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Review. Draw the Lewis structure for CH 3 OH. 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?

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  • 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

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
We are learning to…

  • Identify the shapes of small molecules;

  • Describe and apply the VSEPR theory;

8 1 the shape of small molecules
8-1 The Shape of Small Molecules

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

  • The bondangle is the geometric angle between two adjacent bonds.

8 1 the shape of small molecules why do we use models
8-1 The Shape of Small MoleculesWhy 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



Vsepr theory
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 electronsare arranged as far apart from each other as possible (due to repulsion of electrons).

  • This results in geometries that minimizethe energy of molecules.

Vsepr theory electron group geometry molecular geometry
VSEPR Theory: Electron Group Geometry & Molecular Geometry

  • This leads to the Molecular Geometry, which shows how bondedatomsare arranged around that same central atom. -This is due to charge repulsion of the electrons that surround the atoms.

Common molecular shapes
Common Molecular Shapes





  • BENT


  • 180º bond angle

  • CO2 or O2

  • 2 lone pairs of electrons

Trigonal planar

  • 120º bond angle

  • 1 central atom surrounded by 3 other atoms.

  • No loan pair of electrons in the central atom.

  • BCl3

  • 3 electron domains

Trigonal pyramidal

  • 107º bond angle

  • 1 lone pair of electrons in the central atom.

  • NH3

  • 4 electron domains


  • 109.5º bond angle

  • 1 central atom surrounded by 4 other atoms.

  • No unshared electrons

  • CH4

  • 4 electron domains


  • 105º bond angle

  • 2 lone pairs of electrons in the central atom.

  • OF2

  • 4 electron domains

Exit task
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 bonds

  • In 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)

(Source: Textbook and Teaching resources; also Kotz & Treichel, “Chemistry & Chemical Reactivity,”

5th Ed., Thomson, Brooks/Cole, 2003, pp 354-356.)

8 2 polarity

  • Recall 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 molecules
Properties of Polar Molecules

  • Formaldehyde

    • 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.