The Shapes of Molecules VSEPR Theory

Two Dimensions, Three Dimensions Electron dot diagrams and structural formulas are not complete representations of molecules. They do not take into account the three-dimensional shapes of molecules. The 3D shapes of molecules are very important when considering the physical and chemical properties of substances.

Any theory of chemical bonding must be valid in 3 dimensions, not just the 2D representations shown on paper. Spectroscopic and x-ray crystallographic data of molecules must be accounted for in any valid theory of chemical bonding.

Valence Shell Electron Pair Repulsion VSEPR theory – when electron pairs repel, molecules adjust their shapes so that the valence electron pairs are as far apart as possible.

The classic tetrahedral shape of methane is often used as a demonstration CH4 has four bonding pairs of electrons and no unshared pairs. Bond pairs are furthest apart when the angle between the central carbon and its hydrogens is 109.5. This theoretical result has been confirmed experimentally countless times.

If we substitute N for C as the central atom, we gain one more valence electron. What effect does this have on the structure of ammonia (NH3)? Ammonia has an unshared pair of electrons the unshared pair has no bonding atom, and is held more closely to the nucleus.

the unshared pair repels the bonding pairs, pushing them closer to each other (H – N – H bond angle = 107) the ammonia molecule has a pyrimidal shape.

Substituting O for N as the central atom, we gain a valence electron and another unshared pair of electrons. the two bonding pairs and the two unshared pairs form a tetrahedral arrangement about the central oxygen the molecule is planar, and bent the two unshared pairs strongly repel the bonding pairs (H – O – H bond angle = 104.5)

Carbon Dioxide In carbon dioxide, the central carbon has no unshared pairs. What is the best bond angle for those O=C=O double bonds?   the electrons are furthest apart from each other when the bond angle equals 180, which is exactly what is found in nature. CO2 is a linear molecule.

Other Interesting Shapes

Molecular Model Lab In this lab, you will experimentally observe the molecular models of some sample molecules. Some models may be two-dimensional, others will be three-dimensional. When drawing the three dimensional models on paper, use the following key:

Drawing 3D Structures Use an empty triangle to represent a bond below the plane of the paper (going in to the paper). Use a darkened triangle to represent a bond above the plane of the paper (coming out of the paper). Use a straight line to represent a bond in the plane of the paper.

Electron Pair Geometry Two electron pairs will arrange themselves linearly with an angle of 180°. Three electron pairs will arrange themselves in a trigonal planar arrangement with an angle between them of 120°. Four electron pairs will arrange themselves tetrahedrally with an angle between them of 109.5°.

Materials The molecular model kit should come with multiple colored balls, short stubby sticks (single bonds) and long flexible sticks (double and triple bonds). Atom Ball Color Hydrogen white Carbon black Oxygen red Chlorine green Nitrogen blue

Procedure For each molecule you will:   Draw a Lewis dot diagram for the species. If possible, obey the octet rule. Construct a model of the molecule. For double and triple bonds, use long flexible sticks. For single bonds, use the short stubby sticks. Considering the electronegativity of the atoms, the shape of the molecule, and the pure negativity of the unshared electrons, answer the questions on the chart.

Data Table Dot Diagram Picture Shape Polarity Notes and/or Questions Molecule Dot Diagram Picture Shape Polarity Notes and/or Questions CH4 Is this molecule symmetrical? CH3Cl Which atom is negatively charged compared to the others? CF4 The polarity of the molecule depends on the shape, not the bonds. NH3 Remember the pair of unbonded electrons. How will it affect its shape?

Notes and/or Questions Molecule Dot Diagram Picture Shape Polarity Notes and/or Questions H2O Why is this molecule extremely polar? O2 Double bonded molecule N2 Triple bonded molecule PH3 Is this more or less polar than NH3? Why?

Notes and/or Questions Molecule Dot Diagram Picture Shape Polarity Notes and/or Questions H2O2 You have this substance in your medicine cabinet CO2 Two double bonds CH3OH An alcohol C2H4 Double bond between the carbon atoms What is the shape of this molecule? C2H2 Triple bond between the C atoms HF This compound is nearly ionic H2S The red ball can be used for sulfur...Why?

Questions What is a dipole? Which molecular shapes will make a molecule with polar covalent bond become a non-polar molecule? Why do molecules with four single bonds with carbon have a tetrahedral shape? Methanol (CH3OH) has a boiling point of 70◦C. Methane (CH4) has a boiling point of –160◦C. Explain this difference in physical properties in terms of electronegativity differences between bonded elements. Which element should have a higher boiling point, ammonia (NH3), or phosphine (PH3)? Explain your reasoning.