Ionisation energies

CIE A-Level Chemistry· 6 min read

1. Definitions and Key Factors★★☆☆☆⏱ 15 min

📘 Definition

First Ionisation Energy

I_1

The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions.

Example:

For sodium: $Na (g) \to Na^{+} (g) + e^{-} Δ H = I_{1}$

📘 Definition

Successive Ionisation Energies

A series of ionisation energies corresponding to the sequential removal of each electron from a gaseous atom/ion after the first electron is removed.

Example:

Second ionisation energy of sodium: $Na^{+} (g) \to Na^{2 +} (g) + e^{-} Δ H = I_{2}$

Four main factors affect the magnitude of any ionisation energy:

Nuclear charge: More protons = stronger attraction for outer electrons, increasing ionisation energy.
Atomic radius: Larger distance between nucleus and outer electron = weaker attraction, decreasing ionisation energy.
Shielding: Inner electrons repel outer electrons, reducing net nuclear attraction, decreasing ionisation energy.
Sub-shell stability: Electrons in full or half-full sub-shells have extra stability, increasing ionisation energy.

📐 Worked Example

Write the balanced equation for the third ionisation energy of aluminium.

1
Recall that the nth ionisation energy removes one mole of electrons from the (n-1)+ gaseous ion. For the third ionisation energy, we start with Al²⁺ (gaseous).
2
Write the full balanced equation:
3
$\text{Al}^{2+}(g) \rightarrow \text{Al}^{3+}(g) + e^-$

Exam tip:

Always include (g) state symbols for all species in ionisation energy equations — examiners regularly penalise missing gaseous states.

2. Trends in First Ionisation Energies★★★☆☆⏱ 20 min

Two key periodic trends are observed for first ionisation energy:

Down a group: Each new element has an extra inner electron shell, so shielding and atomic radius increase. These effects outweigh the increase in nuclear charge, so first ionisation energy decreases down a group.
Across a period: Nuclear charge increases, while electrons are added to the same outer shell so shielding stays almost constant. Attraction increases, atomic radius decreases, so first ionisation energy generally increases across a period. Small dips in the trend occur between Group 2→13 and Group 15→16.

📐 Worked Example

Explain why the first ionisation energy of chlorine is higher than that of sulfur.

1
Both elements are in Period 3, so their outer electrons are in the n=3 shell, meaning shielding is identical for both.
2
Chlorine has one more proton than sulfur, so its nuclear charge is higher.
3
Higher nuclear charge creates a stronger attraction between the nucleus and outer electrons.
4
More energy is required to remove an electron, so first ionisation energy of chlorine is higher.

3. Successive Ionisation Energies and Electron Configuration★★★☆☆⏱ 18 min

Large proportional jumps in successive ionisation energies indicate that the next electron is being removed from a new inner electron shell, which is much closer to the nucleus. This pattern allows us to deduce the number of outer electrons, and therefore the group number of the unknown element.

📐 Worked Example

The successive ionisation energies (kJ mol⁻¹) for an element are: 738, 1451, 7733, 10540, 13630. Deduce the group of this element.

1
Look for the largest proportional jump between successive values. The jump between the second (1451) and third (7733) ionisation energy is a 5x increase, much larger than any other change.
2
This large jump confirms that after removing two electrons, the next electron is removed from an inner shell closer to the nucleus.
3
This means there are two outer electrons, so the element belongs to Group 2 of the periodic table.

4. Exam Phrasing★★★★☆⏱ 10 min

✓ Quick check

Test your understanding:

Which of the following describes where the first large jump occurs for a Group 15 element?
- After 3 electrons
- After 5 electrons
- After 15 electrons
- After 2 electrons
Reveal answer
After 5 electrons —
Correct: A Group 15 element has 5 outer electrons, so the big jump comes after all 5 outer electrons are removed.

5. Common Pitfalls

Wrong move:

Forgetting to include (g) state symbols in ionisation energy equations

Why:

CIE examiners always penalise missing state symbols, as ionisation energy is only defined for gaseous species

Correct move:

Always write (g) after every species in any ionisation energy equation

Wrong move:

Explaining decreasing IE down a group by only mentioning increasing nuclear charge

Why:

Nuclear charge does increase, but the effects of increased shielding and atomic radius are larger and dominate the trend

Correct move:

Always state that increased shielding and atomic radius outweigh the increase in nuclear charge when explaining trends down a group

Wrong move:

Removing inner shell electrons first when analysing successive ionisation energies

Why:

Electrons are always removed from the highest energy (outermost) shell first

Correct move:

Outer electrons are removed first, so large jumps occur when moving to an inner shell, after all outer electrons are removed

Wrong move:

Attributing the IE drop between Mg and Al to increased nuclear charge of Al

Why:

While nuclear charge is higher, the key factor is the energy level of the outer electron

Correct move:

Explain the drop by noting Al's outer electron is in the higher energy 3p sub-shell, further from the nucleus, so less energy is required for removal

6. Quick Reference Cheatsheet

Concept	Key Point	Trend Summary
First IE	Energy to remove 1 mol e⁻ from gaseous atoms
Successive IE	Always increase; big jump = new inner shell
Down a group	I₁ decreases	Shielding ↑, radius ↑ > nuclear charge ↑
Across a period	I₁ generally increases	Nuclear charge ↑, shielding constant, radius ↓
Exception 1: Group 2 → 13	I₁ drops	Outer e⁻ in higher energy p sub-shell
Exception 2: Group 15 → 16	I₁ drops	Paired p electron has extra repulsion

7. Frequently Asked

Why do successive ionisation energies always increase?

Each subsequent electron is removed from an increasingly positive ion, so remaining electrons experience a stronger attraction to the nucleus, requiring more energy for removal.

Why is first ionisation energy of Al lower than Mg?

Aluminium's outer electron is in the higher energy 3p sub-shell, which is further from the nucleus than magnesium's 3s outer electron, so less energy is required to remove it.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 1
Deduce group from ionisation data
2023 · 2
Explain trend across Period 3
2021 · 1
Write ionisation energy equation

Going deeper

syllabusCIE 9701 2025-2026 SyllabusSee page 32 for learning outcomes

What's Next

Ionisation energies are a core foundational concept for CIE A-Level Chemistry, heavily tested in both multiple choice and structured questions. Understanding how ionisation energy trends work will help you explain other periodic properties like electronegativity, atomic radius, and reactivity, and supports the model of electron sub-shell structure that underpins all of chemical bonding and reactivity. This topic connects directly to the study of periodicity as a whole, which is a major topic in the second half of your AS Level course. Mastering ionisation energy explanations now will help you access full marks on many common exam questions later.