Successive Ionisation Energies

Successive ionisation energy questions are data questions: spot the big jump, translate it into “valence electrons”, then link the jump to inner-shell removal.

Use your course’s Atomic Structure topic navigation to review electron configurations first. Then connect successive ionisation energies to trends in the Periodic Table topic.

Definitions (Must Know)

A. Successive ionisation energies

The successive ionisation energies are the energies required to remove one mole of electrons, one electron at a time, from one mole of gaseous atoms/ions.

X(g) → X⁺(g) + e⁻ X⁺(g) → X²⁺(g) + e⁻ X²⁺(g) → X³⁺(g) + e⁻

B. Valence (outer-shell) electrons

Valence electrons are the electrons in the highest occupied shell (outer shell). These are removed first.

Key Ideas (What Earns Marks)

• Each successive ionisation energy is larger than the previous (electron removed from a more positive ion → greater attraction).
• A large jump indicates removal of an electron from an inner shell (closer to nucleus, less shielding).
• The number of electrons removed before the first big jump equals the number of valence electrons.

Successive Ionisation Energies: Three Valence Electrons

Data table

Ionisation number	Energy
IE₁	578
IE₂	1817
IE₃	2745
IE₄	11577

Detailed Explanations

A. How to deduce group (workflow)

1. Look for the first big jump in values.
2. Count how many electrons were removed before that jump.
3. That count is the number of valence electrons → group (for main-group elements).

Mini example: a big jump between IE₃ and IE₄ means 3 valence electrons → Group 13 (main group).

B. Why a “big jump” happens

Successive ionisation energies rise because the ion becomes more positive each time, so the remaining electrons are attracted more strongly.

The first large jump happens when you move from removing valence electrons to removing an inner-shell electron. Inner-shell electrons are closer to the nucleus and experience less shielding, so the attraction is much greater and much more energy is needed.

C. Deducing position and electron configuration

For a main-group element, the first large jump locates the boundary between the valence shell and the next inner shell. This gives the number of valence electrons and hence the group.

To deduce a fuller electron configuration or the exact position of an element, combine the jump pattern with all the supplied information:

• period given + number of valence electrons → position in the Periodic Table;
• complete successive-ionisation series → shell populations, because large jumps mark changes to lower principal quantum numbers;
• proton number given → exact electron count and configuration.

For example, large jumps after the 1st and 9th electrons in a complete series are consistent with shell populations 2,8,1: one valence electron, eight in the next shell and two in the innermost shell. The element is in Period 3, Group 1.

Remove an element’s electrons one at a time and plot each measured ionisation energy; switch to “Across periods” to compare first ionisation energies.

Reset

11 protons and 12 neutrons in the nucleus, 11 electrons in shells 2, 8, 1: a sodium-23 atom. No electrons removed yet.

Energy to remove the last electron

— kJ/mol

Build an atomChlorine isotopesChromium and copperSodium's ionisation energies

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BuildIsotopesElectronsIonisation

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Protons

Electrons

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Try this

0 of 4 done

1. Build an atom of carbon-12, then change it into carbon-14. (not done yet)

2. Make a sodium ion, Na⁺, and a chloride ion, Cl⁻. (not done yet)

3. In the mass spectrum of chlorine, make the two peaks the same height. (not done yet)

4. Remove sodium's electrons one at a time until the energy needed jumps. (not done yet)

Worked Examples

Modelled example 1

Deduce Group from a Large Jump

Core

Problem

An element shows a large jump between IE₂ and IE₃. Deduce its group for a main-group element.

Study the worked solution

1. Locate the jump

   Method

   Count the ionisations completed before the large increase.

   Reason

   The electrons removed before the jump belong to the outer shell.

   Working

   Two electrons are removed before IE₃.

2. Infer the valence count

   Method

   Assign two electrons to the outer shell.

   Reason

   The next electron requires much more energy because it comes from an inner shell.

   Working

   Valence-electron count = 2.

3. State the group

   Working

   The element is in Group 2.

Guided practice 2

Interpret a Successive-Ionisation Data Row

About 7 min

Problem

Successive ionisation energies for an element are shown in kJ mol⁻¹.

IE₁	IE₂	IE₃	IE₄
578	1817	2745	11577

(a) Deduce the group. (b) Explain why there is a large jump where it occurs.

Try this before viewing the solution

Jump, group and shell explanation

Hints

Hint 1: compare adjacent values

Find the first increase that is much larger than the preceding increases.

Hint 2: count outer electrons

Count how many electrons have been removed before that jump.

View solution step by step

1. Find the first large jump

   Method

   Compare each adjacent pair of values.

   Reason

   The increase from 2745 to 11577 is much larger than the earlier increases.

   Working

   The first large jump is between IE₃ and IE₄.

2. Deduce the group

   Method

   Count the three electrons removed before the jump.

   Reason

   A main-group atom with three valence electrons is in Group 13.

   Working

   Group 13.

3. Explain the large jump

   Method

   Contrast the fourth electron with the first three.

