The Periodic Table S3.1.7 Discontinuities in Ionization Energy Trends IB DP Chemistry Study Notes - New Syllabus 2025
The periodic table: Classification of elements- IB DP Chemistry- Study Notes
IITian Academy excellent Introduction to the Particulate Nature of Matter – Study Notes and effective strategies will help you prepare for your IB DP Chemistry 2025 exam.
- IB DP Chemistry 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Chemistry 2025 SL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
- IB DP Chemistry 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 1
- IB DP Chemistry 2025 HL- IB Style Practice Questions with Answer-Topic Wise-Paper 2
Structure 3.1.7 – Discontinuities in First Ionization Energy
Structure 3.1.7 – Discontinuities in First Ionization Energy
Definition of First Ionization Energy
The first ionization energy is the energy required to remove one mole of electrons from one mole of gaseous atoms:
X(g) → X⁺(g) + e⁻
General Trend Across a Period
As you move across a period (left to right) in the periodic table:
- The nuclear charge increases (more protons).
- The outer electrons remain in the same energy level.
- Shielding remains constant (no additional inner shells).
- Atomic radius decreases (electrons are pulled closer).
- Attraction between nucleus and outer electrons increases.
Result: More energy is needed to remove an outer electron → ionization energy increases across a period.
Drop in Ionization Energy Between Periods
When moving from the end of one period to the start of the next:
- The outer electron enters a new energy level (further from nucleus).
- Increased shielding from inner shells weakens nuclear attraction.
- Despite increased nuclear charge, ionization energy drops.
Graphical Representation
A graph of first ionization energies from hydrogen (Z = 1) to neon (Z = 10), or from lithium to argon, shows:
- A general increase in ionization energy across the period.
- Two notable discontinuities (Be → B and N → O).
- A sharp drop between the end of one period and start of the next (e.g., Ne → Na).
Example
Why does the first ionization energy decrease from nitrogen to oxygen across Period 2?
▶️Answer/Explanation
- Nitrogen has a half-filled 2p subshell with all electrons unpaired.
- Oxygen has one pair of electrons in a 2p orbital, increasing repulsion.
- The repulsion between paired electrons makes it easier to remove one.
- Hence, oxygen has a lower ionization energy than nitrogen.
Explanation of Discontinuities in Terms of Energy Sublevels
What Are Energy Sublevels?
Each principal energy level (n = 1, 2, 3…) is divided into sublevels (s, p, d, f) with slightly different energies. Electrons fill sublevels in order of increasing energy, following the Aufbau principle.
Evidence from Ionization Energy Data
The first ionization energy values show two clear discontinuities across a period. These discontinuities support the existence of sublevels within the same principal energy level.
Discontinuities in the Trend
Discontinuity 1: Between Beryllium and Boron (Be → B)
- Be configuration: 1s² 2s² → removes a 2s electron.
- B configuration: 1s² 2s² 2p¹ → removes a 2p electron.
- The 2p electron is higher in energy and less strongly attracted to the nucleus than a 2s electron.
- Result: Boron has a lower ionization energy than expected.
This demonstrates that the 2s and 2p sublevels have different energies — clear evidence of sublevel structure within energy levels.
Discontinuity 2: Between Nitrogen and Oxygen (N → O)
- N configuration: 1s² 2s² 2p³ → each 2p orbital has 1 electron (no pairing).
- O configuration: 1s² 2s² 2p⁴ → one 2p orbital has a pair of electrons.
- Repulsion between paired electrons makes one easier to remove.
- Result: Oxygen has a lower ionization energy than expected.
This drop in ionization energy cannot be explained by overall nuclear charge alone. It supports the idea of electron pairing within orbitals and reinforces the structure of sublevels and orbitals.
Summary Table: Trend vs. Discontinuities
| Element | Configuration | Ionization Energy (kJ/mol) | Reason for Discontinuity |
|---|---|---|---|
| Be | 1s² 2s² | 900 | Electron removed from stable 2s orbital |
| B | 1s² 2s² 2p¹ | 801 | Electron removed from higher-energy 2p orbital |
| N | 1s² 2s² 2p³ | 1402 | Stable half-filled 2p orbitals (no repulsion) |
| O | 1s² 2s² 2p⁴ | 1314 | Electron pair in 2p causes repulsion → easier removal |
Example
What does the drop in first ionization energy from Be to B suggest about atomic structure?
▶️Answer/Explanation
- Beryllium: 2s² configuration → electron removed from 2s sublevel.
- Boron: 2s² 2p¹ → electron removed from 2p sublevel.
- 2p electrons are at a higher energy and more shielded than 2s.
- This explains the drop in ionization energy and provides evidence that 2s and 2p are distinct sublevels within the same principal energy level.
Example
How does the difference in ionization energy between N and O provide evidence for sublevel structure?
▶️Answer/Explanation
- Nitrogen (2p³) has all unpaired electrons in separate orbitals → no repulsion.
- Oxygen (2p⁴) has one paired orbital → electron-electron repulsion.
- The repulsion reduces the energy needed to remove one electron.
- Therefore, the drop in ionization energy is due to orbital-level effects, showing sublevel and pairing structure within the atom.