Chemistry

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Topic

6

Microscopic World II

Unit 23 Shapes of molecules Unit 24 Bond polarity and intermolecular forces

Key

C o ncepts

Microscopic World II

Shapes of molecules • Covalent molecules with non-octet structures • Shapes of molecules • The VSEPR theory • Molecular crystals of buckminsterfullerene (C60) • Carbon nanotubes

Bond polarity and intermolecular forces • Electronegativity and bond polarity • Van der Waals’ forces • Factors affecting the strength of van der Waals’ forces • Hydrogen bonding • Surface tension and viscosity of liquids



Topic 6

Microscopic World II

Unit 23 Shapes of molecules



Unit 23
23.1 23.2 23.3 23.4 23.5 23.6 23.7

Shapes of molecules
Covalent molecules with non-octet structures Shapes of molecules of methane, ammonia and water Shapes of some other molecules Influence of the nature of electron pairs on bond angles in molecules of methane, ammonia and water Shapes of some polyatomic ions Shapes of molecules with multiple bonds Molecular crystals of buckminsterfullerene (C60)

23.1 – 23.7 Summary
1 Examples of covalent molecules with non-octet structures are as follows: Molecule Electron diagram F BF3 B F F SF4 F Molecule Electron diagram F S F F

Cl PCl5 Cl

Cl P Cl Cl SF6

F F

F S F

F F

2 Thefollowingtablesummarizestherelationshipbetweenthearrangementofelectron pairsaroundthecentralatomofamoleculeorpolyatomicionandtheshapeofthe species. Total number of electron pairs* in the outermost shell of the central atom of a molecule or polyatomic ion 2 linear

Arrangement of electron pairs*

Shape of the molecule or polyatomic ion

Example(s)

B

A linear B

B

BeCl2 CO2 CS2

3 trigonal planar

A B B trigonal planar B B A B B

BF3

CH4 SiF4 NH4
+

tetrahedral

PCl4+

NH3 4 tetrahedral B A B B PH3 NCl3 H3O+

trigonal pyramidal

H2O A B B H2S NH2–

V-shaped



Topic 6

Microscopic World II

Unit 23 Shapes of molecules



Total number of electron pairs* in the outermost shell of the central atom of a molecule or polyatomic ion

Arrangement of electron pairs*

Shape of the molecule or polyatomic ion

Example(s)

Students need to give good drawings of three-dimensional structures. Use the conventions commonly used in the representation of threedimensional structures. ♦ Do NOT confuse the terms ‘tetrahedral’ and ‘trigonal planar’ in the description of shapes of species.

B B 5 trigonal bipyramidal B A B trigonal bipyramidal B B 6 octahedral B A B B B SF6 PCl6 –

B

PCl5

The shape of a PH3 / NCl3 molecule is trigonal pyramidal, NOT trigonal planar.

P H H H Cl

N Cl Cl

octahedral

The shape of an NH4+ ion is tetrahedral, NOT square planar. ✘ H N H H H +

* When using the VSEPR theory, multiple bonds can be counted as single bonds.

3 a) The C60 molecule is a spherical molecule with the carbon atoms arranged at the 60 vertices of 32 interconnecting pentagons and hexagons. Each carbon atom forms three single covalent bonds with other carbon atoms. b)C60 is as soft as graphite and is a semiconductor. c) At room temperature, molecules in solid C60 are closely packed and bound by weak van der Waals’ forces. d)Interest in the fullerenes has led to the discovery of a related group of carbon structures referred to as nanotubes.

Questions may ask students to draw the shapes of unfamiliar species. e.g. XeF2 F 120°

Examtips

Examinationquestionsmayaskstudentstodrawtheelectrondiagrams of common ions. e.g. CO32– x x x x x x xx

Xe

F O
x x

2–

Cxx Ox xOx x
x x x x

key: electron from an external source –...
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