Review

Draw the Lewis structure for CH3OH. Include polarity of the bonds.

How many unpaired electrons in the center atom of oxygen?

Chapter 8 “Molecular Shape”

Are we living in a two dimensional world?

Lewis structures show 2-dimensional pictures of molecules. In real life molecules are 3-dimensional.

We are learning to…

Identify the shapes of small molecules;Describe and apply the VSEPR theory;

8-1 The Shape of Small Molecules

The shape of a molecule (molecular geometry) is described by the geometric figure formed when the atomic nuclei are imagined to be joined in straight lines.

The bond angle is the geometric angle between two adjacent bonds.

8-1 The Shape of Small Molecules

Why do we use models?

Just as architects use models to see how a building will look when finished, chemists use models to help visualize how atoms of molecules are arranged in space.

Ball & stick Space-filling Wire

EXAMPLES

VSEPR Theory

The VSEPR theory helps explain the shapes of simple molecules.

Valence Shell Electron Pair Repulsion (VSEPR) theory states that, in small molecules, the pairs of valence electrons are arranged as far apart from each other as possible (due to repulsion of electrons).

This results in geometries that minimize the energy of molecules.

VSEPR Theory: Electron Group Geometry & Molecular Geometry

This leads to the Molecular Geometry, which shows how bonded atoms are arranged around that same central atom.

-This is due to charge repulsion of the electrons that surround the atoms.

Common Molecular Shapes

LINEARTRIGONAL PLANARTETRAHEDRAL TRIGONAL PYRAMIDAL BENT

LINEAR

180º bond angleCO2 or O2

2 lone pairs of electrons

TRIGONAL PLANAR

120º bond angle1 central atom surrounded by 3 other atoms.No loan pair of electrons in the central atom.BCl33 electron domains

TRIGONAL PYRAMIDAL

107º bond angle1 lone pair of electrons in the central atom.NH3

4 electron domains

TETRAHEDRAL

109.5º bond angle1 central atom surrounded by 4 other

atoms.No unshared electronsCH4

4 electron domains

BENT

105º bond angle2 lone pairs of electrons in the central

atom.OF2

4 electron domains

Exit Task

Draw the structure of NBr3.

How many lone pairs of electrons in the center atom?

What is the geometry of NBr3?

What is one factor that determines the geometric shape of a molecule?

PROPERTIES OF BONDSIn our models the bonds are all the same length,

but this is NOT true in reality.As one moves down a Group of the Periodic Table,

the atoms form longer bonds.Atoms get larger moving down a Group.

Multiple bonds are shorter and stronger than single bonds.The more electrons in a bond, the greater the

attraction to the positive nuclei of a bond.Electrons act as the ‘electrical glue’ between the two

nuclei.

Characteristics of Various Bonds)

BONDBOND TYPE DISTANCE (nm) STRENGTH (kJ/mol)

H-H Single 0.075 436

C-H Single 0.109 413

F-F Single 0.128 155

Cl-Cl Single 0.198 242

Br-Br Single 0.228 193

I-I Single 0.266 151

O-O Single 0.132 146

O=O Double 0.121 498

S=S Double 0.189

C-O Single 0.143 358

C=O Double 0.121 745

C Ξ O Triple 0.113 1046

C-N Single 0.147 305

C=N Double 0.138 615

C Ξ N Triple 0.116 887

C-C Single 0.154 347

C=C Double 0.134 614

C Ξ C Triple 0.120 839

(Source: Textbook and Teaching resources; also Kotz & Treichel, “Chemistry & Chemical Reactivity,” 5th Ed., Thomson, Brooks/Cole, 2003, pp 354-356.)

8-2 POLARITYRecall what we learned about polar & non-polar bonds.

Are electrons shared equally in all bonds? What happens if they are not shared equally?

Because of the polarity of bonds and how they are arranged around a central atom, molecules may also be polar or non-polar.

Dipole: a polar molecule, one that has a positive end and a negative end.

What determines the polarity of a molecule?Together the shape of a molecule and the

polarity of its bonds determine if a molecule is polar or non-polar.

Properties of Polar MoleculesFormaldehyde

A dipole because of the imbalance of polar bonds. A gas, but very soluble in water.

Carbon Dioxide Not polar because the effect of the two polar bonds cancel. Gas at room temperature due to lack of attraction between

molecules; also soluble in water.Water

A dipole because the bent shape of the molecule does not cancel the polar bonds.

The shape of the water molecule has a major impact on it’s properties, such as melting, boiling points, solubility and physical states at room temperature.

Large Molecules Dipoles are critical to the functions of life. Proteins, DNA/RNA, carbohydrates, lipids, etc.