Successive Ionization Energies & Shell Evidence
This is Higher Level (HL) content.
What Are Successive Ionization Energies?
Successive ionization involves stripping electrons from an atom one by one until only the bare nucleus remains. As each electron is removed from an increasingly positive ion, the electrostatic ratio of protons to electrons increases. So each subsequent ionization requires more energy.
Big Jumps in Successive Ionization Energy
The increase in successive IE is not perfectly linear: dramatic, non-linear jumps occur when an electron is removed from a new, inner principal energy level closer to the nucleus and experiencing vastly reduced electron shielding.
Worked Example: Identifying the Group
Deducing Group Number from Successive IE Data
Problem: An unknown element X has successive ionization energies (kJ mol⁻¹): 738, 1451, 7733, 10543, 13630. Identify its group.
The big jump occurs between IE₂ and IE₃ (from 1451 to 7733). This proves the atom has 2 valence electrons. Element X belongs to Group 2 (e.g. Magnesium).
Analysis:
• IE₁ → IE₃: relatively steady incremental increases (578, 1817, 2745)
• IE₃ → IE₄: massive jump (2745 → 11578). Nearly 4× larger
Conclusion: The 4th electron is being pulled from a full, stable inner shell. Element X has 3 valence electrons → Group 13.
Successive Ionization Energies (log scale)
The position of the first big jump tells you the number of valence electrons, which corresponds to the group number.
Transition Metals and Variable Oxidation States
In transition metals, the successive ionization energies required to remove 3d electrons show relatively small, gradual increases after the 4s electrons are removed. This gentle energetic gradient is the root cause of their defining characteristic: variable oxidation states.
First Ionization Energy Discontinuities
When first IE values are plotted across a period, the general trend is an increase from left to right (due to increasing nuclear charge). However, there are two key discontinuities that the IB expects you to explain.
Drop from Group 2 to Group 13 (s to p Sublevel)
Example: Be (900 kJ mol⁻¹) → B (801 kJ mol⁻¹)
The 2p electron in Boron is in a higher energy subshell and is shielded by the 2s² electrons, making it easier to remove than the 2s electron in Beryllium.
Drop from Group 15 to Group 16 (Electron Spin Pairing)
Example: N (1402 kJ mol⁻¹) → O (1314 kJ mol⁻¹)
In Oxygen, the 4th 2p electron must pair up with an existing electron in the 2px orbital. Inter-electron repulsion between paired electrons lowers the energy required to remove it compared to Nitrogen's singly occupied, half-filled 2p³ subshell.
Explaining IE Discontinuities in Exam Answers
The IB requires you to explain IE discontinuities in terms of subshells (s vs p) and electron pairing / repulsion. Never say "Oxygen is an exception to the octet rule" : mark schemes explicitly reward the subshell shielding and electron-pair repulsion arguments.
Explore the Interactive Periodic Table
See ionization energies and electron configurations for every element across the periodic table.
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