AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to: define first ionisation energy write equations…
Full specification wording
Electron configuration: Students should be able to: define first ionisation energy write equations for first and successive ionisation energies explain how first and successive ionisation energies in Period 3 (Na–Ar) and in Group 2 (Be–Ba) give evidence for electron configuration in sub-shells and in shells.
First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions. Write equations with state symbols, e.g. Na(g) → Na⁺(g) + e⁻. Across Period 3, values rise overall as nuclear charge increases while shielding stays similar, but there are two dips. Aluminium falls below magnesium because its outer electron is in a 3p sub-shell, higher in energy and shielded by the 3s pair. Sulfur falls below phosphorus because its 3p⁴ arrangement forces two electrons into one orbital, causing mutual repulsion. Down Group 2, values fall as radius and shielding increase. A large jump between successive values marks a change to a shell nearer the nucleus.
What earns the marks
- Defining first ionisation energy as the energy required to remove one mole of electrons from one mole of gaseous atoms.
- An equation showing one electron removed with gaseous state symbols on both species.
- The large jump between successive values identifying a change of shell.
- The dip at aluminium explained by the outer electron occupying a 3p sub-shell of higher energy.
- The dip at sulfur explained by repulsion between the paired electrons in one 3p orbital.
Common mistakes and how to put them right
Writing the equation without gaseous state symbols.
Showing two electrons removed in a first ionisation energy equation.
Explaining the aluminium dip by extra shielding from an additional shell rather than the 3s pair and higher 3p energy.
Attributing the sulfur dip to a change in nuclear charge instead of paired-electron repulsion.
Exam tips
- Count the jumps, not the values: the number of electrons before the first big jump gives the group number, except for Group 0 where it is 8.
- Name the sub-shell, 3s or 3p, whenever you explain a Period 3 dip; a vague reference to energy levels will not score.
- For the second ionisation energy always start the equation from the 1⁺ ion, never from the atom.
Try the lesson
The start of the Electron configuration: Students should be able to: define first ionisation energy write equations for first and successive ionisation energies explain how first and successive ionisation energies in Period 3 (Na–Ar) and in Group 2 (Be–Ba) give evidence for electron configuration in sub-shells and in shells. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement correctly defines the first ionisation energy of an element?
Listen
First ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions. Write equations with state symbols, e.g. Na(g) → Na⁺(g) + e⁻. Across Period 3, values rise overall as nuclear charge increases while shielding stays similar, but there are two dips. Aluminium falls below magnesium because its outer electron is in a 3p sub-shell, higher in energy and shielded by the 3s pair. Sulfur falls below phosphorus because its 3p⁴ arrangement forces two electrons into one orbital, causing mutual repulsion. Down Group 2, values fall as radius and shielding increase. A large jump between successive values marks a change to a shell nearer the nucleus. A strong answer earns marks for these points. defining first ionisation energy as the energy required to remove one mole of electrons from one mole of gaseous atoms. an equation showing one electron removed with gaseous state symbols on both species. the large jump between successive values identifying a change of shell. the dip at aluminium explained by the outer electron occupying a 3p sub-shell of higher energy. Exam tip: Count the jumps, not the values: the number of electrons before the first big jump gives the group number, except for Group 0 where it is 8.
Key words
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first ionisation energy
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ionisation equations
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Period 3 trend
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Group 2 trend
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See every definition
- first ionisation energy
- The energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions.
- ionisation equations
- Equations with state symbols showing one electron removed, for example Na(g) → Na⁺(g) + e⁻.
- Period 3 trend
- Values rise overall as nuclear charge increases while shielding stays similar, but there are two dips.
- Group 2 trend
- Values fall down the group as radius and shielding increase.
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Test your understanding of Electron configuration: Students should be able to: define first ionisation energy write equations for first and successive ionisation energies explain how first and successive ionisation energies in Period 3 (Na–Ar) and in Group 2 (Be–Ba) give evidence for electron configuration in sub-shells and in shells.
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