2.2.1Organic Chemistry
Explaining Group 2 Trends — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 5 of 42
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
Part 1 of 5Group 2 Structure
Outer electrons
- Outer electrons All Group 2 elements have two electrons in their outer s subshell, written ns².
Ion formed
- Ion formed They lose both outer electrons to form 2+ ions with the structure of a noble gas.
Metals
- Metals They are reactive metals that form ionic compounds with non-metals.
Oxidation number
- Oxidation number In compounds the oxidation number is always +2.
Position in table
- Position in table They are the s-block elements in the second column of the Periodic Table, reactive and stored away from moisture.
Part 2 of 5Atomic Radius and Ionisation Energy
Atomic Radius and Ionisation Energy
Atomic radius
- Atomic radius Increases down the group because each element has an extra electron shell.
Shielding
- Shielding More inner shells shield the outer electrons from the attraction of the nucleus.
First ionisation energy
- First ionisation energy Decreases down the group because outer electrons are further away and more shielded.
Nuclear charge
- Nuclear charge The increase in protons is outweighed by the larger distance and extra shielding.
Ionic radius
- Ionic radius The 2+ ions are smaller than their atoms but still increase in size down the group as more shells are present.
| Element | First ionisation energy (kJ mol⁻¹, approx.) |
|---|---|
| Be | 900 |
| Mg | 738 |
| Ca | 590 |
| Sr | 550 |
| Ba | 503 |
Part 3 of 5Reactivity Trend
1
Group 2 metals react by losing their two outer electrons.
2
Down the group, the outer electrons are lost more easily because ionisation energy decreases.
3
A lower energy requirement makes oxidation of the metal easier.
4
Therefore reactivity increases down the group, so barium is more reactive than magnesium.
5
Always link reactivity to ease of electron loss in explanations, not to the number of electrons lost.
Part 4 of 5Physical Properties
Melting points
- Melting points Melting points generally decrease down the group, though magnesium does not follow the pattern perfectly.
Metallic bonding
- Metallic bonding Larger ions and the same two delocalised electrons per atom give weaker metallic attraction.
Density
- Density Density generally increases down the group but with exceptions, so exam answers should avoid claiming a perfect trend.
Appearance
- Appearance All are silvery-grey metals that tarnish in air as an oxide layer forms.
Part 5 of 5Exam Checklist & Tips
2.2.1 – Key Points for the Exam
- Ionisation energy Energy needed to remove an electron from each atom in a mole of gaseous atoms.
- Shielding Repulsion of outer electrons by inner electrons, reducing nuclear attraction.
- Atomic radius Distance from nucleus to outer electrons.
- Reactivity How readily a metal loses electrons to form positive ions.
Common Mistakes
- Saying reactivity increases because atoms have more electrons to lose; each loses two.
- Explaining the trend by nuclear charge alone and forgetting shielding and distance.
- Stating that melting points decrease perfectly smoothly down the group.
- Confusing atomic radius with ionic radius.
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