1.7.5Unit 1. Structure, Bonding and Introduction to Organic Chemistry
VSEPR Theory — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 25 of 36 Unit quiz
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.
Section 1 of 4Principles of the Theory
VSEPR stands for valence shell electron pair repulsion.
Basic idea
- Basic idea Regions of electron density around a central atom repel each other and so arrange themselves as far apart as possible.
Bonding pairs
- Bonding pairs Each pair of electrons in a single bond is one region, and a double or triple bond also counts as a single region.
Lone pairs
- Lone pairs Non-bonding pairs of outer electrons are also regions of electron density and take part in the repulsion.
Shape name
- Shape name The shape describes only the positions of the atoms, not the lone pairs, while the electron arrangement includes both.
Section 2 of 4Basic Electron Arrangements
Basic Electron Arrangements
Two regions
- Two regions The regions point in opposite directions to give a linear shape with a 180° bond angle, as in BeCl₂.
Three regions
- Three regions The regions spread flat to give a trigonal planar shape with 120° bond angles, as in BF₃.
Four regions
- Four regions The arrangement is tetrahedral with bond angles of 109.5°, as in CH₄.
Five regions
- Five regions The arrangement is trigonal bipyramidal with angles of 90° and 120°, as in PCl₅.
Six regions
- Six regions The arrangement is octahedral with all angles equal to 90°, as in SF₆.
Section 3 of 4Effect of Lone Pairs
1
A lone pair is held only by one nucleus and so is more spread out and closer to the central atom than a bonding pair.
2
This means that the order of repulsion is lone pair–lone pair, then lone pair–bonding pair, then bonding pair–bonding pair.
3
In ammonia, one lone pair pushes the three bonding pairs closer together, giving a pyramidal shape with an angle of about 107°.
4
In water, two lone pairs squeeze the bonds closer still, giving a bent shape and an H–O–H angle of about 104.5°.
5
A positive ion such as NH₄⁺ has no lone pairs, so it is perfectly tetrahedral at 109.5°.
Section 4 of 4Exam Checklist & Tips
1.7.5 – Key Points for the Exam
- VSEPR A theory that predicts molecular shapes from the repulsion between electron pairs around a central atom.
- Lone pair A pair of outer electrons not used in bonding.
- Bond angle The angle between two bonds at a central atom.
- Bonding pair A shared pair of electrons forming a covalent bond.
- Region of electron density A bonding pair, lone pair or multiple bond treated as one unit in VSEPR.
Common Mistakes
- Giving the shape of water as linear or tetrahedral instead of bent.
- Counting a double bond as two regions of repulsion.
- Saying that lone pairs reduce bond angles because they are more massive.
- Using 109.5° for ammonia or water without correcting for lone pairs.
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