Explain ionisation-energy trends and successive jumps
Use nuclear charge, shielding, radius and electron repulsion to interpret periodic trends and infer electron structure from successive values.
AS Level · Cambridge International Chemistry 9701, 2025–2027 syllabus. Electronic configurations are limited to ground-state atoms and ions of elements H to Kr. Written practice is self-checked; it does not replace cumulative exam practice or human subject-teacher review.
Before you begin
- Use atomic symbols and the Periodic Table, and distinguish an atom from an ion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Why does first ionisation energy generally decrease down a group?
What you will learn
- Explain trends in ionisation energy across a period and down a group
- Explain successive-ionisation patterns and subshell effects
- Infer electron configurations and Periodic Table position from data
Compare nuclear pull, distance and shielding
Across a period, first ionisation energy generally increases: proton number rises while the outer electron enters the same main shell, so nuclear attraction generally strengthens. Down a group, it generally decreases because the outer electron is farther from the nucleus and more shielded by inner shells.
Subshell energy and electron pairing produce exceptions to a perfectly smooth across-period trend. An electron in a higher-energy subshell can be easier to remove; repulsion between paired electrons can also lower an ionisation energy. A precise explanation should name the comparison and the factor that changes, rather than memorising a single arrow.
A large successive jump reveals a shell boundary
Successive ionisation energies rise as electrons are removed from an increasingly positive ion. A much larger jump occurs when removal begins from an inner shell: inner-shell electrons are closer to the nucleus and less shielded by other electrons.
To infer an element's position, locate the large jump after the valence electrons have been removed. If the jump is after the second electron, the atom had two outer-shell electrons and is in Group 2 for the main-group examples considered. Use the number of electrons removed before the jump to infer the configuration, then verify the total against the element's proton number if provided.
Pause and explain
A very large jump occurs between IE₂ and IE₃. How many outer-shell electrons did the atom have?
Worked example
The first four ionisation energies of an element are 590, 1145, 4910 and 6480 kJ mol⁻¹. Deduce its outer-electron count and likely main-group position.
Show the worked solution
- Compare each successive increase.
- The large jump is from IE₂ to IE₃.
- Two electrons were removed before an inner-shell electron is reached.
Answer The atom has two outer-shell electrons and is likely in Group 2. The sharp rise at the third ionisation indicates that the third electron is removed from an inner shell.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain why the first ionisation energy generally increases from sodium to argon, but do not claim that every adjacent value must rise.
Give me a hint
State the overall trend first, then identify factors that can cause local irregularities.
Compare my reasoning
- Proton number rises across Period 3.
- The added electron enters the same main shell, so shielding does not rise by a full shell.
- Subshell energy and paired-electron repulsion can create dips.
The first ionisation energy generally increases because nuclear charge rises while electrons are added to the same main shell, increasing attraction to an outer electron. The trend is not perfectly smooth: subshell energy and repulsion between paired electrons can make individual values lower than a neighbour's.
Look for these in your work
- Overall trend stated
- Nuclear charge and shell shielding compared
- Exceptions qualified
Successive ionisation energies are 738, 1450, 7730 and 10 500 kJ mol⁻¹. Deduce the number of outer-shell electrons and explain the large increase.
Give me a hint
Find the largest change and count removals before it.
Compare my reasoning
- The largest jump is between IE₂ and IE₃.
- Two electrons were removed before the jump.
- The third electron comes from a closer, less shielded inner shell.
The atom has two outer-shell electrons. The third ionisation removes an inner-shell electron, which is closer to the nucleus and less shielded, so much more energy is needed.
Look for these in your work
- Jump located
- Valence-electron count inferred
- Inner-shell explanation
An unknown element has a large jump after its fourth ionisation. Explain what you can infer and state one limitation.
Give me a hint
A jump identifies the number of electrons removed before the inner-shell step, but check which element families the question permits.
Compare my reasoning
- Four electrons were removed before the large jump.
- The atom therefore has four outer-shell electrons in the simple main-group interpretation.
- Do not infer a unique element or period without additional data.
The atom has four outer-shell electrons and is consistent with Group 14 among main-group elements. The jump alone does not identify a unique element or period; additional ionisation data or the proton number is needed.
Look for these in your work
- Four-electron inference justified
- Group inference qualified
- Limitation stated
Common mistakes
- Saying shielding increases across a period by one full shell at every step
- Explaining an exception with nuclear charge alone
- Calling every increase a shell jump
- Inferring a group from the first value instead of the large successive jump
What can you explain now?
Why does first ionisation energy generally decrease down a group?
Compare with the explanation
The outer electron is farther from the nucleus and more shielded
Greater distance and shielding generally reduce attraction.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How does a successive-ionisation jump reveal outer-shell electron count, and which factors explain periodic trends?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.