   Reason

   After three outer electrons are removed, the fourth comes from a lower-n inner shell, closer to the nucleus and with less shielding.

   Working

   Much stronger attraction → much higher energy required.

Common misconception 3

Correct a Jump-Index Error

Find and correct the mistake

Learner claim

A learner says, “A large jump between IE₂ and IE₃ means the element is in Group 3 because the jump ends at the third ionisation energy.” Identify the error and give the correct group.

Count the relevant electrons

Electrons removed before the jump

Two electronsThree electrons

Check my attempt

View solution step by step

1. Count completed removals

   Method

   Count IE₁ and IE₂, which occur before the jump.

   Reason

   The jump appears when IE₃ attempts to remove the first inner-shell electron.

   Working

   Two valence electrons were removed.

2. Correct the group

   Method

   Map two valence electrons to the main-group position.

   Reason

   The group is based on the outer-electron count, not the later ionisation-energy index.

   Working

   Group 2, not Group 3.

Examiner practice 4

Explain a Jump after the First Electron

4 marks

Problem

An element has a large jump between IE₁ and IE₂. Deduce its group for a main-group element and explain what the jump means in terms of electron shells. [4 marks]

Try this before viewing the solution

Group and shell-attraction explanation

View solution step by step

1. Interpret the jump position

   1 mark

   Method

   Recognise that only one electron is removed before the jump.

   Reason

   IE₂ is the first ionisation to remove an inner-shell electron.

   Working

   One valence electron.

2. Deduce the group

   1 mark

   Reason

   A main-group atom with one valence electron belongs to Group 1.

   Working

   Group 1.

3. Compare the electron environment

   1 mark

   Method

   State that the second electron is in an inner shell.

   Reason

   It is closer to the nucleus and experiences less shielding.

   Working

   The inner electron is much more strongly attracted.

4. Link attraction to the jump

   1 mark

   Method

   State that much more energy is needed for the second removal.

   Reason

   More energy is required to overcome the much stronger attraction.

   Working

   IE₂≫ IE₁.

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Credit the valence count, group and two-part inner-shell explanation separately.

Mark points

1. 1 mark: Infers one valence electron before the jump.
2. 1 mark: Deduces Group 1.
3. 1 mark: Identifies a closer, less-shielded inner-shell electron.
4. 1 mark: Links stronger attraction to much higher energy.

Calculate my mark

Challenge 5

Predict the Jump from a Configuration

Minimal support

Problem

An element has electron configuration 1s² 2s² 2p⁶ 3s² 3p². Predict where the first large jump in its successive ionisation energies occurs and explain your prediction.

Try this before viewing the solution

Valence count, jump position and explanation

Hints

Hint 1: count the outer-shell electrons

Only the n = 3 electrons are removed before the first inner-shell electron.

Hint 2: name both indices

Four electrons are removed as IE₁ through IE₄.

View solution step by step

1. Count the valence electrons

   Method

   Add the 3s and 3p electron counts.

   Reason

   The n = 3 shell is the outer occupied shell.

   Working

   3s² 3p² gives four valence electrons.

2. Predict the jump

   Method

   Place the jump after those four electrons have been removed.

   Reason

   The fifth electron is the first to come from the n = 2 inner shell.

   Working

   The first large jump is between IE₄ and IE₅.

3. Explain the energy increase

   Method

   Compare the inner-shell electron with the removed valence electrons.

   Reason

   It is closer to the nucleus and less shielded, so it experiences much stronger attraction.

   Working

   Inner-shell removal requires much more energy.

Next step

Try the Period 3 identification Mind Stretcher independently.

Common Mistakes

• Using the jump position as “number of shells” (it’s about valence vs inner-shell electrons).
• Forgetting that the first big jump corresponds to moving to a lower principal quantum number (inner shell).
• Naming an exact element from one jump without enough information about its period or proton number.

Use the topic check in Practise and check below to practise and check your understanding.

Exam Tips

• In explanations, use keywords: “inner shell”, “less shielding”, “closer to nucleus”, “greater attraction”.
• If asked to identify the element, combine the group deduction with given data (e.g. atomic number / mass / period).
• The “big jump → group” shortcut is for main-group elements. Transition-metal patterns require their actual configurations: 4s electrons are removed before 3d electrons.

Mind Stretchers

Mind stretcher 1Extension

An element in Period 3 shows a large jump between IE₂ and IE₃. Identify the element and write its electron configuration.

Show Answer

Mark scheme:

• A big jump after removing 2 electrons means 2 valence electrons → Group 2.
• Period 3, Group 2 is magnesium, Mg.
• Electron configuration: 1s² 2s² 2p⁶ 3s².

Mind stretcher 2Extension

Explain why IE₄ is usually much larger than IE₃ when the first big jump is between IE₃ and IE₄.

Show Answer

Mark scheme:

• The first three electrons removed are valence electrons (outer shell).
• After three electrons are removed, the next electron comes from an inner shell (lower n), closer to the nucleus and with less shielding.
• Attraction is much stronger, so the energy required is much larger.