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1.3.6Additional lessons

Successive Ionisation Energies (HL) — IB Chemistry HL Revision Notes & Topical Questions

5 min read 4 parts Lesson 13 of 172

📚 IB Chemistry HL 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 4Definition and Trend

Successive ionisation energies measure the removal of electrons one at a time from the same atom.

Definition
  • Definition The nth ionisation energy is the energy needed to remove one mole of electrons from one mole of gaseous ions with charge (n−1)+.
Always increases
  • Always increases Each removal is harder because the ion is more positive while the nuclear charge stays the same.
Radius
  • Radius As electrons are removed the remaining electrons are drawn closer, so they are held more strongly.
Equation
  • Equation For the second ionisation of magnesium: Mg⁺(g) → Mg²⁺(g) + e⁻.

Part 2 of 4Large Jumps

Large Jumps
Big increase
  • Big increase A very large jump occurs when electrons start to be removed from a new inner shell.
Reason
  • Reason Inner-shell electrons are closer to the nucleus and experience less shielding, so they are held far more strongly.
Valence electrons
  • Valence electrons The number of electrons removed before the big jump equals the number in the outer shell.
Group
  • Group The number of valence electrons shows the group number of a main-group element.
IonisationMagnesium / kJ mol⁻¹
1st738
2nd1451
3rd7733
4th10543

Part 3 of 4Using the Data

The data for magnesium can be used to deduce its group.

1
List the successive ionisation energies in order and look at the ratio of each one to the previous.
2
Between the 2nd (1451) and 3rd (7733) values there is a sharp rise of about five times.
3
This shows that two electrons are removed easily from the outer shell before the inner shell is reached.
4
Magnesium therefore has two valence electrons and is in group 2.
5
A graph of log₁₀(ionisation energy) against number removed shows the jump clearly and compresses the large range of values.

Part 4 of 4Exam Checklist & Tips

1.3.6 – Key Points for the Exam

  • Successive ionisation energy The energy for removing each electron in turn from a gaseous atom.
  • Shielding Reduction in attraction between the nucleus and outer electrons caused by inner electrons.
  • Valence electrons Electrons in the outermost shell of an atom.
  • Logarithmic scale A scale that shows ratios, useful for data covering a wide range.

Common Mistakes

  • Saying the big jump happens between the first and second ionisation of every element.
  • Explaining the increase by an increase in the shielding, when shielding does not increase.
  • Stating that the number of values before the jump gives the period rather than the group.
  • Forgetting that the 2nd ionisation energy is for a 1+ ion, not a neutral atom.

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