What you will learn

• Explain how successive ionisation energies provide evidence for electronic shells.
• Locate a large jump and count the electrons removed before it.
• Deduce outer-shell electron numbers and, with suitable context, main-group membership.
• Use several jumps in a complete sequence to reconstruct shell populations.
• Read logarithmic axes and recognise what a short dataset cannot establish.

A1.3.2 concerns evidence from successive ionisation energies for the electronic structure of an atom. Definitions and equations were covered in A1.3.1. Trends between different elements are reserved for A1.3.3.

1. What is the experimental evidence?

Successive ionisation energies measure the energy needed for each further one-electron removal from an element’s increasingly positive gaseous species. The horizontal axis of a successive-energy graph counts electrons removed from one element; it does not run across the periodic table.

The values increase because the remaining electrons are more strongly bound as the species becomes more positive. However, the increase is not uniform. Some neighbouring values differ much more sharply than others. These large jumps provide evidence that electrons occupy distinct shells rather than a continuous, uniform distribution around the nucleus.

An outer-shell electron is generally farther from the nucleus and shielded by inner-shell electrons. Once that outer shell is emptied, the next electron must be removed from an inner shell. It is closer to the nucleus and experiences less shielding from inner electrons, so much more energy is required.

A large jump between IEₙ and IEₙ₊₁ indicates that n electrons were removed before reaching a more strongly bound inner shell.

Do not confuse the increase between every pair of successive values with a particularly large shell-boundary jump. Use the overall pattern, the scale of the change and any supplied electronic or periodic-table context.

2. Count before the jump, not after it

Suppose the first substantial shell-boundary jump is between the second and third ionisation energies. The first two electrons were removed relatively easily from the outer shell; the third is the first electron removed from an inner shell. Therefore, the neutral atom had two outer-shell electrons.

The jump starts after two removals, not after three. The higher value belongs to the first electron of the next, more tightly bound set.

For a main-group element, the first clear shell-boundary jump can identify its outer-shell population: one suggests Group 1; two suggests Group 2; three suggests Group 13; four suggests Group 14, and so on. Use the group-numbering convention requested; Groups 13–18 are sometimes described in school shorthand as Groups 3–0 or 3–8.

Limit this rule: do not automatically label every atom with two highest-shell electrons “Group 2”. Transition metals such as iron also have two 4s electrons in the neutral atom, while 3d electrons participate in further removals. Their sequences need subshell and configuration context. A brief dataset alone may not establish the element’s identity.

3. Read tables and graphs accurately

First confirm what the numbers represent: individual ionisation energies, not cumulative totals. Check units and read the ionisation stage numbers. Compare adjacent values and identify the unusually large change associated with entering an inner shell.

A ratio can help when later values are large. For magnesium, IE₂/IE₁ ≈ 1451/738 = 1.97, while IE₃/IE₂ ≈ 7733/1451 = 5.33. This supports the larger relative increase between the second and third removals. There is no universal numerical ratio threshold that defines every shell boundary.

Logarithmic axes: a long sequence can span several orders of magnitude. On a log₁₀ scale, equal vertical distances represent equal multiplicative factors: values 10³, 10⁴ and 10⁵ are equally spaced even though their differences are not equal. Read the tick labels rather than treating the axis as linear. If the graph plots log₁₀(IE / kJ mol⁻¹), a vertical value of 4 corresponds to IE = 10⁴ = 10,000 kJ mol⁻¹.

When asked to explain a jump, write the electronic reason. “The line gets steeper” describes a graph; it does not explain why the next electron is harder to remove.

4. Use several jumps to reconstruct shells

A complete sequence contains one ionisation stage for each electron originally present in the neutral atom. For magnesium, twelve stages correspond to twelve electrons. The major principal-shell boundaries occur between IE₂ and IE₃, and between IE₁₀ and IE₁₁.

1. Stages 1–2 remove two electrons from n = 3.
2. Stages 3–10 remove eight electrons from n = 2: 10 − 2 = 8.
3. Stages 11–12 remove two electrons from n = 1: 12 − 10 = 2.

Removal order: 2 → 8 → 2 (outer to inner).
Conventional shell arrangement: 2, 8, 2 (inner to outer).

The numbers happen to read the same way for magnesium. That is not true for all atoms: a 13-electron main-group atom with major boundaries after stages 3 and 11 has removal groups 3 → 8 → 2, but shell arrangement 2, 8, 3.

Within a principal shell, electrons occupy different subshells and interact with one another. Smaller structure in a sequence can reflect subshell changes and electron interactions. Do not interpret every increase as the start of a new principal shell, or claim that one large jump alone specifies every orbital occupancy.

5. Worked examples: make and justify deductions

6. Common misconceptions and exam tips

• “Jump from IE₃ to IE₄ means four outer electrons.” It means three were removed before entering the inner shell.
• “Electrons enter a new shell at the jump.” Ionisation removes electrons; the next removal comes from a different shell.
• “The nuclear charge increases each time.” Proton number stays fixed. The ion becomes more positive as electrons are removed.
• “A large jump means a larger electron.” Electrons are identical; their environments and binding energies differ.
• “A short sequence reveals every shell and subshell.” It may only reveal an outer-shell boundary. State what extra information is needed.
• “Equal spacing on a log axis means equal energy differences.” It means equal multiplicative factors.
• “Two outer electrons always means Group 2.” Apply the simple group link with appropriate main-group context.

Exam answer sequence: quote the boundary → count electrons before it → state the shell deduction → explain stronger attraction using smaller distance and reduced shielding → use period/group context only when supplied or justified.

Use “evidence supports” rather than claiming that a single jump proves a complete orbital diagram. If identifying an element, explain both the electron-count evidence and the additional information that fixes its position.

Your revision summary

• Successive energies refer to repeated removal from one element’s gaseous species.
• A particularly large jump indicates removal has reached a more tightly bound inner shell.
• A jump between IEₙ and IEₙ₊₁ comes after n electron removals.
• Differences between boundary positions give electron populations in successive shells.
• Conventional shell arrangements are written inner to outer, opposite the removal sequence.
• Read axis scales and avoid claiming more than the dataset establishes.

Magnesium values are rounded to whole kJ mol⁻¹ for teaching, consistent with NIST Atomic Spectra Database values 7.646236, 15.035271 and 80.1436 eV for the first three removals (1 eV per particle ≈ 96.485 kJ mol⁻¹). NIST magnesium ionisation-energy data. Datasets explicitly labelled illustrative are original teaching examples.

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