  • Electronegativity across It increases across a period because the stronger nuclear charge attracts bonding electrons more strongly.
Electronegativity down
  • Electronegativity down It decreases down a group because bonding electrons are further from the nucleus and more shielded.
Melting point, metals
  • Melting point, metals Across Na, Mg, Al melting point rises as metallic bonding strengthens with more electrons per atom.
Melting point, non-metals
  • Melting point, non-metals Giant covalent silicon is very high, while P, S and Cl form molecules with weak forces and low melting points.

Part 4 of 6Writing Good Explanations

Nuclear charge
  • Nuclear charge State that the number of protons increases across a period, so attraction on valence electrons increases.
Shielding
  • Shielding Inner electrons shield valence electrons, and this changes little across a period but increases down a group.
Distance
  • Distance Valence electrons in higher shells are further from the nucleus, so they are held less strongly.
Compare clearly
  • Compare clearly Use the words more, less, greater and smaller to compare, and refer to both elements.

Part 5 of 6Worked Example

Question Explain why the first ionisation energy of sodium (496 kJ mol⁻¹) is lower than that of magnesium (738 kJ mol⁻¹).

  1. Both elements lose a 3s electron and have the same number of inner shells, so shielding is similar.
  2. Magnesium has 12 protons and sodium 11, so the nuclear charge is greater.
  3. The 3s electron in magnesium is attracted more strongly and the atom is slightly smaller, so more energy is needed to remove it.

Answer Magnesium has a higher nuclear charge with similar shielding, so its outer electron is held more strongly.

Part 6 of 6Exam Checklist & Tips

3.1.3 – Key Points for the Exam

  • Ionisation energy The energy needed to remove one mole of electrons from one mole of gaseous atoms.
  • Shielding The reduction in attraction from the nucleus due to inner electrons.
  • Atomic radius Half the distance between nuclei of two bonded atoms of an element.
  • Electronegativity The ability of an atom to attract bonding electrons.

Common Mistakes

  • Saying the nuclear charge decreases down a group.
  • Explaining trends only by distance and forgetting shielding.
  • Forgetting the drops at Al and S in period 3 ionisation energies.
  • Stating that an anion is smaller than its atom.

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