Chapter 10 Structures of Solids and Liquids

10.2

Shapes of Molecules and Ions (VSEPR Theory)

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VSEPR Theory

In the valence-shell electron-pair repulsion theory (VSEPR), the electron groups around a central atom • • • are arranged as far apart from each other as possible have the least amount of repulsion of the negatively charged electrons have a geometry around the central atom that determines the molecular shape

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MOLECULAR GEOMETRY AND VSEPR

V

alence

S

hell

E

lectron

P

air

R

epulsion

•

VSEPR is used to predict the molecular geometries of compounds based on the number of electron groups attached to the center atom of the molecule.

•

Most important factor in determining the geometry is minimizing the relative repulsion between electron pairs.

•

Molecules’ or ions’ geometry reflects arrangement that favors minimal electron repulsion.

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Shapes of Molecules

The three-dimensional shape of a molecule • is the result of bonded groups and lone pairs of electrons around the central atom • is predicted using the VSEPR (valence-shell electron-pair repulsion) theory

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Two Electron Groups

• In BeCl 2 • • two electron groups are bonded to the central atom Be (exception to the octet rule) repulsion is minimized when the two electron groups are opposite each other at 180 ° the shape of the BeCl 2 molecule is linear

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Two Electron Groups with Double Bonds

• • • In CO 2 two electron groups are bonded to C (electrons in a double bond count as one group) minimal repulsion occurs when the two electron groups are opposite each other (180 ° ) the shape of the CO 2 molecule is linear

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Three Electron Groups

• In BF 3 three electron groups surround the central atom B (B is an exception to the octet rule) • • minimal repulsion occurs when the three electron groups are at angles of 120 ° the shape of the BF 3

trigonal planar

molecule is

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Two Electron Groups and a Lone Pair

In SO 2 • three electron groups, two bonded groups and one lone pair, surround the S atom • repulsion is minimized with three electron groups at angles of 120 ° , a trigonal planar arrangement.

• with two O atoms bonded to S and one lone pair of electrons, the shape of the SO 2 molecule is bent (120 

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• Three dimensional arrangement of the atoms in a molecule.

Bond Angle 180 o ~120 o ~109 o Copyright © Cengage Learning. All rights reserved

• Linear structure – atoms in a line  Carbon dioxide Copyright © Cengage Learning. All rights reserved

• Trigonal planar – atoms in a triangle  Boron trifluoride Copyright © Cengage Learning. All rights reserved

• Tetrahedral structure  Methane Copyright © Cengage Learning. All rights reserved

Section 12.9

VSEPR Model

• • VSEPR: Valence Shell Electron-Pair Repulsion.

The structure around a given atom is determined principally by minimizing electron pair repulsions .

Copyright © Cengage Learning. All rights reserved

Four Electron Groups

In a molecule of CH 4 •

four electron groups

are bonded to a C atom • repulsion is minimized by placing the four electron groups at angles of 109 ° , • a

tetrahedral arrangement

• the shape with four bonded atoms is

tetrahedral Basic Chemistry Copyright © 2011 Pearson Education, Inc.

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Three Bonding Atoms and One Lone Pair

In a molecule of NH 3 •

four electron groups

, three bonding groups, and one lone pair, surround a N atom • repulsion is minimized with four electron groups at angles of 109 ° , which is a tetrahedral arrangement of electron groups • with three bonded atoms and one lone pair of electrons, the shape is

trigonal pyramidal Basic Chemistry Copyright © 2011 Pearson Education, Inc.

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Two Bonding Atoms and Two Lone Pairs

In a molecule of H 2 O, •

four electron groups

, • • • two groups bonded to H atoms and two lone pairs, surround the O atom four electron groups minimize repulsion in a tetrahedral arrangement • the shape of the H 2 O molecule with two bonded atoms is

bent (109



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Shapes with Two or Three Electron Groups

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Shapes with Four Electron Groups

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Predicting Molecular Shape (VSEPR Theory)

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Learning Check

The shape of a molecule of N 2 O (N N O) is 1) linear 2) trigonal planar 3) bent (120 ° )

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Solution

STEP 1 Draw the electron-dot formula.

In the electron-dot structure with 16 e  , octets are acquired using two double bonds to the central N atom. • • • • : N : : N : : O :

STEP 2 Arrange the electron groups around the

central atom to minimize repulsion. Repulsion is minimized with two electron groups at 180 ° , a linear arrangement.

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Solution (continued)

STEP 3 Use the atoms bonded to the central atom to determine the molecular shape.

The shape of a N 2 O molecule with two bonded atoms and no lone pairs on the central N is linear.

• • • • : N=N=O : linear, 180° (1)

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Learning Check

State the number of electron groups, lone pairs, bonded atoms, and use VSEPR theory to determine the shape of the following molecules or ions: 1) tetrahedral 2) pyramidal 3) bent A. PF 3 B. H 2 S C. CCl 4 D. PO 4 3-

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Solution

A. PF 3 4 electron groups, 3 bonded atoms, 1 lone pairs, pyramidal (2) B. H 2 S 4 electron groups, 2 bonded atoms, 2 lone pairs, bent (3) C. CCl 4 4 electron groups, 4 bonded atoms, 0 lone pairs, tetrahedral (1) D. PO 4 3 4 electron groups, 4 bonded atoms, 0 lone pairs, tetrahedral (1)

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