When atoms gain or lose electrons, they become charged particles called ions.
The energy needed to remove those electrons is known as ionisation energy — a key idea in atomic structure and periodicity.
Definition
Ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous positive ions.
For example:
This process requires energy because the electron is attracted to the positively charged nucleus.
Understanding the Concept
Electrons are held in energy levels by electrostatic forces. To remove an electron, enough energy must be supplied to overcome this attraction. The greater the attraction between the nucleus and the outer electrons, the higher the ionisation energy.
Factors Affecting Ionisation Energy
Factor | Effect on Ionisation Energy | Explanation |
Nuclear charge | ↑ increases | More protons = stronger attraction between nucleus and electrons. |
Atomic radius | ↓ decreases | Larger distance between nucleus and outer electrons = weaker attraction. |
Electron shielding | ↓ decreases | Inner electrons repel outer ones, reducing the nuclear pull. |
Electron sub-shell structure | varies | Half-filled or full sub-shells are more stable → slightly higher ionisation energy. |
Trends Across the Periodic Table
Across a Period (→)
Ionisation energy increases because:
Nuclear charge increases,
Electrons are added to the same energy level (so little extra shielding),
Atomic radius decreases.
Down a Group (↓)
Ionisation energy decreases because:
Atomic radius increases,
More inner shells cause greater shielding,
Outer electrons are farther from the nucleus and easier to remove.
Successive Ionisation Energies
If you keep removing electrons from the same atom, each one requires more energy than the last. This is because, as electrons are removed, the remaining electrons experience a stronger attraction to the nucleus.
Example – for aluminium:
There’s a large jump after the second ionisation, showing magnesium has two outer electrons.
Why Ionisation Energy Matters
In Chemistry ionisation energy helps explain:
Periodic trends and how the periodic table is organised,
Reactivity (especially in Group 1 and Group 7 elements),
Bond formation and why metals form positive ions while non-metals gain electrons,
Spectroscopic evidence for energy levels within atoms.
Real-World Connection
Ionisation energy isn’t just theoretical — it’s measured using techniques like photoelectron spectroscopy, which fires photons at atoms to knock out electrons. The data confirms the arrangement of electrons in shells and sub-shells — proving the structure you learn in class is grounded in experimental evidence.
Key Takeaways
Ionisation energy = energy to remove an electron from a gaseous atom.
Depends on nuclear charge, atomic radius, shielding, and sub-shell structure.
Increases across a period, decreases down a group.
Successive ionisation energies reveal number of outer electrons and group position.
📝 Sample Exam Question (6 marks)
The graph shows the successive ionisation energies for sodium.
Use the pattern to explain why this element is in group 1, period 3.
Model answer
Group 1 - There are 11 points on graph which relates to the 11 electrons in sodium. This also means that there are 11 protons the atomic number 11.
After electron 1 is removed, there is a huge jump in ionisation energy. This relates to the 1 electron in outermost shell. This electron has the lowest ionisation energy because it is furthest from the nucleus and also encounters the most shielded because it is the outermost shell. This 1 outer electrons means that sodium is in group 1.
Period 3 - After the first electron is removed there is a huge jump in ionisation energy which indicates that the next electron is removed from a different shell to the first one. Electrons 2-9 on the graph have similar energy which means they must be in the same shell. After electron 9 is removed there is a huge jump in ionisation energy which indicates that the next electron is removed from a different shell to electron 9.
There are 2 jumps in the graph (from shell 1 to 2 and then from shell 2 to 3) which indicates that there are 3 shells of electrons therefore sodium is in period 3.
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