🟣 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.
🔑 The Big Jumps
The increase is not perfectly linear. Massive, discontinuous jumps in required energy occur when an electron is removed from a new, inner principal energy level that is significantly closer to the nucleus and less shielded by inner electrons.
Worked Example: Identifying the Group
Problem
An unknown element X has the following successive ionization energies (kJ mol⁻¹):
578 → 1817 → 2745 → 11578 → 14831 → 18378
Identify the group of element X.
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 → 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: Group 2 → Group 13
Example: Be (900 kJ mol⁻¹) → B (801 kJ mol⁻¹)
Explanation: Group 13 elements remove an electron from a 2p sublevel, which is at a higher energy than the 2s sublevel from which Group 2 loses its electron. The 2p electron is also partially shielded by the 2s electrons, so it is easier to remove.
Drop: Group 15 → Group 16
Example: N (1402 kJ mol⁻¹) → O (1314 kJ mol⁻¹)
Explanation: Group 16 has a paired electron in one of the 2p orbitals. The inter-electron repulsion between two electrons sharing the same orbital makes one easier to remove than the unpaired electrons in Group 15's half-filled p subshell.
⚠️ Examiner Tip
The IB requires you to explain IE discontinuities in terms of the energy of the electron being removed, not in terms of "special stability" of half-filled subshells. Focus on sublevel, shielding, and electron-electron repulsion in your answer.
Explore the Interactive Periodic Table
See ionization energies and electron configurations for every element across the periodic table.