1.2.1Physical Chemistry
Ionisation Energy — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 5 of 42
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.
Section 1 of 5Definitions and Equations
Ionisation energy measures how strongly an atom holds on to 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 For sodium: Na(g) → Na⁺(g) + e⁻, with state symbols showing that the atoms and ions are gaseous.
Second ionisation energy
- Second ionisation energy The energy to remove one mole of electrons from one mole of gaseous 1+ ions, for example Mg⁺(g) → Mg²⁺(g) + e⁻.
Units
- Units Ionisation energies are measured in kJ mol⁻¹ and are always positive because energy must be supplied.
Section 2 of 5Factors Affecting Ionisation Energy
Factors Affecting Ionisation Energy
Nuclear charge
- Nuclear charge A larger number of protons attracts the outer electron more strongly, increasing the ionisation energy.
Atomic radius
- Atomic radius The further the outer electron is from the nucleus, the weaker the attraction and the lower the ionisation energy.
Shielding
- Shielding Inner shell electrons repel outer electrons and reduce the attraction felt from the nucleus, lowering the ionisation energy.
Down a group
- Down a group First ionisation energy decreases because added shells increase both distance and shielding more than the extra nuclear charge matters.
Section 3 of 5Successive Ionisation Energies
1
Removing each further electron is harder because the remaining electrons are held by the same nuclear charge but with less repulsion between them.
2
Successive ionisation energies therefore increase steadily within a shell.
3
When the next electron comes from an inner, lower shell closer to the nucleus, there is a very large jump.
4
The position of the large jump shows how many electrons are in the outer shell, and so the group of the element.
5
Plotting the logarithm of the ionisation energy against the electron number shows the shells as steps in the graph.
| Mg ionisation | Value / kJ mol⁻¹ | Comment |
|---|---|---|
| 1st | 738 | Outer 3s electron |
| 2nd | 1451 | Second 3s electron |
| 3rd | 7733 | Large jump: inner shell |
| 4th | 10540 | Inner shell continues |
Section 4 of 5Worked Example
Question
- The ionisation energies of an element X in kJ mol⁻¹ are 738, 1451, 7733, 10540.
- State its group and write the equation for its second ionisation energy.
- The large jump comes between the second and third values, from 1451 to 7733.
- So two electrons are in the outer shell, and the third is taken from a closer inner shell.
- Two outer electrons means the element is in Group 2; the data are those of magnesium.
- Second ionisation energyX⁺(g) → X²⁺(g) + e⁻, which for magnesium is Mg⁺(g) → Mg²⁺(g) + e⁻.
Answer
- Group 2 (magnesium);
- equation: Mg⁺(g) → Mg²⁺(g) + e⁻.
Section 5 of 5Exam Checklist & Tips
1.2.1 – Key Points for the Exam
- First ionisation energy Energy to remove one electron from each of one mole of gaseous atoms.
- Successive ionisation energy The energy for each further electron removed, from the ion formed in the previous step.
- Shielding Repulsion of outer electrons by inner electrons, reducing the effective nuclear attraction.
- Gaseous state Atoms are gaseous so that no energy is used to overcome forces between particles.
Common Mistakes
- Leaving out state symbols from the ionisation energy equation.
- Writing the equation with a negative sign for the electron or as an exothermic change.
- Explaining a trend using only nuclear charge and ignoring distance and shielding.
- Using atoms instead of 1+ ions in the equation for the second ionisation energy.
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