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Predicting Molecule Shapes using Electron Pair Repulsion Theory, Assignments of Mathematics

Mathematics

The electron pair repulsion theory, which is used to predict the shapes of simple molecules and ions by considering the repulsions between pairs of electrons. It covers the shapes of various species, including those with and without lone pairs, and provides examples of molecules and ions. The document also explains how to determine the shape based on the number of electron pairs around the central atom.

Typology: Assignments

2019/2020

Uploaded on 04/27/2020

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owais-usman 🇵🇭

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SHAPES OF SIMPLE MOLECULES AND IONS
Electron pair repulsion theory
This is used to predict the shapes of simple molecules and ions by considering the repulsions
between pairs of electrons (lone pair and bond pair) within the molecule. It states that,
“The shape adopted is the one which keeps repulsive forces to a minimum”
To determine the shape, count up the number of covalent bond pairs and lone pairs around the
central atom and work out the shape which keeps the bonds as far apart as possible.
Species without lone pairs
Only bond pair repulsions occur and the basic shapes are regular.
Species with lone pairs
Lone pairs of electrons have a greater repulsive power than bond pairs so their presence will
affect the angles of bonds as they push the bond pairs away. The order of repulsive power is ...
lone pair - lone pair > lone pair - bond pair > bond pair - bond pair
The resulting configuration is based on the number of electron pairs but the actual shape does
not include the lone pairs. A water molecule is angular despite the fact that it has 4 electron
pairs around oxygen. Two of the pairs are lone pairs and are “invisible”.
Shapes of molecules 1
F321
© KNOCKHARDY 2PUBLISHING 008
SHAPE
BOND
ANGLE(S)
BOND
PAIRS
MOLECULE STRUCTURE
2
3
4
5
6
180°
120°
109.5°
90°
90°
120°
Tetrahedral
Trigonal
Octahedral
BeCl2
3
BF
4
CH
5
PF
6
SF
H
H
H
H
C
F
PF
F
F
F
FF
F
F
F
S
F
F
F
F
B
Be Cl
Cl Cl Be Cl
B
FF
F
C
H
H
HH
F
P
F
F
F
F
F
F
F
F
F F
S
bipyramidal
Trigonal
planar
Linear
pf2

Partial preview of the text

Download Predicting Molecule Shapes using Electron Pair Repulsion Theory and more Assignments Mathematics in PDF only on Docsity!

SHAPES OF SIMPLE MOLECULES AND IONS

Electron pair repulsion theory

This is used to predict the shapes of simple molecules and ions by considering the repulsions

between pairs of electrons (lone pair and bond pair) within the molecule. It states that,

“The shape adopted is the one which keeps repulsive forces to a minimum”

To determine the shape, count up the number of covalent bond pairs and lone pairs around the

central atom and work out the shape which keeps the bonds as far apart as possible.

Species without lone pairs

Only bond pair repulsions occur and the basic shapes are regular.

Species with lone pairs

Lone pairs of electrons have a greater repulsive power than bond pairs so their presence will

affect the angles of bonds as they push the bond pairs away. The order of repulsive power is ...

lone pair - lone pair > lone pair - bond pair > bond pair - bond pair

The resulting configuration is based on the number of electron pairs but the actual shape does

not include the lone pairs. A water molecule is angular despite the fact that it has 4 electron

pairs around oxygen. Two of the pairs are lone pairs and are “invisible”.

Shapes of molecules F321 1

© KNOCKHARDY 2PUBLISHING 008

BOND ANGLE(S) SHAPE BOND MOLECULE STRUCTURE PAIRS

2

3

4

5

6

180°

120°

90°

90°

120°

Tetrahedral

Trigonal

Octahedral

BeCl 2

BF 3

CH 4

PF 5

SF 6

H^ H

H

H

C

F

P F

F F

F

F (^) F

F

F

F S F

F

F F B

Cl Be Cl Cl Be (^) Cl

B

F (^) F

F

C

H

H

H H

P F

F

F

F

F

F

F

F

F F F

S

bipyramidal

Trigonal planar

Linear

Ammonia 3 bond pairs and 1 lone pair (total = 4 pairs) so the shape is based on a tetrahedron. As the lone pair-bond pair repulsions are greater than bond pair-bond pair repulsions the H-N-H bond angle is reduced from 109.5° to 107°.

:- shape is PYRAMIDAL

Water 2 bond pairs and 2 lone pairs (total = 4 pairs) so the shape is based on a tetrahedron. The extra lone pair-lone pair repulsion pushes the H-O-H bond angle down further to 104.5°.

:- shape is ANGULAR

Carbon 2 double bond pairs and no lone pairs dioxide For repulsive purposes, double bonds act like single bonds. The shape will be based on two bond pairs repelling each other. The bond angle is 180°.

:- shape is LINEAR

Simple ions

Shapes can be worked out according to the method shown. It allows you to predict the shape but in some cases not the true nature of the bonding.

For ions containing oxygen (e.g. SO 4 2-) some bonds are double and some single. In these cases add an electron to an oxygen atom for every -ive charge on the ion. Single bond these oxygens to the central atom and double bond the rest.

e.g. SO 4 2-

Sulphur has 6electrons in its outer shell. As the ion has a 2- charge, give two of the O’s an electron each to make them O¯ and form a single bond between them and S. The other two O’s are then double bonded to the sulphur. This produces 4 bonds and no lone pairs so the ion is tetrahedral ..

2 F321 Shapes of molecules

© KNOCKHARDY PUBLISHING 2008

Q.1 Determine the shapes of the following molecules and ions.

a) PCl 3 b) AlH 3 c) H 2 S d) SO 2 * e) SO 3 * f) PF 6 ¯ g) AlH 4 ¯

* double bonds are treated as single bonds for repulsion purposes (e.g. CO 2 is linear)

H N H

H

H

H

H

N

H

H

O

H H

O

N N-

N N N

N+

H^ H

H

H

H N H N+

H

N-

H H

NH 3 NH 4 + NH 2 -

SHAPE PYRAMIDAL^ TETRAHEDRAL ANGULAR

ELECTRON PAIRS

BOND PAIRS LONE PAIRS 1

3 4 2 (^0 )

BOND PAIRS LONE PAIRS

BOND PAIRS LONE PAIRS

If the species is an ion ... Add one electron for each negative charge or remove one electron for each positive charge

Draw out the OUTER electronic configuration of the central atom.

Pair up the electrons of the central species with those of the atom(s) surrounding it. Count the electron pairs.

O C O O C O