QCE Chemistry

Successive Ionisation Energy and Evidence for Shells

Published 2026-10-20 · Updated 2026-10-20

Successive ionisation energy questions trip up a surprising number of Queensland Chemistry students, not because the concept is difficult, but because students misread the data tables or forget what a "large jump" actually means. QCAA assessors frequently include these questions in the external assessment, and knowing exactly how to interpret the numbers, and what they prove, can be the difference between a strong and a weak response.

What the syllabus asks

This concept sits within Unit 1: Chemical Fundamentals. You need to understand what successive ionisation energies are, why they always increase, and, most importantly, how the pattern of large jumps in the data provides direct experimental evidence for the existence of electron shells. You also need to be able to use a table of successive ionisation energies to identify how many valence electrons an element has, and therefore which group it belongs to.

The idea, explained

What is ionisation energy?

Ionisation energy (IE) is the energy required to remove one mole of electrons from one mole of gaseous atoms or ions. The first ionisation energy (IE1) removes the first electron from a neutral gaseous atom. The second ionisation energy (IE2) removes a second electron from the resulting +1 cation, and so on.

In equation form:

• M(g) → M+(g) + e⁻ corresponds to IE1
• M+(g) → M2+(g) + e⁻ corresponds to IE2

Why do successive ionisation energies always increase?

Each time you remove an electron, the remaining electrons are being held by the same nucleus but with fewer electrons around it. The effective nuclear attraction on each remaining electron is stronger, so every successive electron is harder to remove than the one before it. This means IE1 < IE2 < IE3 < ... for every element, without exception.

The big jumps: evidence for shells

Although every ionisation energy rises, the jumps between them are not all the same size. When the next electron to be removed must come from a shell that is closer to the nucleus (a lower principal energy level), there is a very large jump in ionisation energy compared with removing electrons within the same shell. These large jumps are the key to understanding shell structure.

Sodium (Na, Z = 11) is the classic example. Its electron configuration is 1s2 2s2 2p6 3s1, which gives the shell arrangement 2, 8, 1. The successive ionisation energies (in kJ mol⁻¹) follow this pattern:

IE1: 496, IE2: 4562, IE3: 6912 ... IE9: 28932, IE10: 141367, IE11: 159075

• IE1 (496 kJ mol⁻¹) removes the single 3s electron from the outer shell. It is relatively low energy.
• There is a huge jump to IE2 (4562 kJ mol⁻¹) because the next electron must come from the n = 2 shell, which is much closer to the nucleus.
• IE2 through IE9 rise steadily as eight electrons are removed from the same n = 2 shell.
• There is another enormous jump from IE9 to IE10, because the next electron must come from the n = 1 shell, the innermost shell of all.

The pattern, one small IE, then a big jump, then eight medium IEs, then another enormous jump, matches the shell arrangement 2, 8, 1 exactly. This is direct experimental evidence that electrons in sodium are arranged in three shells containing 2, 8, and 1 electrons respectively.

Using IE data to find group number

If you are given successive ionisation energies for an unknown element, you can identify its group by following these steps:

1. Look for the position of the first very large jump in the data.
2. Count how many electrons were removed before that jump. This equals the number of valence electrons.
3. The number of valence electrons tells you the group (1 valence electron → Group 1, 2 valence electrons → Group 2, and so on).

For example, an element with IE1 = 738, IE2 = 1451, and IE3 = 7733 kJ mol⁻¹ shows a large jump between IE2 and IE3. Two electrons are removed easily before the jump, so the element has 2 valence electrons and belongs to Group 2. This matches magnesium.

What the exam asks

In the external assessment, QCAA assessors may give you a table of successive ionisation energies and ask you to identify the group of an unknown element, explain why a particular jump is large, or describe what the pattern of jumps tells us about electron arrangement. Always refer explicitly to the change in shell (principal energy level) when explaining a large jump, and always connect the number of electrons removed before the jump to the number of valence electrons.

Common mistakes

• Thinking successive IEs can decrease. They cannot. Every successive IE is larger than the one before it because the ion becomes more positively charged each time, increasing nuclear attraction on the remaining electrons.
• Confusing a large jump with the valence electrons being removed after the jump. The valence electrons are the ones removed before the first large jump, not after it.
• Saying the jump occurs because the electron is "further from the nucleus". It is the opposite: the large jump occurs because the next electron is in a shell closer to the nucleus, making it much harder to remove.
• Forgetting to write the equation correctly. IE2 starts from the +1 cation (M+(g)), not from the neutral atom. QCAA assessors look for this precision.
• Assuming every rise in IE is a shell change. IEs rise steadily within a shell too. Only a very large, sudden jump signals a change to a lower principal energy level.

Mastering this topic is very achievable with the right practice. Avocado is an AI-native tutoring service built specifically for Queensland students, and its Successive Ionisation Energy and Evidence for Shells lesson walks you through data interpretation step by step, so you can approach the external assessment with confidence.