2.4.1Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Ionisation Energy – Groups 1 and 2 — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 26 of 53 Unit quiz
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 5Electron Configurations and Ions
Group 1 and 2 metals lose their outer s electrons to form positive ions with a noble gas configuration.
Group 1
- Group 1 Alkali metals have one electron in the outer s subshell, ns¹, and lose it to form M⁺ ions.
Group 2
- Group 2 Alkaline earth metals have two outer electrons, ns², and lose both to form M²⁺ ions.
Reducing agents
- Reducing agents Because they lose electrons readily, the metals are oxidised and so act as reducing agents.
First ionisation energy
- First ionisation energy The energy needed to remove one mole of electrons from one mole of gaseous atoms, giving gaseous 1+ ions.
Second ionisation energy
- Second ionisation energy The energy needed to remove one mole of electrons from one mole of gaseous 1+ ions, giving gaseous 2+ ions.
Section 2 of 5Trends Down the Group
Both the first and the second ionisation energies fall as you go down either group.
Trends Down the Group
Atomic radius
- Atomic radius Each element has one more electron shell than the one above, so the outer electron is further from the nucleus.
Shielding
- Shielding More inner shells shield the outer electrons from the positive nucleus, which weakens the attraction felt.
Nuclear charge
- Nuclear charge The nuclear charge rises down the group, but extra shielding and distance outweigh this effect.
Result
- Result Outer electrons are held less strongly, so less energy is needed to remove them and reactivity increases.
| Element | First ionisation energy / kJ mol⁻¹ |
|---|---|
| Lithium | 520 |
| Sodium | 496 |
| Potassium | 419 |
| Magnesium | 738 |
| Calcium | 590 |
Section 3 of 5Group 1 Compared with Group 2
1
Across a period nuclear charge increases while shielding stays nearly the same, so Group 2 first ionisation energies exceed those of Group 1.
2
A sodium atom loses one electron easily, but a Na⁺ ion has a full inner shell very close to the nucleus.
3
The second ionisation energy of sodium is therefore very large, so sodium forms only Na⁺ ions.
4
A magnesium atom has two outer electrons, and the total energy for both is still small compared with the lattice energy released.
5
Energy released when ions form a lattice compensates for the ionisation energies, so Group 2 compounds contain M²⁺ ions.
Section 4 of 5Worked Example
Question Using first and second ionisation energies of 738 and 1451 kJ mol⁻¹ for magnesium, calculate the energy needed to convert 1 mol of Mg(g) into Mg²⁺(g).
- Write the two stepsMg(g) → Mg⁺(g) + e⁻ and Mg⁺(g) → Mg²⁺(g) + e⁻.
- The energies add because the steps occur one after the other.
- Total = 738 + 1451.
Answer 2189 kJ mol⁻¹ is needed to form one mole of Mg²⁺(g) ions.
Section 5 of 5Exam Checklist & Tips
2.4.1 – Key Points for the Exam
- First ionisation energy The energy to remove one electron from each atom in one mole of gaseous atoms.
- Second ionisation energy The energy to remove one electron from each ion in one mole of gaseous 1+ ions.
- Shielding The repulsion of outer electrons by inner electrons, which reduces the nuclear attraction.
- Alkali metals The Group 1 elements, which form 1+ ions.
- Alkaline earth metals The Group 2 elements, which form 2+ ions.
Common Mistakes
- Saying nuclear charge decreases down the group; it increases, but shielding and distance outweigh it.
- Omitting state symbols (g) from the definition of ionisation energy.
- Describing ionisation energy per atom rather than per mole.
- Claiming the second ionisation energy is smaller than the first; it is always larger.
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