1.1.6Physical Chemistry
Ionisation Energy — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 6 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Defining Ionisation Energy
Ionisation energy is a measure of how strongly an atom holds its outer electrons.
First ionisation energy
- First ionisation energy The energy needed to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
Equation
- Equation The first ionisation of magnesium is Mg(g) → Mg⁺(g) + e⁻, with state symbols required.
Units
- Units Ionisation energies are quoted in kJ mol⁻¹ and are always positive because energy must be supplied.
Second ionisation energy
- Second ionisation energy This is the energy to remove one mole of electrons from one mole of gaseous 1+ ions: Mg⁺(g) → Mg²⁺(g) + e⁻.
Part 2 of 5Factors Affecting Ionisation Energy
Factors Affecting Ionisation Energy
Nuclear charge
- Nuclear charge A larger number of protons attracts the outer electrons more strongly, so more energy is needed to remove one.
Distance
- Distance Electrons further from the nucleus feel a weaker attraction, which lowers the ionisation energy.
Shielding
- Shielding Inner shells repel outer electrons and reduce the pull of the nucleus, lowering the ionisation energy.
Net effect
- Net effect The attraction between nucleus and outer electron depends on all three factors acting together.
Part 3 of 5Successive Ionisation Energies
1
Successive ionisation energies are measured by removing electrons one at a time from the same atom.
2
Each one is larger than the last because the ion becomes more positive and holds its remaining electrons more tightly.
3
There is a large jump when the next electron must come from an inner shell closer to the nucleus.
4
For magnesium the values 738, 1451 and 7733 kJ mol⁻¹ show a huge jump after the second electron.
5
This jump shows that magnesium has two electrons in its outer shell, placing it in Group 2.
Part 4 of 5Writing Equations
Third ionisation of aluminium
- Third ionisation of aluminium Al²⁺(g) → Al³⁺(g) + e⁻ is the equation, with the 2+ ion on the left.
Atoms only
- Atoms only The first ionisation equation begins with a gaseous atom, not with an ion or a molecule.
Electron
- Electron The electron removed is shown as e⁻ on the right-hand side, and the charge must balance.
Diatomic elements
- Diatomic elements For elements such as chlorine the atoms, Cl(g), are used rather than the Cl₂ molecule.
Part 5 of 5Exam Checklist & Tips
1.1.6 – Key Points for the Exam
- First ionisation energy Energy required to remove one electron from each atom in one mole of gaseous atoms.
- Shielding The repulsion of outer electrons by inner-shell electrons, which reduces the effective nuclear attraction.
- Nuclear charge The total positive charge of the protons in the nucleus.
- Successive ionisation energies The energies needed to remove electrons one after another from the same atom.
Common Mistakes
- Leaving out state symbols in an ionisation energy equation, which are required.
- Writing the equation for the second ionisation energy starting from a neutral atom instead of the 1+ ion.
- Describing ionisation energy as the energy to remove all of the outer electrons.
- Forgetting that the definition refers to one mole of gaseous atoms.
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