Ionisation energies
Ionisation energies
First ionisation energy and attraction
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Periodicity connection: ionisation energies are included here to show how electron structure explains experimental evidence.
First ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms, forming one mole of gaseous 1+ ions. Keep all those details in an exam definition.
Energy is needed to overcome the electron's attraction to the nucleus, so ionisation is endothermic. The unit is kJ mol⁻¹. A second ionisation starts from gaseous 1+ ions; later ionisations start from increasingly positive ions.
- First ionisation energy
- The energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
X an atom of the element in the gas state; X+ a gaseous ion of the element with charge +1; e− an electron; (g) gas.
| Factor | Consequence |
|---|---|
| More protons | Greater nuclear charge usually strengthens attraction. |
| Greater atomic radius | The removed electron is further from the nucleus, weakening attraction. |
| More inner-electron shielding | Weaker attraction to the electron being removed. |
| Higher-energy subshell | Less additional energy may be needed to remove its electron. |
| Paired electrons in an orbital | Electron repulsion can make one easier to remove. |
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
- Define first ionisation energy. [3]
- Write the first-ionisation equation for potassium, including state symbols. [1]
- State the units and whether the process is endothermic or exothermic. [2]
Total: 6 marks
Show answer & marking points
- Energy to remove one electron from each atom [1] in one mole of gaseous atoms [1], forming one mole of gaseous 1+ ions [1].
- K(g) → K⁺(g) + e⁻. [1]
- kJ mol⁻¹ [1]; endothermic [1].
Ionisation-energy trends and exceptions
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Periodicity connection: explain each trend through the attraction between the nucleus and the electron being removed.
Across a period: nuclear charge increases while electrons enter the same main shell. Inner-shell shielding changes little, the radius decreases and attraction strengthens. First ionisation energy generally increases.
Down a group: the outer electron is further away and more shielded. These effects outweigh the increased nuclear charge, so attraction weakens and first ionisation energy generally decreases.
For exceptions, check the subshell and electron pairing. Removing a higher-energy p electron, or an electron repelled by its partner in a p orbital, can require less energy than the simple trend suggests.
| Decrease | Configurations | Explanation |
|---|---|---|
| Be → B | Be 1s² 2s²; B 1s² 2s² 2p¹ | B loses a higher-energy, more shielded 2p electron rather than 2s. |
| N → O | N ends 2p³; O ends 2p⁴ | O has one paired p orbital. Pair repulsion makes an electron easier to remove. |
| Mg → Al | Mg [Ne] 3s²; Al [Ne] 3s² 3p¹ | Al loses a higher-energy, more shielded 3p electron rather than 3s. |
| P → S | P ends 3p³; S ends 3p⁴ | S has a paired p orbital; pair repulsion lowers the energy needed. |
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
Be is 1s² 2s²; B is 1s² 2s² 2p¹. N ends in 2p³; O ends in 2p⁴.
- Explain why B has a lower first ionisation energy than Be. [3]
- Explain why O has a lower first ionisation energy than N. [2]
- Explain why first ionisation energy generally decreases down a group. [4]
Total: 9 marks
Show answer & marking points
- B loses a 2p electron rather than a 2s electron [1]. It is higher in energy / more shielded [1], so less strongly attracted and easier to remove [1].
- O has a paired 2p orbital [1]; repulsion within that pair makes an electron easier to remove [1].
- The outer electron occupies a higher shell [1], further from the nucleus [1], with greater shielding [1]. These effects outweigh the greater nuclear charge, reducing attraction and energy needed [1].
Using successive ionisation energies
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Periodicity connection: a sequence of ionisation energies can reveal the atom's shell structure.
The energies rise as electrons are removed from an increasingly positive ion. A much larger jump marks the start of removal from an inner shell, where electrons are closer to the nucleus and less shielded.
- Find the first large jump.
- Count the electrons removed before that jump. This is the outer-shell electron count.
- For a main-group element, link that number to the group.
- Use any further large jumps to deduce the populations of inner shells.
| Outer-shell electrons | Group |
|---|---|
| 1 or 2 | Group 1 or 2 |
| 3, 4, 5, 6, 7, 8 | Groups 3, 4, 5, 6, 7, 0 respectively |
A large jump after three removals
Al has successive IEs 578, 1817, 2745 and 11 578 kJ mol⁻¹.
Identify where the largest jump occurs and explain what it shows about the number of outer-shell electrons and the group of the element.
Worked answer
- The large jump is between IE₃ and IE₄.
- Three outer electrons have been removed before the jump.
- The fourth electron comes from an inner shell.
Read more than one shell
All 13 successive ionisations of an atom show large jumps after removals 3 and 11.
Deduce the electron arrangement by shells and identify the element. Explain how the jumps support your answer.
Worked answer
- First 3 removals: outer shell contains 3 electrons.
- Next 8 removals: second shell contains 8 electrons.
- Last 2 removals: innermost shell contains 2 electrons.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
A Period 3 element has successive ionisation energies 738, 1451, 7733 and 10 543 kJ mol⁻¹. Period 3 elements: Na, Mg, Al, Si, P, S, Cl, Ar.
- Locate the large jump and state the outer-shell electron count. [2]
- Identify the element. [1]
- Explain the jump. [3]
- Write the equation for its third ionisation energy. [1]
Total: 7 marks
Show answer & marking points
- Between IE₂ and IE₃ [1]; two outer-shell electrons [1].
- Magnesium. [1]
- The third electron comes from an inner shell [1], closer to the nucleus with less shielding [1], so it is much more strongly attracted and harder to remove [1].
- Mg²⁺(g) → Mg³⁺(g) + e⁻. [1]