2.2.1Physical Chemistry
Trends in Group 2: The Alkaline Earth Metals — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 56 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 4Group 2 Basics
Group 2 elements are Be, Mg, Ca, Sr and Ba, each with two electrons in the outer s sub-shell.
Electron configuration
- Electron configuration Every Group 2 atom has outer configuration ns², for example magnesium is 1s² 2s² 2p⁶ 3s².
Ion formation
- Ion formation They lose both outer electrons to form M²⁺ ions, reaching a stable noble-gas configuration.
Reducing agents
- Reducing agents Because they lose electrons, Group 2 metals are oxidised in reactions and act as reducing agents.
Oxidation state
- Oxidation state In every compound the Group 2 metal has oxidation state +2.
Part 2 of 4Trends Down the Group
Trends Down the Group
Atomic radius
- Atomic radius Radius increases down the group because each element has an extra electron shell further from the nucleus.
Ionisation energy
- Ionisation energy Both first and second ionisation energies decrease because outer electrons are further away and more shielded.
Reactivity
- Reactivity Reactivity increases down the group because less energy is needed to remove the two outer electrons.
Melting point
- Melting point Melting points decrease overall down the group, but not smoothly, since magnesium melts at a lower temperature than calcium.
| Element | First IE (kJ mol⁻¹) | Second IE (kJ mol⁻¹) |
|---|---|---|
| Be | 900 | 1757 |
| Mg | 738 | 1451 |
| Ca | 590 | 1145 |
| Sr | 550 | 1064 |
| Ba | 503 | 965 |
Part 3 of 4Explaining Reactivity
1
Going down the group, the outer electrons occupy shells that are further from the positive nucleus.
2
Additional inner shells increase shielding, which offsets the growing nuclear charge.
3
The attraction between the nucleus and the outer electrons therefore becomes weaker.
4
Less energy is needed to remove both electrons, so the metals are oxidised more easily.
5
Greater ease of electron loss means that calcium, strontium and barium are more reactive than magnesium and beryllium.
Part 4 of 4Exam Checklist & Tips
2.2.1 – Key Points for the Exam
- Alkaline earth metals The elements of Group 2, which form M²⁺ ions and alkaline hydroxides.
- Second ionisation energy The energy needed to remove one electron from each ion in one mole of gaseous 1+ ions to form 2+ ions.
- Reducing agent A substance that loses electrons and so reduces another species.
- Shielding Repulsion of outer electrons by inner electrons, reducing the effective nuclear attraction.
Common Mistakes
- Saying reactivity increases down the group because there are more electrons, instead of explaining weaker attraction.
- Forgetting that the ion is M²⁺, so both electrons are lost.
- Describing melting point as decreasing smoothly when the trend is irregular.
- Stating that nuclear charge decreases down the group; it increases but shielding and distance outweigh it.
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